Prior to parathyroid surgery what tests must be performed?
Parathyroid tumours vary in their position in the neck. For many years the adage was that the only localisation necessary was the name of an experienced parathyroid surgeon! The problem has always been that in most cases parathyroid surgery is straightforward, with only 20% of cases representing a difficult problem. However, modern imaging has now progressed so much that it is churlish not to consider its use in both first and second operations.
There are six separate pre-operative imaging tests:
1. Ultrasound
2. Radioactive scans
3. CT scans
4. MRI scans
5. Parathyroid autobiography
6. Selective venous sampling
Ultrasound
Longitudinal US scan of right neck with an intrathyroidal parathyroid adenoma, 7.5 MHz probe
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Using high frequency probes both single adenoma and multiple parathyroid enlargements may be located. Normal parathyroids are not usually seen and there is a 25% failure rate to detect significant parathyroid pathology. False positive results are due to thyroid nodules and lymph nodes, but the most common cause is operator inexperience. Rarely parathyroid tumours lie totally within the substance of the thyroid and pre-operative ultrasound may demonstrate such tumours very clearly.The weight of the parathyroid tumour is the most important factor in its visualation. The Mayo Clinic have shown that tumours of 1000 mgs or more are visualized in over 95% of cases.Tumours weighing less thhan 200 mgs are visualized in less than 5% of cases.Large parathyroid tumours may be missed by ultrasound because they are behind the oesophagus.
Some centres combine ultrasound with fine needle aspiration of the presumed parathyroid. We do not recomnend this because it is easy even with the finest needle to break the capsule of the parathyroid and spill parathyroid cells which will seed in the local soft tissues and make long term cure difficult or even impossible.
Radioactive Parathyroid Scans
Sestamibi subtraction scan demonstrating a left inferior parathyroid adenoma
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A sestamibi localisation scan - click to enlarge
Sestamibi scans will demonstrate pathology in over 80% of cases. Sestamibi is a small protein attached to technetium 99, a mildly radioactive compound (the radioactivity poses no danger to the patient). It is injected into the patient, travels round the body and is then taken up by the parathyroids more readily than the thyroids. A scan is then performed by a detector that picks up the radioactivity, allowing good localisation of the parathyroid glands. Some cellular factors influence the uptake of parathyroid tumours by Tc-Mibi. The most important factor influencing uptake is the weight of the parathyroid tumour. Biertho and co-workers have classified TC-Mibi uptake from 0 (false negative) to 3 (high uptake) and type I (equivocal uptake). The 0-type tumours have a median weight 250 mgs and account for of 8% of tumours. The type I uptake had a median weight oF 340 mgs and accounts for 26% of tumours. This information is very helpful to the surgeon since it suggests in negative or equivoal scans the weight of the tumour will most likely be less than 500mgs.The scans are less helpful in cases of 4 or more gland disease (parathyroid hyperplasia), and cannot be used in pregnancy.
CT and MRI Scanning
CT scan of an ectopic parathyroid adenoma at the carotid bifurcation
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These two imaging techniques can be discussed together. Both are particularly useful when previous surgery has failed and reinvestigation is required, but are not indicated before first operations. Normal parathyroids are not usually seen by either imaging method. Parathyroid adenomas are more consistently detected than parathyroid hyperplasia. MRI may be used in pregnancy.
Parathyroid Arteriography and Selective Venous Sampling
These invasive tests entail the passage of catheters from the groin into the arteries and veins of the neck. A map of the blood supply to the neck is made and the abnormal parathyroid tissue identified. This invasive technique is more than 95% accurate even in difficult cases where previous surgery has failed. Arteriography should be combined with neck venous blood sampling (locating the veins with the highest parathyroid hormone level) and is mandatory before any parathyroid neck re-exploration. It is very important to have a venous map of the neck because very often particularly in patients who have had previous surgery a tumour on one side of the neck may drain by a rogue vein into the other side of the neck. If one is not aware of this the wrong side of the neck could be explored. In sick patients unfit for surgery arteriography can be used to destroy the parathyroid tumour.
It cannot be stressed too strongly that even though all the pre-operative tests are negative, as long as the blood tests confirm hyperparathyroidism there is still an excellent chance that an experienced parathyroid surgeon will cure the patient. Expert parathyroid surgeons would expect a success rate of 98%. It must be remembered that even in the best hands there is still risk of failure.
Highly selective arteriogram of right-sided parathyroid adenoma - click to enlarge
