Cushing’s Syndrome results from chronic exposure to excessive levels of glucocorticoids: its investigation and management remains an on-going challenge in clinical endocrinology. Although the condition is considered rare, the clinical spectrum of the disease is broad, and its investigation is frequently required given the high prevalence of many of its non-specific symptoms such as obesity, muscle weakness and depression. Clinicians are now considering the diagnosis in its earlier manifestations, often before the development of the more classic signs and symptoms as described by Harvey Cushing early in the last century. In its severe form and when untreated, the metabolic upset of Cushing's syndrome is associated with a high mortality, approximately 50% at five years [1]. However, more subtle excesses of cortisol may have significant effects on glycaemic control and blood pressure, and may therefore be an important cause of morbidity.
The glucocorticoid cortisol is secreted from the zona fasciculata and reticularis of the adrenal gland under the stimulus of adrenocorticotropin (ACTH) from the pituitary gland. ACTH in turn is secreted in response to corticotropin releasing hormone (CRH) and vasopressin from the hypothalamus. Cortisol exerts negative feedback control on both CRH and vasopressin in the hypothalamus, and ACTH in the pituitary. In normal individuals, cortisol is secreted in a circadian rhythm; levels fall during the day from a peak at 07.00h-08.00h to a nadir at around midnight: they then begin to rise again at 02.00h. It is the loss of this circadian rhythm, together with loss of the normal feedback mechanism of the hypothalamo-pituitary-adrenal (HPA) axis, which results in chronic exposure to excessive circulating cortisol levels and that gives rise to the clinical state of endogenous Cushing's syndrome [2].
Table 1. Aetiology of Cushing's syndrome
|
ACTH-dependent
|
|
ACTH-independent
|
The etiology of Cushing's syndrome can broadly be divided into two categories; ACTH-dependent and ACTH-independent (Table 1). Of the ACTH-dependent forms, pituitary-dependent Cushing's syndrome, Cushing's disease, is the most common, accounting for 60-80% of all cases. Ectopic sources of ACTH derive from multiple tumour types (Table 2), the most frequent being small-cell lung carcinoma. Of the ACTH-independent causes exogenous glucocorticoids is the most common cause. Excessive autonomous cortisol secretion can occur from an adrenal adenoma or carcinoma. In addition, rarer forms of Cushing's syndrome include ectopic CRH production, macronodular adrenal
hyperplasia secondary to abnormal hormone receptor expression (e.g. gastric inhibitory polypeptide, catecholamines, vasopressin, interleukin-1, etc.), McCune-Albright syndrome and primary pigmented nodular adrenal disease [3-6]. The latter is associated in approximately 50% of cases with Carney complex, an autosomal dominant disorder characterised by mesenchymal tumours, particularly atrial myxomas, pigmented skin lesions, endocrine and peripheral nerve tumours.
Table 2. Aetiology of the ectopic ACTH syndrome in patients seen at St. Bartholomew's Hospital 1969-2001
|
Site of secretion |
Female |
Male |
|---|---|---|
|
Bronchial carcinoid tumor |
11 |
2 |
|
Small cell lung carcinoma |
1 |
5 |
|
Medullary thyroid carcinoma |
3 |
|
|
Pancreatic carcinoid tumor |
1 |
2 |
|
Thymic carcinoid tumor |
1 |
|
|
Disseminated carcinoid tumor |
1 |
|
|
Mesothelioma |
1 |
|
|
Pancreatic carcinoma |
1 |
|
|
Colonic carcinoma |
1 |
|
|
Phaechromocytoma |
1 |
|
|
Gall bladder carcinoma |
1 |
|
|
Total |
16 |
16 |
Pseudo-Cushing's states are conditions in which a patient presents with clinical features suggestive of true Cushing's syndrome with some biochemical evidence of hypercortisolaemia. Depression and alcohol abuse are two such states, where treatment of the underlying disorder results in resolution of the Cushingoid state. There may also be diagnostic confusion in certain types of simple obesity: such cases can cause considerable diagnostic difficulty to the physician.
The clinical manifestations associated with hypercortisolaemia are variable and differ widely in severity. The classical impression of the disease as the association of gross obesity of the trunk with wasting of the limbs, facial rounding and plethora, hirsutism with frontal balding, muscle weakness, spontaneous bruising, vertebral fractures, hypertension and diabetes mellitus, is rarely seen nowadays in its most obvious form. Other symptoms include lethargy, depression, acne, easy bruising, loss of libido and menstrual irregularity [7]. The signs that are most useful in discriminating Cushing's syndrome from pseudo-Cushingoid states are myopathy, thin skin and easy bruising [8]. Sequential photographs of the patient over many years can be extremely helpful in demonstrating progression to a Cushingoid state. In children, weight gain associated with growth retardation should highlight the possibility of the diagnosis [9].
The rapid onset of the symptoms of profound weakness, associated with hyperpigmentation, often with little or no weight gain and an absense of a gross Cushingoid appearance, is almost certainly due to the ectopic ACTH syndrome, most often from small cell lung carcinoma. However, other forms of the ectopic ACTH syndrome, particularly associated with carcinoid tumors, may be clinically indistinguishable from patients with other forms of hypercortisolism [10]. Severe hirsutism and virilisation strongly suggest an adrenal carcinoma [7].
Some cases of ACTH-dependent Cushing's syndrome occur in a periodic or cyclical form, with intermittent and variable cortisol secretion, the symptoms and signs waxing and waning according to the active periods of the disease. These patients can cause particular diagnostic difficulty, as it is imperative that the diagnostic tests are performed in the presence of hypercortisolaemia to allow accurate interpretation. Patients may 'cycle in' or 'cycle out' over periods of months or years; if at presentation they are eucortisolaemic,they will need regular re-evaluation to allow full investigation at the appropriate time [11]. Cyclicity can occur with all causes of Cushing’s syndrome.
