How is carcinoid syndrome diagnosed?
The clinical features of full-blown carcinoid syndrome are obvious. However, as many of these tumours are non-functioning, diagnosis is often made post-mortem, incidentally at appendectomy when the tumour is found during an operation to remove the appendix for another reason, or if the tumour itself causes direct physical problems such as bowel obstruction or bleeding, where the patient is investigated for the cause of that.
However, if carcinoid tumours are suspected a number of investigations can be performed firstly to confirm the presence of the tumour, and secondly, to locate any tumour.
Confirming the presence of the tumour
Urine analysis
An increased level of 5-hydroxyindoleacetic acid (5-HIAA) in a 24-hour urine collection, in the presence of symptoms, confirms the diagnosis. 5-HIAA is a breakdown product of serotonin, one of the main hormones produced by carcinoid tumours.
Liver Function Tests
These are blood tests looking at the levels of specific enzymes and chemicals originating from the liver including:
- alanine transaminase (ALT)
- aspartate transaminase (AST)
- alkaline phosphatase (Alk Phos)
- bilirubin
Abnormalities in the blood levels of these compounds can indicate liver damage which may be due to secondary tumour deposits from a malignant carcinoid tumour.
Other Blood Tests
These include analysing blood levels of the hormones produced by carcinoid tumours such as serotonin, neurotensin and substance P.
Liver biopsy
This is quite an invasive procedure and so not done as a first choice investigation. Essentially, using a special liver biopsy needle, cells from the liver are removed in order to look at them under the microscope. Carcinoid tumour cells which maybe present in the liver have a characteristic appearance allowing the pathologist to diagnose carcinoid syndrome in the presence of symptoms and locate secondary tumours.
ECG and echocardiogram
ECGs look at the electrical activity of the heart. Characteristic patterns on the ECG can indicate some the heart changes that can occur in carcinoid syndrome (such as fibrosis and valve problems). More accurate is echocardiography where the heart is visualised using ultrasound to allow the radiographer to actually see any changes occurring to the heart.
Locating the tumour
Standard techniques
Chest X rays, Abdominal X rays, Ultrasound, Barium meals and CT scans are all used initially to try to locate a tumour.
Radioisotope scanning
Octreotide scanning, which is done in the Nuclear Medicine Department (not involving X-rays) can often detect small carcinoid tumours, even if other imaging modalities have failed to locate it.
An alternative scan is the MIBG scan. This is where a compound called meta-iodobenzyl guanidine (MIBG), with a harmless radioactive iodine atom attached (forming the compound 131I-MIBG) is injected into the patient. The compound travels round the blood and adheres mainly to the surface of metastatic deposits in about 50% patients. The patient is then scanned to detect the location of the radioactivity allowing accurate localisation of sites of spread of a malignant tumour.