Let me ask you a question. What are metanephrines since they are so important to the diagnosis
of pheochromocytoma? I’ll give you a second to think about this. That’s right. Metanephrines are
catecholamine metabolites. Let’s take a look at how this works. As a reminder, catecholamines
include substances called norepinephrine or epinephrine. These medications or substrates
may actually be familiar to you in the sepsis lectures. Norepinephrine and epinephrine undergoes
series of oxidative reactions by a very, very important enzyme called monoamine oxidase.
Do you recall where monoamine oxidase may be pertinent in other series? Norepinephrine
and epinephrine subsequently turn into their metabolite after these oxidative reactions into
normetapinephrines or metanephrines. This is what we measure in the urine or the plasma.
Remember, monoamine oxidases are very important and MAO inhibitors in your psychiatry lectures.
Now, let’s move on to diagnostic imaging. Endocrine societies of America recommend the first line
imaging as a cross-sectional CT scan of the abdomen/pelvis. Remember, importantly for the vast
majority of detection of tumors by CT scan, it’s only sensitive for lesions greater than 1 cm.
You’ll notice by the green arrow pointing at an organ just above the kidney on the left side,
this is an adrenal gland. Typically speaking, the adrenal gland is a small organ.
This is about three times the normal size. In pregnant women or patients where it’s difficult
to identify based on CT scan of the abdomen/pelvis, MRI can be a very important modality.
In this image, take a look at the white arrow. The white arrow once again points to the similar
left adrenal gland above the kidney. Above the MRI image is a surgical specimen of the removed
adrenal gland. Now, let’s think upon a scenario where the CT scan and the MRI cannot localize
where the adrenal gland is or where the pheochromocytoma is. I like to introduce you to an
MIBG scan. This is a radioactive scan using radiotracer iodine-123 scintigraphy. MIG is a substrate
for norepinephrine transport. Therefore, any hyperactivity of the transporter such as a
pheochromocytoma would light up as a hot spot on this MIBG scan. Now, if you’re presented
with a clinical scenario and the question is what is the next diagnostic image study of choice?
Remember, don’t pick the MIBG scan first because we recommend use of MIBG scan only
if the CT and MRI cannot locate the lesion.