As stated above, hypercortisolaemia together with the loss of the normal circadian rhythm of cortisol secretion, and disturbed feedback of the HPA axis, are the cardinal biochemical features of Cushing's syndrome. Tests to confirm the diagnosis are based upon these principles. To screen for Cushing's syndrome, tests of high sensitivity should be used initially, so as to avoid missing milder cases. Tests of high specificity can then be employed to exclude false positives. It is important to realise that the validation of the published test criteria employed have been on specific assays, and thus test responses should ideally be validated on the local assay used before the results can be interpreted in particular patients. This is aided by supra-regional and nationwide inter-assay quality control assurance [2]. Exogenous oestrogens will increase cortisol-binding globulin and therefore total cortisol levels. Hence, in all investigations relying on a serum cortisol assay that measures total cortisol, hormone replacement therapy or the oral contraceptive pill should be stopped some time prior to investigation. We currently wait for a period of 6 weeks, although it is likely that a shorter time off treatment may still be effective.
Measurement of urinary free cortisol (UFC) is a non-invasive test that is widely used in the screening of Cushing's syndrome. Under normal conditions, 10% of plasma cortisol is 'free' or unbound and physiologically active. Unbound cortisol is filtered by the kidney, with the majority being reabsorbed in the tubules, and the remainder excreted unchanged. Thus, 24-hour UFC collection produces an integrated measure of serum cortisol, smoothing out the variations in cortisol during the day. In a series of 146 patients with Cushing's syndrome, UFC measurement was shown to have a sensitivity of 95% for the diagnosis [12]. However, within this series 11% had at least one of four UFC collections within the normal range, which confirms the need for multiple collections. The 24 hour UFC is of little value in the differentiation from pseudo-Cushing's states [13;14], although obesity per se does not appear to confound the results [15]. The major drawback of the test is the potential for an inadequate 24-hour urine collection, and written instructions must be given to the patient. In addition, creatinine excretion in the collection should be measured to assess completeness, and should equal approximately 1g/ 24 hours in a 70kg patient (variations depend on muscle mass). This should not vary by more than 10% between collections in the same individual [7]. In summary, UFC measurements have a high sensitivity if collected correctly, and several completely normal collections make the diagnosis of Cushing's syndrome very unlikely. Values greater than fourfold normal are rare except in Cushing's syndrome. For intermediate values the specificity is somewhat lower, and thus patients with marginally elevated levels require further investigation [2].
This test works on the principle that in normal individuals administration of an exogenous glucocorticoid results in suppression of the HPA axis, whilst patients with Cushing's syndrome are resistant, at least partially, to negative feedback. Dexamethasone is a synthetic glucocorticoid that is 30 times more potent than cortisol, and with an extremely long duration of action. It does not cross-react with most cortisol assays. The original low-dose dexamethasone test (LDDST) described by Liddle in 1960 measured urinary 17-OHCS after 48 hours of dexamethasone 0.5mg 6 hourly [16]. However, the simpler measurement of a single plasma or serum cortisol has been validated in various series and gives the test a sensitivity of between 95% and 100% [17-19]. The overnight LDDST was first proposed by Nugent et al in 1965; this measured a 09.00h plasma cortisol after a single dose of 1mg dexamethasone taken at midnight [20], and is thus considerably easier to perform. Since then, various doses have been suggested for the overnight test, between 0.5 and 2mg, and various diagnostic cut-offs have been used [21-23]. There appears to be no advantage in discrimination between 1mg and 1.5mg or 2mg [24]. Although higher doses have been tried, the increased suppression in some patients with Cushing's syndrome significantly decreases the sensitivity of the test [25]. In a comprehensive review of the LDDST, both the original 2 day test and the overnight protocol appear to have comparable sensitivities (98%-100%) using the criteria of a post-dexamethasone serum cortisol of <50nmol/l (1.7 μg/dL) [26]. However, the specificity is greater for the 2 day test (95%-100%) compared to the overnight test (88%) [26]. Despite the greater simplicity of the overnight test, we would still advocate the 48 hour test, even as an out-patient, as long as written instructions are given to the patient. If the overnight test is used, we suggest that a dose of dexamethasone 1mg at midnight and a threshold of <50nmol/l (1.7 μg/dL) at 09.00h will rarely lead to the diagnosis being missed, but false positives remain significant.
Factors such as variable absorption and increased metabolism can influence dexamethasone test results. A history of symptoms of malabsorption and a careful drug history should be taken prior to using the test in a patient. Hepatic enzyme inducers such as carbamazepine, phenytoin, phenobarbitone and rifampicin will reduce plasma dexamethasone concentrations [27], and will usually render the test uninterpretable.
Salivary cortisol measurement accurately reflects the plasma free cortisol concentration. Due to the simple non-invasive collection procedure which can conveniently be performed at home, and the fact that salivary cortisol is stable for days at room temperature, it offers a number of attractive advantages over blood collection, particularly in children. Loss of the circadian rhythm of cortisol secretion by measuring night-time salivary cortisol has been studied at a number of centres as a screening test for Cushing’s syndrome. The diagnostic value cut-off varies between studies because of different assays and the comparison groups studied (0.13 mg/dL (3.6 nmol/L) to 0.55 mg/dL (15.2 nmol/L)) [28-30] [31] [32] [33-36]. Normal values also differ between adults and paediatric populations, and may be affected by other comorbidities such as diabetes [37]. From these studies the sensitivity and specificity of this test appears to be relatively consistent at different centres, ranging from 92% to 100%, and 93% to 100% respectively. In summary, late-night salivary cortisol appears to be a useful and convenient additional screening test for Cushing's syndrome, particularly in the outpatient setting. However, local normal ranges need to be validated based on the assay used and population studied.
