Cushing syndrome

Definition

Prolonged exposure to high levels of cortisol secondary to exo or endogenous sources.

Causes for CS

Most common is from tumour

Causes of endogenous hypercortisolism

Name	Site of pathology	Secretion product	Percent of cases	ACTH levels
Cushing disease	Pituitary corticotroph adenoma	ACTH	60–80%	Slightly elevatedᵃ
Cooke cell adenoma	Ectopic ACTH production; most are lung tumors, others: pancreas...	ACTH	1–10%	Very elevated
ㅤ	Adrenal (adenoma or carcinoma)	Cortisol	10–20%	Low
ㅤ	Hypothalamic or ectopic secretion of corticotropin-releasing hormone (CRH) producing hyperplasia of pituitary corticotrophs; pseudo-Cushing’s state	CRH	Rare	Elevated

ᵃ ACTH may be normal or slightly elevated; normal ACTH levels in the presence of hypercortisolism are considered inappropriately elevated

Biochemical

Special test

Tests for hypercortisolism

To determine if hypercortisolism (Cushing’s syndrome, CS) is present or not,

Regardless of aetiology.
Usually only needed if the screening 24-hr urine free cortisol is equivocal.
Overnight low-dose dexamethasone (DMZ) suppression tests:

Overnight low dose suppression test (Dexamethasone suppression test (DST)):

In healthy individual

A supraphysiologic dexamethasone dose (1mg) inhibits vasopressin and ACTH secretion, thereby decreasing cortisol levels

Pros

Can be done for shift workers and patients with disrupted circadian rhythm due to uneven sleep schedules

Cons

May not be reliable in women treated with oral oestrogen → use UFC

Procedure

Give DMZ 1mg PO @ 11P.M.
Draw serum cortisol the next day at 8 A.M.

Results:

Cortisol < 1.8 mcg/dl (50 nmol/L)

Note:

This is the currently accepted normal value;
Previously it was 5 mcg/dl, reduce test sensitivity

A negative result strongly predicts CS absence
Cushing’s syndrome is ruled out (except for a few patients with CS who suppress at low DMZ doses, possibly due to low DMZ clearance)

Cortisol 1.8–10 mcg/dl: (50-276 nmol/L)

Indeterminate, retesting is necessary

Cortisol > 10 mcg/dl: (276 nmol/L)

CS is probably present.
False positives: can occur in

Pseudo-Cushing’s state

Mech: ectopic CRH secretion produces hyperplasia of pituitary corticotrophs that is clinically indistinguishable from pituitary ACTH-producing tumours (requires further testing)
Seen in:

15% of obese patients
25% of hospitalized and chronically ill patients
High oestrogen states
Uremia
Depression

The combined DMZ-CRH test can be used to identify this (see reference.

Increased in metabolism of DMZ

Alcoholics
Patients on phenobarbital or phenytoin, St John's Wort

Mech: increased metabolism of DMZ caused by induced hepatic microsomal degradation (CYP3A4 inducers)

Rapid absorption/ malabsorption of dexamethasone due to increased gut transit time, chronic diarrhea, or celiac disease
Increased corticosteroid binding globulin (CBG) levels from oral estrogens, pregnancy, or chronic active hepatitis, which may increase total cortisol levels

2 day low-dose test

Used when overnight test is equivocal:

Procedure

24 hr urine collection prior to test
Give DMZ 0.5mg PO q 6 hrs for 2 days starting at 6 A.M.;
24 hr urine collections on the 2nd day of DMZ administration.

