USMLE Step 1 Questions: Neuroanatomy
A 17-year-old high school student had been recently experiencing visual problems. When questioned by the physician, she mentioned that she had stopped menstruating several months previously. A MRI-scan of her head showed an abnormal mass just inferior to the hypothalamus.
1. The MOST likely diagnosis is:
- Thalamic tumor
- Aneurysm of the posterior cerebral artery
- Frontal meningioma
- Tumor of the adenohypophysis
- Midbrain tumor
ANSWER
The answer is: D. An abnormal mass below the hypothalamus would most likely indicate a tumor of the pituitary gland. Biopsy of the pituitary can be performed through the sphenoid sinus. Abnormal secretion of gonadotrophic hormones and/or prolactin in response to a tumor of the anterior pituitary can often result in cessation of the menstrual cycle (amenorrhea). A frontal meningioma might compress the frontal lobe, resulting in motor dysfunction and personality changes from compression of the motor cortices and prefrontal cortex, respectively. A tumor of the midbrain, such as a pinealoma, could compress the cerebral aqueduct, resulting in a noncommunicating hydrocephalus.
Along with the internal carotid artery, the abducent nerve does travel through the center of the cavernous sinus; however, because the pituitary is prevented from lateral expansion into the cavernous sinus by the sphenoid bone, the abducent nerve will not be compressed. A pituitary tumor can compress the optic chiasm, leading to bitemporal hemianopia (loss of temporal visual fields). In addition, a pituitary tumor could potentially result in increased secretions of ACTH and/or TSH, leading to increased glucocorticoid secretion and thyroid hyperactivity (goiter), respectively.