Before the introduction of salivary cortisol measurement a midnight serum cortisol was the only reliable method used to determine loss of the circadian rhythm of cortisol secretion. It is still useful as a second line test in cases of diagnostic difficulty. However, it is a burdensome test that requires that the patient should have been an in-patient for at least 48 hours to allow acclimatisation to the hospital environment. The patient should not be forewarned of the test, and should be asleep prior to venepuncture, which must be performed within 5-10 minutes of waking the patient. A single sleeping midnight plasma cortisol <50nmol/l (1.7 μg/dL) effectively excludes Cushing's syndrome [19], but false positive results do occur, particularly in the critically ill, in acute infection, heart failure, and in the pseudo-Cushing's state associated with depression [38]. An awake midnight cortisol of greater than 207 nmol/l (7 μg/dL) shows 94% sensitivity and 100% specificity for the differentiation of Cushing's syndrome from pseudo-Cushing's states [39].
When the diagnosis remains in doubt other tests may be necessary. The most promising of these are the combined dexamethasone-CRH test [13;40] and the desmopressin test [41;42] [43]. However, there remains some doubt as to whether these tests offer real advantages over standard tests [44], and they require further validation. The insulin tolerance test has been used to help distinguish Cushing's syndrome from pseudo-Cushing's states. The normal cortisol rise to insulin-induced hypoglycaemia is blunted in the majority of patients with Cushing's syndrome, but is usually preserved in depression-associated pseudo-Cushing's [45].
Once Cushing's syndrome has been diagnosed, the next step is to differentiate between ACTH-dependent and ACTH-independent causes. Nowadays, the simplest method of differentiation is the measurement of plasma ACTH. Rapid collection and processing of the sample is essential as ACTH is susceptible to degradation by peptidases so that the sample must be kept in an ice water bath and centrifuged, aliquoted, and frozen within a few hours to avoid a spuriously low result. Consistent ACTH measurements of less than 5 pg/mL or 5 ng/L (1.1pmol/L) essentially confirm ACTH-independent Cushing's syndrome. Conversely, if levels are consistently greater than 15 pg/mL (3.3pmol/L), Cushing's syndrome is ACTH-dependent. Intermediate levels are less discriminatory, but a lack of ACTH response to the CRH test (see below) may be particularly helpful. It is important to stress that all investigations into the differential diagnosis of Cushing's syndrome must be carried out in the presence of hypercortisolaemia.
Imaging of the adrenal glands is the mainstay in differentiating between the various types of ACTH-independent Cushing's syndrome. High-resolution computed tomography (CT) scanning of the adrenal glands is the investigation of choice and is accurate for masses greater than 1 cm and allows evaluation of the contralateral gland [46]. In certain circumstances MRI may be useful for the differential diagnosis of adrenal masses; the T2-weighted signal is progressively less intense in phaeochromocytoma, carcinoma, adenoma, and finally normal tissue [47]. Adrenal tumours typically appear as a unilateral mass with an atrophic contralateral gland [48]. If the lesion is greater than 5 cm in diameter it should be considered to be malignant until proven otherwise, and imaging characteristics should not be relied upon. Bilateral adenomas can be present [48]. In PPNAD the adrenal glands appear normal or slightly lumpy from multiple small nodules but are not generally enlarged [49]. Exogenous administration of glucocorticoids results in adrenal atrophy and very small glands may be a clue as to this entity. AIMAH is characterized by bilaterally huge (>5 cm) adrenals with a nodular pattern [50;51]. Confusion can arise as the CT appearance of the adrenals in AIMAH may be similar to the appearance seen in ACTH-dependent forms of Cushing's syndrome, where adrenal enlargement is present in 70% of cases [52], but the two can usually be distinguished by the ACTH level and the degree of adrenal enlargement. Some patients with Cushing's disease can also develop a degree of adrenal autonomy which can cause biochemical confusion [53;54].
This has been one of the most significant challenges in the investigation of Cushing's syndrome in the past, although advances over the last 15 years have greatly improved our diagnostic potential. Cushing's disease accounts for by far the majority of cases of ACTH-dependent Cushing's syndrome, between 85% and 90% in most series. This depends on gender, and in our series of 115 patients with ACTH-dependent Cushing's syndrome, of the 85 women, 92% had Cushing's disease; this percentage was 77% in the 30 men [55]. Therefore, even before one starts investigation, the pretest probability that the patient with ACTH-dependent Cushing’s syndrome has Cushing's disease is very high, and any investigation must improve on this pretest likelihood. However, as transsphenoidal pituitary surgery is widely accepted as the primary treatment of Cushing's disease, testing should be designed to avoid inappropriate pituitary surgery in patients with ectopic ACTH production. Thus, any test should ideally be set with 100% specificity for the diagnosis of Cushing's disease.
Levels of serum cortisol and ACTH tend to be higher in the ectopic ACTH syndrome, but there is considerable overlap of values, producing poor discrimination (Fig 1) [55;56]. Higher levels of cortisol saturate the renal enzyme 11β-hydroxysteroid dehydrogenase type 2, allowing it to exert a potent effect as a mineralocorticoid. This results in the hypokalaemia and an associated alkalosis seen in the majority of cases of ectopic ACTH syndrome [57]. It is important to measure the plasma bicarbonate, as hemolysed samples may have spuriously normal serum potassiums. However, 10% of cases of Cushing's disease also have hypokalaemia [56]; therefore, as a useful discriminator it has a high sensitivity but lower specificity for the ectopic ACTH syndrome. Very high serum cortisol levels are also associated with a fall in serum albumin, and this is therefore a more common finding in patients with the ectopic ACTH syndrome [58].
Figure 1. Serum cortisol levels during circadian studies in normal individuals and patients with ACTH-dependent cushing's syndrome. In a normal individual, the serum cortisol level falls naturally during the day: in Cushing's syndrome this circadian rhytmicity is lost.