Results

Suppressed urinary 17-hydroxycorticosteroids (OHCS) to less than 4mg/24 hrs

Normal patients

Higher amounts of OHCS in urine

Whereas ≈ 95% of patients with CS have abnormal response

Distinguishing Cushing’s disease from ectopic ACTH secretion

Aim

To distinguish primary Cushing’s disease (CD) (pituitary ACTH hypersecretion) Vs ectopic ACTH production and adrenal tumours or Pseudo Cushing Syndrome

May be required since 40% of CD patients have a normal MRI
Psychiatric disorders, alcohol use disorder, polycystic ovary syndrome, and obesity may activate the HPA axis
Concomitant medications could result in steroid cross-reactivity or otherwise interfere with laboratory test results

Random serum ACTH:

If< 5 ng/L indicates ACTH independent CS (e.g. adrenal tumour)

Not sensitive or specific due to variability of ACTH levels

Abdominal CT:

In ACTH-dependent cases: will see

Unilateral adrenal mass with adrenal tumours OR
Normal OR
Bilateral adrenal enlargement

High-dose dexamethasone (DMZ) suppression test:

Aka: Overnight high-dose test

Up to 20% of patients with CD do not suppress with high-dose DMZ.
Phenytoin may also interfere with high-dose DMZ suppression

Methods

Obtain a baseline 8 A.M. plasma cortisol level
Then give DMZ 8mg PO @ 11P.M.
Measure plasma cortisol level the next morning at 8 A. M.

Results

In 95% of CD cases plasma cortisol levels are reduced to <50% of baseline
In ectopic ACTH or adrenal tumours cortisol will be unchanged

Metyrapone (Metopirone®) test:

General

A reversible inhibitor of 11β-hydroxylase -- block cortisol synthesis -- this stimulates ACTH secretion -- which in turn increases plasma 11-deoxycortisol levels

Procedure

Performed on an inpatient basis.
Give 750mg metyrapone (suppresses cortisol synthesis) PO q 4 hrs for 6 doses.

Results

Most patients with CD will have

Rise in 17-OHCS in urine of 70% above baseline
An increase in serum 11-deoxycortisol 400-fold above baseline

Corticotropin-releasing hormone (CRH) stimulation test:

Procedure

Give CRH 0.1 mcg/kg IV bolus
Check plasma ACTH and cortisol levels

Results

CD further increased plasma ACTH and cortisol levels
Ectopic ACTH and adrenal tumours do not

Inferior petrosal sinus sampling (IPSS)

Aka cavernous sinus sampling is preferred by some
General information:

IPS sampling is not needed when the following criteria of CD are met

ACTH-dependent Cushing’s disease
Suppression with high-dose dexamethasone test
Visible pituitary adenoma on MRI

IPSS should not be used to diagnose hypercortisolism because the central-to-peripheral ACTH gradient in healthy controls and pseudo-CS overlaps that seen in patients with CD
While IPSS has high diagnostic accuracy for localization to the pituitary gland, it is not sufficiently reliable for tumour lateralization to the right or left side of the gland

15–30% of the time this test falsely lateralizes the tumour due to the communication through the circular sinus

May also determine likely side of a microadenoma within the pituitary (thus may be able to avoid bilateral adrenalectomy, which requires lifelong gluco- and mineralo-corticoid replacement and risks Nelson’s syndrome in 10–30%).
Complication rate: 1–2%, includes puncture of the sinus wall

Indicated

All patients with lesions <6 mm should have IPSS and those with lesions of 10 mm do not need IPSS

To localize tumour NOT aim to lateralize

Procedure

Done by interventional neuroradiologist.
Using a microcatheter

Each sinus is catheterised with the micro-catheter, a similar distance from the gland

Measurements done on each side

Baseline, at 2, 5 & 10 minutes after stimulation with IV CRH (100 µg IV)
Measure

Peripheral ACTH levels
Central ACTH levels

Results

Pre CRH

>1.7:1 central:peripheral = pituitary source
<1.5:1 central:peripheral = ectopic source

Post CRH

>3.3:1 central:peripheral = pituitary source
<1.8:1 central:peripheral = ectopic source

Lateralisation (not aim to lateralize

>1.4:1 interpetrosal gradient

Complications are rare:

Deep cerebral venous thrombosis
Pulmonary embolism
Venous subarachnoid haemorrhage (SAH)
Brainstem ischaemic injury

Desmopressin test

Mech

Is based on the finding that ACTH-secreting adenomas express vasopressin V1b (V3) receptors, producing a rise in plasma ACTH after desmopressin injection.

