A 58-year-old man has been referred to intensive care by the anaesthetic registrar in orthopaedic theatre. He has had a knee replacement but has developed a persistent vasopressor requirement despite extensive intravenous fluids. His past medical history is significant for type 2 diabetes, hypertension, osteoarthritis and vitiligo. His blood gases reveal the following: 


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|---------------------------|:-------:|--------------------|
|Sodium|137 mmol/L|135 - 145|
|Potassium|6.2 mmol/L|3.5 - 5.3|
|Fasting Glucose|3.9 mmol/L|3.5 - 5.5|


What is the definitive investigation for his underlying disease?

Options:
explanation: This patient is presenting with an addisonian crisis postsurgery, as evidenced by refractory hypotension, hyperkalaemia, low-normal sodium and the past medical history of autoimmune disease. A 9 a.m. cortisol < 50 is usually sufficient to make the diagnosis and > 550 to rule out the diagnosis. In between, further testing is required. For primary adrenal insufficiency (Addison's disease), this test will be a short synacthen test.
name: Short synacthen test
label: a
answer: yes
explanation: Cortisol levels fluctuate during the course of the day and with stress, such as that induced by surgery. Although the levels of a random cortisol may be within a normal range, it is still possible that the patient could have a diagnosis of adrenal insufficiency since even apparently normal values may be insufficient for that patient at that particular moment in time. Therefore, a random cortisol will not provide adequate definitive diagnostic information.
name: Random cortisol level
label: d
answer: no
explanation: Assessment of adrenal size and architecture can be useful for the evaluation of adrenal insufficiency, especially if there is concern that there may be an underlying cause, such as malignancy or infection. This is unlikely in this patient since there is no suggestion of there being an underlying cause; it is most likely that this patient has autoimmune Addison's disease, especially given the history of vitiligo.
name: CT abdomen
label: b
answer: no
explanation: Secondary adrenal insufficiency is caused by insufficient production of adrenocorticotropic hormone (ACTH) from the pituitary gland. Assessment of the size and architecture of the pituitary gland could be useful if this was suspected. This usually comes to light during a short synacthen test, where the baseline ACTH is evaluated, which would be low in the case of secondary adrenal insufficiency. Secondary adrenal insufficiency responds normally to short synacthen with a rise in cortisol; however, prolonged secondary adrenal insufficiency may result in underactive adrenal glands that do not respond well to short synacthen; in this case, a long synacthen test will be needed to stimulate cortisol release.
name: MRI pituitary
label: c
answer: no
explanation: Aldosterone is also produced by the adrenal glands, alongside cortisol. It can also be deficient in Addison's disease and fludrocortisone replacement may be required alongside hydrocortisone in these cases. However, the first step in suspected Addison's disease is to evaluate the cortisol insufficiency since this is the most classical feature. A short syncathen test evaluates cortisol levels in response to exogenous ACTH administration and is the definitive initial test. In the case of Addison's disease (primary adrenal insufficiency), there will not be a rise in cortisol levels after ACTH administration.
name: Aldosterone and renin levels
label: e
answer: no

Correct answer: a


Learning point: The Short Synacthen Test is used to diagnose Addison's disease by evaluating the adrenal glands' ability to produce cortisol in response to synthetic ACTH stimulation, with a low or absent cortisol response indicating adrenal insufficiency.