As with the LDDST, the high dose dexamethasone suppression test (HDDST) was originally proposed by Liddle to differentiate between cortisol-secreting adrenal tumors and Cushing's disease, [16]. The HDDST’s role in the differential diagnosis of ACTH-dependent Cushing’s syndrome is based on the same premise: that most pituitary corticotroph tumors retain some albeit reduced responsiveness to negative glucocorticoid feedback, whereas ectopic ACTH-secreting tumours like adrenal tumours typically do not. The test is performed according to the same protocol as the LDDST either as 2mg 6 hourly for 2 days, or as an overnight using a single dose of 8mg of dexamethasone at 23.00h. Overall, only about 80% of patients with Cushing's disease will show suppression of cortisol to less than 50% of the basal value and there are high number of false positives tests (~10-30%) seen in ectopic Cushing’s syndrome [59-61] [62-64]. Shifting the criteria can only increase sensitivity with a loss of specificity, and vice-versa. Therefore, the test achieves worse discrimination than the pretest probability of Cushing's disease. In addition, a recent study has shown that suppression to HDDST can be inferred by a > 30% suppression of serum cortisol to the 2-day LDDST [65]. Therefore, we do not recommend routine use of the HDDST except when bilateral inferior petrosal sinus sampling is not available, and then only as part of a combined testing strategy.
The use of the CRH (corticotrophin-releasing hormone) test for the differential diagnosis of ACTH-dependent Cushing's syndrome is based on the premise that pituitary corticotroph adenomas retain responsivity to CRH, while ectopic ACTH tumours lack CRH receptors and therefore do not respond to the agent. CRH either 1 μg/kg or 100 μg synthetic ovine (oCRH) or human sequence CRH (hCRH) is given as a bolus injection and the change in ACTH and cortisol measured. Human sequence CRH has qualitatively similar properties to oCRH, although it is shorter acting with a slightly smaller rise in plasma cortisol and ACTH in normal and obese patients, and in those with Cushing's disease [66]; this may be related to the more rapid clearance of the human sequence by endogenous CRH-binding protein [67]. The availability differs worldwide with oCRH predominant in North America but hCRH elsewhere. Adverse effects encountered are brief facial flushing and a metallic taste in the mouth. However, there may be a slight fall in blood pressure, and this may be significant in patients with adrenocortical insufficiency.
Different centres have used differing protocols, including type of CRH and sampling time-points, and thus there is little consensus on a universal criterion for interpreting the test. In the largest published series of the use of oCRH, an increase in ACTH by at least 35% from a mean basal (-5 and -1 minutes) to a mean of 15 and 30 minutes after oCRH in 100 patients with Cushing's disease and 16 patients with the ectopic ACTH syndrome gave the test a sensitivity of 93% for diagnosing Cushing’s disease, and was 100% specific. The best cortisol criterion proved less discriminatory [68]. Conversely, in the largest series of the use of hCRH in 101 patients with Cushing's disease and 14 with the ectopic ACTH syndrome, the best criterion to differentiate Cushing's disease from ectopic ACTH syndrome was a rise in cortisol of at least 14% from a mean basal (-15 and 0 minutes) to a mean of 15 and 30 minutes, giving a sensitivity of 85% with 100% specificity. The best ACTH response was a maximal rise of at least 105%, giving 70% sensitivity and 100% specificity [55]. In a multicentre analysis from Italy, both hCRH or oCRH were used in 148 patients with Cushing's disease and 12 with the ectopic ACTH syndrome. A maximal 50% increase in ACTH and cortisol levels were considered as consistent with Cushing's disease, excluding all patients with the ectopic ACTH syndrome and thus giving 100% specificity. The sensitivity and specificity for the ACTH response were comparable for the two types of CRH (sensitivity: 85% vs 87% for oCRH and hCRH respectively). However, the sensitivity for the cortisol response was significantly greater with oCRH than with hCRH (sensitivity: 67% vs 50% for oCRH and hCRH respectively) [69]. The authors do not report in this paper or an associated publication [70] whether time-point combinations other than the maximal were analysed for the rise in cortisol. Indeed, our data show that if the maximal rise in cortisol is used the sensitivity falls to 71% [55]. These results again demonstrate that specific criteria need to be developed for each test, and cannot readily be extrapolated from other similar but non-identical agents.
In summary, the CRH test is a useful discriminator between causes of ACTH-dependent Cushing's syndrome, but which cut-off to use should be evaluated at individual centres, and caution should be exercised as there will undoubtedly be patients with the ectopic ACTH syndrome who respond outside these cut-offs.
Both vasopressin and desmopressin (a synthetic long-acting vasopressin analogue without the V1-mediated pressor effects) stimulate ACTH release in Cushing’s disease, probably through the corticotroph-specific V3 (or V1b) receptor. Hexarelin (a growth hormone secretagogue) stimulates ACTH release probably occurs through stimulation of vasopressin release in normal subjects [71], and by stimulation of aberrant growth hormone secretagogue receptors in corticotroph tumors [72]. These peptides have been utilised in a similar manner to CRH to try and improve the differentiation of ACTH-dependent Cushing’s syndrome, but have unfortunately proved inferior [73-75]. A combined desmopressin and hCRH stimulation test initially looked promising [76], but a further study of this combined test showed significant overlap in the responses [77]. The inferior discriminatory value of these stimulants is most likely due to the expression of both vasopressin and growth hormone secretagogue receptors by some ectopic ACTH-secreting tumours [78] [79].
Given the suboptimal diagnostic utility of individual non-invasive tests, a number of groups have evaluated combined test strategies. The CRH and high-dose dexamethasone test results when combined have a diagnostic accuracy greater than that of either test alone, yielding 98% to 100% sensitivity, and an 88% to 100% specificity [60;61;80]. A similar high accuracy has been obtained by combining the results of the LDDST and the CRH test [17-19;65] . However, it should be remembered that responses to both CRH and high-dose dexamethasone are more frequently discordant in Cushing's disease due to a macroadenoma [81].