Pros

High specificity for Cushing disease
Less complex
Less expensive than the Dex-CRH test,

Cons

Good diagnostic performance in distinguishing CS from pseudo-CS in some studies
When both tests (low dose 2-day dexamethasone test with-CRH stimulation test and desmopressin test) are done, they showed excellent agreement.

Test for: Assessing cortisol reserve

Cosyntropin stimulation test (short synacthen test)

Procedure

Draw a baseline cortisol level

Fasting is not required;
Test can be performed at any time of day

Give cosyntropin (Cortrosyn®) (a potent ACTH analogue) 1 ampoule (250 mcg) IM or IV
Check cortisol levels at

30mins (optional) and
60mins

Results

Normal response:

Peak cortisol level > 18 mcg/dl (496.62nmol/L) AND an increment >7 mcg/dl (193.13nmol/L) , or a peak >20 (551.8nmol/L) mcg/dl regardless of the increment (we use nmol/L)
Normally increase should be > 200 nmol/L and 30 min value > 600 nmol/L)
Rules out primary and overt secondary adrenal insufficiency, but may be normal in mild cases of reduced pituitary ACTH or early after pituitary surgery where adrenal atrophy has not occurred. In these cases further testing may be positive:

Metyrapone test or ITT

Subnormal response:

Indicates adrenal insufficiency.

Primary adrenal insufficiency:

Low cortisol + increased pituitary ACTH secretion

Secondary adrenal insufficiency

Low cortisol + dec. ACTH
Chronically reduced ACTH causes adrenal atrophy and unresponsiveness to acute stimulation with this exogenous ACTH analogue

Insulin tolerance test (ITT)

"Gold standard” for assessing integrity of the hypothalamic-pituitary adrenal axis.

Cumbersome to do.

Abnormal in 80% of CS.

Assesses ACTH, cortisol & GH reserve

Rationale: an appropriate cortisol increment in response to insulin-induced hypoglycemia suggests patient will also be able to respond to other stresses (acute illness, surgery…)

Contraindications:

Seizure disorder
Ischemic cardiac disease
Untreated hypothyroidism

Protocol:

Pre-test preparation:

Stop oestrogen replacement for 6 weeks prior to test.
Have 50ml of Dextrose 50% and 100mg IV hydrocortisone available during test

Give regular insulin 0.1 U/kg IV push
Draw blood

Glucose
Cortisol
GH

0, 10, 20, 30, 45, 60, 90 and 120mins
(Monitor blood sugar by fingerstick during test, and give IV glucose if patient becomes symptomatic).

If fingerstick blood sugar is not <50 mg/dl (2.8mmol/L) by 30 minutes and patient is asymptomatic, give additional regular insulin 5U IVP.
There must be 2 specimens after adequate hypoglycaemia

Results:

If adequate hypoglycaemia (< 40mg/dl/2nmol/L) was not accomplished:

Cortisol or GH deficiency cannot be diagnosed

Normal:

Cortisol increment> 6 mcg/dl to a peak> 20

Peak cortisol = 16–20:

Steroids needed only for stress

Peak cortisol < 16:

Glucocorticoid replacement needed

Cushing syndrome: increment <6

Laboratory tests for CS diagnosis

Test	Cutoff level	Sensitivity (%)	Specificity (%)	Advantages/Instructions for testing	Disadvantages/Pitfalls
1-mg DST	1.8 µg/dL (50 nmol/L)	98	81	- High negative predictive value - Easy for healthcare provider to administer	- False positives common - Variable dexamethasone metabolism can confound results - Oral estrogen can increase CBG
24-hr UFC	Assay-specific reference range	91	81.5	- Wide range for normal values	- Cumbersome for patient to undertake - Variability could be 50% between samples, thus 2–3 collections are needed
LNSC	Assay-specific reference range	97	97.5	- Easy for patient to perform - Patients should be cautioned not to eat, drink, smoke, or brush their teeth for 15 min prior to collecting saliva samples	- Intra-patient variability - Cut-offs vary significantly based on reference laboratory - Potential for contamination with topical hydrocortisone - Not available in all centers

