| Cervical
Ultrasonography.
Ultrasonography offers important anatomical information, including
the size and depth of the enlarged parathyroid gland as well as its
relationship to surrounding structures such as the thyroid gland,
esophagus, and carotid artery. With the currently available U/S scanners,
abnormally large parathyroid glands are confidently detectable although
normal ones are rarely imaged. Adenomas appear hypoehoic compared
to the adjacent normal tissue, and measure approximately 1cm in length
in symptomatic patients. When present in normotropic glands they cannot
be missed on U/S. Furthermore, U/S is relatively simple to perform,
well-tolerated, painless, requires no contrast material, and is the
least expensive of all four modalities. Its sensitivity ranges from
70 to 90% because it is user dependent.
Computed Tomography. The accuracy
of CT in the detection of an adenoma is highly dependent on its
size: the larger the adenoma the better it is visualized. Use of
intravenous contrast material enhances the rate of detection but
imparts disadvantages to this technique and makes it more costly.
CT is not generally recommended for the initial evaluation of primary
hyperparathyroidism.
MRI is another imaging technique
suitable for demonstrating parathyroid adenomas since it offers
excellent soft tissue contrast without the need for intravenous
contrast material. It has been established that MRI can locate abnormally
enlarged glands at a rate equal to or even better than scintigraphy
or sonography (2). It is however a lengthy and costly procedure
which generally limits its use.
Sestamibi Scan. Sestamibi is a synthetic
isonitrile compound, 2-methoxy-isobutyl isonitrile (MIBI). It is
labeled with Tc-99 to form a cationic complex. Uptake of this tracer
is seen in both parathyroid and thyroid glands at 8-10 min, but
to a lesser extent in the thyroid, probably because sestamibi is
sequestered intracellularly in the mitochondria, and parathyroid
adenomas specifically are rich in mitochondria compared with the
surrounding thyroid tissue. One method described by Kumar at Derbyshire
Royal Infirmary, London, involved injecting each patient with 400
MBq of Tc-99 sestamibi intravenously, and obtaining anterior early
images 10 min post-injection, and later at 3 hours. Then immediately
after the late anterior image, the patient was injected with 100
MBq of Tc-99m pertechnetate and an additional
|
image was taken about 5 min later. This allowed the background
uptake of sestamibi within the thyroid to be subtracted from the
parathyroid uptake, thus enhancing the visibility of parathyroid
adenomas on the scan (6).
Sestamibi
scanning can be performed safely with no side effects since the
exposure to radiation is minimal. Furthermore, its sensitivity ranges
from 80 to 90%, except in patients with hyperplasia.
The
use of Sestamibi-SPECT scintigraphy (SPECT is single photon emission
computerized tomography) can be helpful in determining the precise
location of an enlarged parathyroid gland because it allows the
reconstruction of a 3 dimensional image.
SURGICAL APPROACHES
When
choosing between medical and surgical therapy for primary hyperparathyroidism,
the medical community must balance the risks of continued surveillance
with that of operative intervention. But literature review unequivocally
reveals that parathyroidectomy is the treatment of choice in patients
with symptomatic I HPTH. However there is a high degree of variability
in choosing medical or surgical therapy in asymptomatic patients.
In
1990, the National Institutes of Health (NIH) convened a consensus
panel composed of endocrinologists, surgeons, radiologists, epidemiologists,
and primary health care providers to determine the optimal management
of patients with primary hyperparathyoidism. They recommended that
patients with asymptomatic disease should be referred for parathyroidectomy
if:
1.
The serum calcium is greater than 12mg/d,
2.
The creatinine clearance is reduced by more than 30%
3.
The 24-hour urinary calcium level is greater than 400mg/dL
4.
The bone mass is reduced by more than 2SDs compared with age-matched
controls
5.
The patient is less than 50 years of age
6.
The patient is poorly compliant or prefers surgery.
If
these recommendations are strictly followed, approximately 50% of
asymptomatic patients will be referred for surgery (3).
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