This procedure involves placement of sampling catheters in the inferior petrosal sinuses that drain the pituitary. Blood for measurement of ACTH is obtained simultaneously from each sinus and a peripheral vein at two time points before and at 3-5 minutes and possibly also 10 minutes after the administration of ovine or human CRH (IV 1 μg/kg or 100μg respectively). A central (inferior petrosal) to peripheral plasma ACTH gradient of 2:1 or greater pre-CRH, or a gradient of 3:1 post-CRH is consistent with Cushing's disease. Results from early series show these criteria to be 100% sensitive and specific for Cushing’s disease [82;83]. However, it is now clear that false negative tests and to a smaller degree false positive test results do occur [84-86]. In order to minimise these it is important to ensure the patient is actively hypercortisolaemic at the time of the study [87], and that catheter position is confirmed as bilateral and any anomalous venous drainage noted by venography before sampling [88]. Overall, BIPSS is probably the most accurate diagnostic tool in the investigative armoury of ACTH dependent Cushing's syndrome with a sensitivity of approximately 94% and a specificity fractionally short of 100% for the diagnosis of Cushing’s disease. There appears to be no discriminatory difference between ovine or human sequence CRH. Recent data suggest that where is CRH is unavailable desmopressin 10 μg may be used instead [89].
It should be noted that the procedure is technically difficult, and should only be performed by radiologists experienced in the technique. The most common complications are transient ear discomfort or pain, and local groin haematomas. More serious transient and permanent neurological sequelae have been reported, including brainstem infarction, although these are rare (<1%), and most have related to a particular type of catheter used [90] [91]; if there are any early warning signs of such events the procedure should be immediately halted. Patients should be given heparin during sampling to prevent thrombotic events [79]. CRH itself is generally tolerated well, although patients may experience brief facial flushing and a metallic taste in the mouth. One case of CRH inducing pituitary apoplexy in a patient with Cushing’s disease has been reported [92]. There appears to be advantage in trying to sample the cavernous sinus.
BIPSS is also useful to lateralise microadenomas within the pituitary using the inferior petrosal sinus ACTH gradient (IPSG), with a basal or post-CRH inter-sinus ratio of at least 1.4 being the criteria for lateralisation used in all large studies [83;84;93;94]. In these studies the diagnostic accuracy of localisation as assessed by operative outcome varied between 59% and 83%. This is improved if venous drainage is assessed to be symmetric [95]. The accuracy of lateralisation appears to be higher in children (90%), a situation where imaging is often negative [96]. There is some discrepancy between studies as to whether CRH improves the predictive value of the test [97]. If a reversal of lateralisation is seen pre- and post-CRH, the test cannot be relied upon [98].
Imaging of the pituitary is an important part of the investigation of ACTH-dependent Cushing's syndrome to identify a possible pituitary lesion and to aid the surgeon during exploration. However, the results must be used in conjunction with the biochemical assessment as approximately 10% of normal subjects may have pituitary incidentalomas on MRI [99]. In addition, the majority of cases of Cushing's disease are due to microadenomas that may be only a few millimeters in diameter (the mean diameter has been quoted as 6mm), and therefore some will not be seen on current imaging techniques. Computerised tomography (CT) imaging typically shows a hypodense lesion that fails to enhance post-contrast. On MRI, 95% of microadenomas exhibit a hypointense signal with no post-gadolinium enhancement (Fig 2); however, as the remaining 5% have an isointense signal post-gadolinium, pre-gadolinium images are essential [100]. CT has a sensitivity of only approximately 40%-50% for identifying microadenomas, and is thus significantly inferior to MRI (sensitivity 50%-60%)[70;101;102], and it should therefore be reserved for patients in whom MRI is contraindicated or unavailable.
Figure 2. Magnetic resonance imaging (MRI), showing a right sided hypointense (post-gadolinium) corticotroph adenoma (arrow).
Preoperative localisation to one side of the pituitary gland by MRI had been advocated as better than BIPSS with a positive predictive value of 93% [85;103]. Other groups have found MRI less effective [104], [84]. In addition, as noted above, we have found MRI often to be unhelpful in the paediatric age group, and BIPSS to be of significant value in these patients [96].
Visualising an ectopic ACTH source can be a challenge, but in general patients should begin with imaging of the chest and abdomen with CT and/or MRI, bearing in mind likely sites (Table 2). The most common site of the secretory lesion is the chest, and although small cell lung carcinomas are generally easily visualised, small bronchial carcinoid tumors that can be less than 1cm in diameter often prove more difficult. Fine-cut high-resolution CT scanning with both supine and prone images can help differentiate between tumors and vascular shadows [2]. MRI can identify chest lesions that are not evident on CT scanning, and characteristically show a high signal on T2-weighted and short-inversion-time inversion-recovery images [105].
The majority of ectopic ACTH secreting tumours are of neuroendocrine origin and therefore may express somatostatin receptor subtypes. Depending on the receptor subtype expression, these tumours may show binding of different somatostatin analogues, e.g. octreotide (Novartis) or lanreotide (Ipsen), and therefore radiolabelled somatostatin analogue (111In-pentetreotide) scintigraphy may be useful to show either functionality of identified tumours, or to try and localise radiologically unidentified tumours [106]. Undoubedly this is a useful technique, but to date there are only sporadic reports that it identifies lesions not apparent using conventional imaging [107;108]. However, a lesion of uncertain pathology is more likely to represent a neuroendocrine tumour, and hence an ectopic source of ACTH, if somatostatin scintigraphy is positive.
As part of an international workshop on Cushing's syndrome in 2002, a consensus statement has been published for the diagnosis and differential diagnosis of Cushing's syndrome. It is recommended that three 24hr-UFC and /or the LDDST are used as the first line screening test. Late-night salivary cortisol requires further evaluation but appears promising. False-positive screening results will be common, and therefore second-line tests should be used as necessary for confirmation. Once the diagnosis of Cushing’s syndrome is unequivocal, ACTH levels, the CRH test, and possibly the HDDST, together with appropriate imaging, are the most useful non-invasive investigations to determine the aetiology. BIPSS is recommended in cases of ACTH-dependent Cushing’s syndrome where the clinical, biochemical, or radiological results are discordant or equivocal [79]. However, our own approach has been to abandon the HDDST and use BIPSS in almost all cases of ACTH-dependent Cushing’s syndrome.