Clinical Considerations and Recommendations

If CD is suspected:

Start with either UFC and/or LNSC; DST could also be an option if LNSC not feasible
Multiple LNSC may be easier for patient collection

If confirming CD:

Use any test
UFC average 2–3 collections
LNSC 2 on consecutive days
DST useful in shift workers, not in women on estrogen-containing OC
Measuring dexamethasone level along with cortisol the morning after 1 mg dexamethasone ingestion improves test interpretability

If CS due to adrenal tumor is suspected:

Start with DST
LNSC has lower specificity in these patients

Monitoring for Recurrence

Test	Cutoff level	Sensitivity (%)	Specificity (%)	Advantages	Disadvantages
LNSC	0.27 µg/dL (7.5 nmol/L)	75–90	93–95	- In most patients abnormal earlier than DST and UFC	- Intra-patient variability - May be normal despite recurrence
24-hr UFC	1.6 × ULN	68	100	- Direct reflection of bioavailable cortisol	- ~50% intra-patient variability - Last test to become abnormal
Desmopressin	Absolute cortisol increments of 7.0–7.4 µg/dL from baseline	68	95	- Earliest test to become positive in some studies - Predicts presence of corticotroph tumor - Can become positive before clinical adenoma recurrence	- Dynamic labor-intensive testing
1-mg DST	1.8 µg/dL (50 nmol/L)	N/A	N/A	- Likely to be abnormal before 24-hr UFC	- Limited evidence specifically assessing utility for recurrence

Distinguishing Between CD and Ectopic ACTH-dependent CS

General

Glucocorticoid (GC) receptors typically retain the ability to inhibit ACTH secretion in the presence of high dexamethasone doses AND
V2 and V1b (V3R), along with CRH receptor are all overexpressed

Ectopic ACTH-dependent CS

Most (but not all) ectopic ACTH-secreting do not express V2 and V1b (V3R), along with CRH receptors.

If a pituitary tumor 10 mm is detected on MRI and dynamic testing results are consistent with CD, IPSS is not necessary for diagnosis

Test

Desmopressin stimulations test
CRH stimulation test

Increased plasma ACTH and increased cortisol following CRH or desmopressin administration usually indicates CD

High-dose DST

Has low accuracy overall
High does dexamethasone suppression test (8mg) does suppress ACTH secreting pituitary tumour

But does not suppress extracranial carcinoid secretion

IPSS

Measures ACTH in pituitary vs peripheral venous drainage, has long been the gold standard to reliably exclude ectopic ACTH production
All patients with lesions <6 mm should have IPSS and those with lesions of 10 mm do not need IPSS
A central-to-peripheral ACTH gradient <2 before or <3 after stimulation suggests an ectopic tumor

Radiology

MRI

Is the imaging method of choice for detecting ACTH-secreting pituitary adenomas.

As most lesions are very small, using standard 1.5T MRI, only approximately 50% of microadenomas are clearly depicted

1/3 of scans in patients with CD still remain negative

Other Sq to

Improve detection

Spoiled gradient–recalled (SPGR) acquisition echo with 1 mm slice intervals

Best

FLAIR
CISS

Improve localization

T1weighted turbo spin echo (TSE) sequences
Higher resolution with 3T or 7T magnets

Can increase the risk of detecting incidentalomas potentially unrelated to the disorder.

Tumor size does not necessarily correlate with degree of hypercortisolism in CD.

Patients with larger adenomas frequently present with milder hypercortisolism

Positron emission tomography (PET)

18F-fluoro-deoxy-glucose (18F-FDG) PET/CT

Possibly as good as fast spin echo MRI in detecting pituitary lesions
But Not as good as SPGR MRI

11C-methionine PET

Might be better than FDG PET