EMReports | Pearls and Pitfalls - Third Cranial Nerve Palsy Considerations

Third Cranial Nerve Palsy Considerations

Larry Mellick, MD, MS, FAAP, FACEP
Professor of Emergency Medicine
Medical College of Georgia

Pearl: Third cranial nerve (CN III) palsy often is associated with an expanding aneurysm from the posterior communicating artery (PCA). But, if the pupil isn’t dilated and the patient is an older diabetic, you should consider diabetic third cranial nerve palsy.

Presentation: A 72-year-old African-American female presented with the chief complaint of double vision in her right eye since 5 a.m. on the morning of presentation. In fact, she fell on her way back from the bathroom. She denied any change in vision of the left eye, headache, weakness, slurred speech, nausea, vomiting, or change in sensation. She did report photophobia in the right eye.

Here past medical history was significant for hypertension, diabetes mellitus, elevated cholesterol, osteoarthritis, and cancer of the right breast that was treated with a mastectomy. She also had a cataract of the right eye. The patient denied use of tobacco, alcohol, or substance abuse.

Medications included fish oil, hydrochlorothiazide, candesartan, metformin, nifedipine, tamoxifen, aspirin, and Aleve.

The physical examination demonstrated a blood pressure of 137/68, pulse 90, respiratory rate 16, temperature 36.2°C, and oxygen saturation on room air of 100%. The patient appeared healthy and had an unremarkable appearance except for moderate eyelid ptosis and deviation of her right eye laterally. (See Figure 1.) The patient was unable to move her right eye past the midline. Visual acuity was intact. The double vision resolved when the left eye was covered. Pupils were 3 mm, equal, and minimally reactive. There was minimal tearing of the right eye. The examination of the corneal fundus with a panoptic ophthalmoscope was incomplete due to the early cataract and patient cooperation. The remainder of the neurologic examination, which included motor function, strength, sensation, and cranial nerves, was all within normal limits.

A CT scan without contrast was normal. The neurology service was consulted and a subsequent CT angiogram (CTA) showed no evidence for aneurysm or vascular malformation or area of abnormal enhancement with the contrast.

Laboratory testing demonstrated a white count of 8.7, Hgb 11.7, and Hct 35.0%. The differential showed 84% segmented neutrophils and 13% lymphocytes. Electrolytes were normal except for a CO₂ of 20 mEq/L and a glucose level of 215 mg/dl. BUN and creatinine were 23 mg/dl and 0.9 mg/dl.

The diagnosis of diabetic 3rd cranial nerve palsy was made, and the patient was discharged to follow up with ophthalmology and neurology.

Discussion: An isolated CN III palsy is a relatively rare event. Since the third cranial nerve (CN) exits ventrally through brainstem and travels in the subarachnoid space next to the posterior communicating artery (PCA), CN III palsy often is associated with an expanding and life-threatening aneurysm. The basilar and superior cerebellar arteries also travel in this area and may have aneurysms that cause an isolated CN III palsy. The fibers controlling pupil size (pupillary constrictor fibers) are located superficially and run medially and dorsally so they are almost always (95% to 97%) affected by any compressive forces.¹

This patient had normal pupils that were symmetrical in size. The sparing of her pupil made the possibility of an aneurysm less likely. The “Rule of the Pupil” must be considered along with some important caveats. That is, if the pupil of the affected eye is dilated, an aneurysm must be ruled out with computed tomography angiography (CTA) or magnetic resonance angiography (MRA); and, if the probability remains high, angiography should be performed.

Cranial nerve III also travels through the cavernous sinus where aneurysms of the internal carotid and basilar arteries can be pupil-sparing as they may not impact the dorsomedial fibers, so it behooves the practitioner to proceed with caution.^1,2

If the patient is at least 50 years old, has diabetes mellitus and hypertension, has palsy that is complete, with undilated pupil, then the most likely diagnosis is diabetic CN III palsy. It is postulated that diabetes and hypertension cause changes in the vasa nervorum that lead to ischemic injury to axons deeper within the nerve bundle. Since pupillary constrictor fibers are superficial and close to the microvascular blood supply, they are less likely to be damaged by ischemia.³ This explains the eye deviation without pupil dilation. If the patient doesn’t meet these criteria, then one must consider an aneurysm. In an MRA study, it was concluded that only the presence of complete external CN III palsy and normal pupil function allowed ischemia to be distinguished clinically from an aneurysm of the posterior communicating artery.⁴ Even though some sources state that diabetic CN III palsy may not require imaging and immediate consultation,⁴ this rare condition is best managed in the emergency department with a conservative approach that includes imaging and consultation with a specialist.

This patient had a normal non-contrasted head CT, a neurology evaluation, and a negative CTA of the head. She was sent home with follow-up by ophthalmology. At follow-up, no improvement was noted.

References:

Woodruff MM, Edlow JA. Evaluation of third nerve palsy in the emergency department. J Emerg Med 2008;35:239-46. Epub 2007 Sep 17.
Lee AG, Hayman LA, Brazis PW. The rvaluation of isolated third nerve palsy revisited: An update on the evolving role of magnetic resonance, computed tomography, and catheter angiography. Surv Ophthalmol 2002;47:137–157.
Yanovitch T, Buckley E. Diagnosis and management of third nerve palsy. Current Opin Ophthalmology 2007;18:373–378.
Kupersmith M, Heller G, Cox T. Magnetic resonance angiography and clinical evaluation of third nerve palsies and posterior communicating artery aneurysms. J Neurosurg 2006;105:228–234.

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