Transsphenoidal surgery is widely regarded as the treatment of choice for Cushing’s disease [109]. The overall remission rate in various large series is in the order of 70%-75%, although higher rates of approximately 90% can be achieved with microadenomas [70;110-112]. Of the patients achieving remission, about 25% of these will have a recurrence by 10 years [113]. Where an adenoma is apparent at transsphenoidal exploration, a selective adenomectomy is performed, and the surgeon may be guided by pre-operative imaging. However, where no tumour is obvious a hemi-hypophysectomy as guided by the IPSS results is often the best course of action, hopefully achieving cure without panhypopituitarism. Where remission is not achieved at the first operation, a re-operation may be attempted, but appears to offer prolonged remission in only around 50% of cases, and with a high risk of hypopituitarism [114]. Hypocortisolaemia and the need for glucocorticoid replacement post-operatively has been well demonstrated to be a positive predictor of long-term remission of Cushing’s disease and should be the aim of surgery [111;115;116], but is not a guarantee of cure [117]. In those patients receiving less than a total hypophysectomy, the suppressed normal corticotrophs usually (but not always) recover by 12 months, and glucocorticoid replacement needs to be continued until such recovery, with regular reassessment of the HPA axis. The procedure is not without risks, and in the European Cushing’s disease survey group of 668 patients, the perioperative mortality was 1.9%, with other major complications occurring in 14.5% [111]. The frequency of reported adverse events varies widely: diabetes insipidus (either temporary or permanent) (3%-46%); hypogonadism (14%-53%); hypothyroidism (14%-40%); cerebrospinal fluid rhinorrhoea (4.6%-27.9%); severe growth hormone deficiency (13%); bleeding (1.3%-5%); and meningitis (0-2.8%) [111;112;115;118]. Transsphenoidal surgery is also a useful procedure in patients with Nelson’s syndrome to reduce tumour size, and ameliorate hyperpigmentation [119].
Adrenalectomy is the definitive treatment for all cases ACTH-independent Cushing’s syndrome. This is either unilateral in the case of an adrenal adenoma or carcinoma, or bilateral in cases of bilateral hyperplasia. In adrenal adenomas cure following surgery in skilled hands approaches 100% [120]. Bilateral adrenalectomy is also an important therapeutic option in patients with ACTH-dependent Cushing’s syndrome not cured by other techniques. However, the development of Nelson’s syndrome in patients with ACTH-secreting pituitary adenomas occurs in between 8% and 38% of cases [121]. The chance of developing Nelson’s syndrome appears to be greater if adrenalectomy is performed at a younger age, and if a pituitary adenoma is confirmed at previous pituitary surgery [113;121]. Prophylactic pituitary radiotherapy probably reduces the risk of developing Nelson’s syndrome [122]. However, it may be best to hold radiotherapy in reserve and undertake regular MRI scanning of the pituitary, especially whern imaging has originally not shown any clear tumour [123]. Others have advocated unilateral adrenalectomy plus pituitary irradiation as an alternative to bilateral adrenalectomy, as it gives similar remission rates to primary transsphenoidal surgery [124], but this should be reserved for selected cases.
Since its introduction in 1992 [125], laparoscopic adrenalectomy, both unilateral and bilateral, has been shown to be a safe procedure and in many centers has become the approach of choice. Its complication rate is lower than with the open approach, and the in-patient stay is significantly reduced. However, the area of controversy that remains is its use in potentially malignant lesions, and in such patients the traditional open approach is preferable [126]. Patients undergoing bilateral adrenalectomy will require lifelong mineralocorticoid and glucocorticoid replacement, and patients after unilateral surgery will require glucocorticoid replacement whilst the suppressed HPA axis reawakens.
If the ectopic ACTH-secreting tumour is benign and amenable to surgical excision, such as in a lobectomy for a bronchial carcinoid tumour, the chance of cure of Cushing’s syndrome is high. However, if significant metastatic disease is present, surgery is not curative although it may still be of benefit in selected cases.
Primary pituitary radiotherapy for the treatment of Cushing’s disease has been shown to produce poor long-term remission rates of only 37% in one series [113]. In contrast, as a second line therapy to failed pituitary surgery better results are achieved with 83% showing long-term remission as defined by the normalisation of the clinical state and biochemical parameters in a series of 30 patients, 88% of these achieving remission within two years, although it can take up to 5 years [127]. The major side effect is hormone deficiency, occurring in 68%, with a lesser incidence of hypogonadism (40%) and hypothyroidism (16%), with only one patient developing hypocortisolaemia in the reported series.
Gamma knife radiosurgery is a relatively new development that has been utilised in patients with Cushing’s disease as a second-line treatment [128], and also in Nelson’s syndrome [129]. In 43 patients treated following failed transsphenoidal surgery, 27 patients (63%) showed remission, although 3 of these later relapsed [130]. As with other forms of radiotherapy, new hormone deficiencies are the major side-effect. It is unclear as to whether the rate of onset of cortisol normalisation is faster than for conventional external radiotherapy, but treatment in a single session, where a clear discrete lesion may be seen, may be logistically advantageous.
Overall, there is no significant evidence that radiotherapy improves survival in adrenocortical carcinoma, although in the literature there are sporadic reports that it may be helpful adjuvant treatment to radical surgery in selected cases and will decrease local recurrence [131-133]. Local radiotherapy following surgical resection of an ectopic ACTH-secreting source may also be beneficial, particularly in non-metastatic thoracic carcinoid tumours [134;135].
