A 74 yo female with hx of diabetes, HTN presents to the ED with new onset painless left sided diplopia for the last 24 hours. No change in visual acuity. The patient has no other symptoms. You examine the eye and left sided ptosis and a “down and out pupil”. The pupil is reactive to light and symmetrical compared to the other eye. No other exam findings. You wonder if this patient could be having an aneurysm.
The oculomotor (third) cranial nerve has two roles:
(1) its supplies innervation to the levator muscles and four of the six extraocular muscles (medial rectus, superior rectus, inferior rectus, and inferior oblique).
(2) it carries parasympathetic fibers from the Edinger-Westphal nucleus in the midbrain to the ciliary body / iris sphincter to constrict the pupil.
The orientation of these fibers and their blood supplies are also important in understanding how to evaluate an isolated third nerve palsy:
1) The parasympathetic nerve fibers run superficial to the somatic motor nerves and are therefore most susceptible to compressive lesions, like an aneurysm.
2) The blood supply to the nerves is also different, such that an ischemic event to the somatic motor nerve can spare the superficial parasympathetic fibers.
With those two facts in mind, you rethink about this patient. An isolated third nerve palsy that involves the extraocular muscles and levator muscles, but spares the parasympathetic fibers is most likely an microvascular ischemic event and not an imminently rupturing aneurysm or other compressive lesion.
Source: Andrew G. Lee. Third cranial nerve (oculomotor nerve) palsy in adults. In UpToDate. Jun 19, 2017.
Venous panel vs. BMP? As a follow up from last weeks question regarding accuracy of the venous panel vs. BMP: Per the director of Mt Sinai labs: The venous panel’s creatinine has a precision up to 6.5 mg/dl using the NOVA machine. If the NOVA records a critical value, a second machine, the Radiometer, is used to verify the result before it is published. Turn around time on the NOVA is ~90 seconds.