The role of medical treatment of Cushing’s syndrome is an important one. It is the routine practice of many groups to pre-treat Cushing's syndrome patients prior to surgical treatment to reverse the hypercortisolaemia and its metabolic sequelae, and to hopefully reduce the complications of the definitive procedure. Similarly, medical treatment is desirable in patients with Cushing's disease whilst waiting for pituitary radiotherapy to take effect. In patients where surgery and/or radiotherapy has failed, medical management is often essential prior to (or long-term as an alternative) bilateral adrenalectomy. It may not always be possible to identify the source of ACTH in certain cases of ACTH-dependent Cushing's syndrome, and therefore medical management is desirable pending re-investigation. Finally, medical therapy is helpful as a palliative modality in patients with metastatic disease causing Cushing's syndrome.
These agents are primarily used as inhibitors of steroid biosynthesis in the adrenal cortex, and thus can be utilised in all cases of hypercortisolemia regardless of cause, often with rapid improvement in the clinical features of Cushing's syndrome. The most commonly used agents are metyrapone, ketoconazole, mitotane and in certain circumstances etomidate.
Metyrapone acts primarily to inhibit the enzyme 11β-hydroxylase, thus blocking the production of cortisol from 11-deoxycortisol in the adrenal gland [136]. The subsequent elevation of 11-deoxycortisol can be monitored in the serum of patients treated with metyrapone. It should be noted that there may be some cross-reactivity from 11-deoxycortisol with some cortisol radioimmunoassays: this may result in an unnecessary increase in the metyrapone dose and subsequent clinical hypoadrenalism [137]. The fall in cortisol is rapid, with trough levels at 2 hours post-dose, and in our unit we usually administer a test dose of 750mg with hourly cortisol estimation for 4 hours [138]. Maintenance therapy is 500-6000 mg/day in 3-4 divided doses daily. Metyrapone has been used to good effect to reduce the hypercortisolaemia in patients with Cushing's syndrome from adrenal tumours, the ectopic ACTH syndrome, and Cushing's disease. In the former, patients can be very sensitive to low doses of this agent, whilst in Cushing’s disease higher doses are often required. In Cushing's disease this can be due to the compensatory rise in ACTH in patients not having received pituitary radiotherapy.
The principal side-effects with metyrapone are hirsutism and acne (as predicted by the rise in adrenal androgens), dizziness and gastrointestinal upset. However, it is hypoadrenalism that remains the most important potential problem, and careful monitoring of treatment and education of the patient is required. Hypokalaemia, oedema and hypertension due to raised mineralocorticoids are infrequent [138], but can require cessation of therapy [139].
Ketoconazole is an imidazole derivative which was originally developed as an oral anti-fungal agent. It is an inhibitor of sex steroids production by its action on C17-20 lyase, and cortisol secretion by 11β-hydroxylase inhibition [140-142]. It also inhibits 17-hydroxylase and 18-hydroxylase activity, amongst other enzymes [143]. It has also been reported to have a direct effect on ectopic ACTH secretion from a thymic carcinoid tumor [144]. Treatment for Cushing's syndrome is usually started at a dose of 200mg twice daily, with an onset of action that is slower than metyrapone. It has been used sucessfully to lower cortisol levels in patients with Cushing's syndrome of various aetiologies including adrenal carcinoma, the ectopic ACTH syndrome, and invasive ACTH-producing pituitary carcinoma, with doses required between 200-1200mg/day in up to 4 divided daily doses [145-147].
The principal side effect of ketoconazole is hepatotoxicity. Reversible elevation of hepatic serum transaminases occurs in approximately 5%-10% of patients, with the incidence of serious hepatic injury at around 1 in 15,000 patients [148]. The hepatotoxicity appears to be idiosyncratic, but has been reported within 7 days of the start of treatment in a patient with Cushing's syndrome [149]. Other adverse reactions of ketoconazole include skin rashes and gastrointestinal upset [150], and one must always be wary of causing adrenal insufficiency [149;151]. Due to its C17-20 lyase inhibition and consequent anti-androgenic properties, ketoconazole is particularly useful in female patients where hirsutism is an issue, which may be worsened with metyrapone. Conversely, gynaecomastia and reduced libido in male patients may be unacceptable and require alternative agents. One further advantage of ketoconazole is its inhibition of cholesterol synthesis, particularly LDL cholesterol [152], and in 34 patients with Cushing's syndrome the mean total cholesterol was reduced from 6.1 to 5.0 mmol/l on ketoconazole [150].
Mitotane (o’p'DDD), an isomer of the insecticide DDD (belonging to the same family of chemicals as the insecticide DDT), was developed following the observation of adrenal atrophy in dogs administered DDD. It reduces cortisol and aldosterone production by blocking cholesterol side-chain cleavage and 11β-hydroxylase in the adrenal gland [153]. Mitotane is used as a treatment for adrenal carcinoma and causes tumour regression and improved survival in some patients [154;155], and has a beneficial effect on endocrine hypersecretion in approximately 75% of patients [156]. It is also utilised in Cushing's syndrome of non-malignant origin, and in this regard lower doses can be utilised (up to 4 g/day versus 12 g/day), thus reducing the incidence of side effects, particularly gastrointestinal [137;157]. At these lower doses the onset of the cortisol lowering effect takes longer (6-8 weeks) than with higher doses. One major problem even with lose dose mitotane is the hypercholesterolaemia (principally an increase in LDL-cholesterol), which appears to be due to the impairment of hepatic production of oxysterols, normally a brake on the enzyme HMG Co A reductase [158]. However, simvastatin, an HMG Co A reductase inhibitor, can reverse the hypercholesterolaemia, and it or a similar agent should be used if necessary in patients treated with mitotane [158]. Other side effects of mitotane include: neurological disturbance; elevation of hepatic enzymes; hypouricaemia; gynaecomastia in men; and a prolonged bleeding time [156;159]. In the long-term, measurement of blood levels can allow dose titration and reduction as appropriate. A therapeutic level of 14-20 μg/ml has been recommended. In general, its use has been limited outside of adrenal carcinoma, in which cases it has recently been shown to prolong life [155].
Etomidate is an imidazole-derived anaesthetic agent which was reported to have an adverse effect on adrenocortical function in 1983 [160]. Compared to the other imidazole derivative ketoconazole, etomidate more potently inhibits adrenocortical 11β-hydroxylase, has a similar inhibition of 17-hydroxylase, but has less of an effect on C17-20 lyase [161]. At higher concentrations it also appears to have an effect on cholesterol side-chain cleavage [162;163]. Following their initial report in 1983 [164], Allolio et al have shown that low-dose intravenous non-hypnotic etomidate (2.5mg/hour) normalised cortisol levels in 5 patients with Cushing's syndrome of various aetiologies [165]. Since then, there have been a number of case reports on the use of etomidate in successfully reducing hypercortisolaemia in seriously ill patients with either Cushing's disease or the ectopic ACTH syndrome [166-169]. It is usually given at a dose of 2.5 - 3.0mg/hour which is adjusted based on the serum cortisol levels. Etomidate is an effective adrenolytic agent that acts rapidly, but is limited in its use by the fact it has to be given parenterally. However, in this situation it may be life-saving.
A number of agents including the 5-HT antagonists cyproheptadine [170] and ritanserin [171], the dopamine agonists bromocriptine [172] and cabergoline [173], the somatostatin analogue octreotide [174], and sodium valproate [175] have been tried in Cushing's disease. The precise mechanism of action of many of these agents is incompletely understood, although most seem to reduce ACTH secretion through an effect on the hypothalamo-pituitary axis. Their efficacy in Cushing's disease seems to be quite variable between individual patients, and therefore we do not recommend their routine use.
Some ectopic ACTH-secreting tumours also express somatostatin receptors and somatostatin analogs may be useful in such tumours when complete surgical resection is impossible [174]. It is possible that the introduction of somatosatin analogues with a broader spectrum of activity for somatostatin receptor sub-types, will increase the therapeutic role of these agents. Such an agent, pasireotide (SOM230, Novartis, Basel, Switzerland) has been shown in vitro to reduce human corticotroph proliferation and ACTH secretion [176], and early results in vivo are encouraging [177].
Retinoic acid has been found to inhibit ACTH-secretion and cell proliferation both in vitro in ACTH-producing tumor cell lines, and cultured human corticotroph adenomas, and in vivo in nude mice [178]. These potential anti-secretory and anti-proliferative activities of this agent in Cushing’s syndrome need to be investigated further.
The thiazolidinedione rosiglitazone, a PPAR-γ agonist, has been shown in supra-pharmacological doses to induce G0/G1 cell-cycle arrest and apoptosis, and suppress ACTH secretion in human and murine corticotroph tumor cells. In addition, the development of murine corticotroph tumours, generated by subcutaneous injection of ACTH-secreting AtT20 cells, were prevented [179]. It appears this is not specific to corticotroph adenomas, but also applies to other forms of pituitary tumor [180]. However, results in human subjects with Cushing's disease have been disappointing [181-184]. This may be because doses used in the animal studies were much higher than the equivalent licensed dose in humans. Its use cannot be recommended.
In the rare causes of Cushing’s syndrome due to bilateral macronodular adrenal hyperplasia (AIMAH) and aberrant receptor expression of GIP, β-adrenergic and LH/hCG receptors, specific receptor antagonists may prove to be useful (Lacroix, 2000 473 /id}.
The glucocorticoid antagonist mifepristone (RU 486) is a potent antagonist of glucocorticoid and progesterone receptors [185]. In humans, mifepristone blocks glucocorticoid-induced negative feedback at the hypothalamo-pituitary level, inducing a rise in ACTH, arginine-vasopressin (AVP) and hence cortisol [186]. Although, it has proven effective in the treatment of hypercortisolaemia [187;188], the major drawback is the lack of biochemical markers to monitor over-treatment, and its long half-life and minimal agonist activity leaves the patient open to hypoadrenalism.
It is important to monitor all patients on medical therapy for Cushing’s syndrome, to assess the effectiveness of treatment, and in particular to avoid adrenal insufficiency. We use the mean of five serum cortisol measurements across the day, although others favour measurement of urinary free cortisol (UFC). A mean serum cortisol between 150 and 300 nmol/l corresponds to a normal cortisol production rate [189], and this range should be the aim of therapy.
The life expectancy of patients with non-malignant causes of Cushing's syndrome, once a uniformly fatal illness, has improved dramatically with effective surgical and medical treatments. From a number of studies in patients with Cushing’s disease treated in the era of transsphenoidal surgery, it appear that after curative transsphenoidal surgery long-term mortality is not significantly different from that in the general population [190-192]. However, another population based study suggested that mortality is marginally increased [193]. This is perhaps not surprising given that increased cardiovascular risk markers and evidence of atherosclerotic disease persist when measured 5 years after remission of Cushing’s disease [194]. There is also some evidence that the outcome from Cushing's disease may be worse in males [195]. The outcome of paediatric Cushing’s disease is excellent if treated at centres with appropriate experience [196].
The prognosis of the potentially malignant causes of Cushing's syndrome is more variable. Adrenal cancer associated with Cushing's syndrome has an extremely poor prognosis. Tumors that produce ectopic ACTH tend to have a poorer prognosis, compared with tumors from the same tissue that do not produce ACTH. Small cell lung cancer, islet cell tumours and thymic carcinoids [197] illustrate this phenomenon. Up to 82% of patients with small cell lung cancer and Cushing’s syndrome die within 2 weeks from the start of chemotherapy [198].
Cushing’s syndrome is a complex endocrine disorder that often requires intensive investigation, best carried out at centres with a long-standing experience. Modern surgical techniques and medical therapies have resulted in an improved outcome for most patients, but it is still a condition that can cause considerable morbidity and increased mortality. Further research is needed into therapies for patients with the most common cause, Cushing's disease that remain uncured by conventional treatment.