The term pupil-sparing should be reserved for situations in which there is normal pupillary function but complete loss of eyelid and ocular motor (somatic) functions of the third nerve. This is the typical finding of an ischemic cranial neuropathy, often associated with pain, which improves (and usually fully resolves) within 3 months.
A complete third nerve palsy with sparing of the pupil is almost always benign and secondary to microvascular disease, often associated with diabetes, hypertension, and/or hyperlipidemia. It is not uncommon, however, for the pupil to react normally and for there to be only minimal impairment of levator palpebrae and extraocular function (a partial third nerve palsy). Although the pupil is normal in this scenario, it should not be considered in the same category as pupil-sparing with otherwise complete oculomotor paresis, given that many other fibers within the third cranial nerve are also “spared.” This distinction is crucial given that some proportion of partial third nerve palsies with normal pupillary function are related to compressive lesions and may later progress to involve the pupil. Management of a partial third nerve palsy without pupillary involvement must be individualized based on the demographics, historical features, and availability of accurate noninvasive imaging modalities. Some clinicians favor noninvasive imaging; others advocate close, frequent observation each day or so for 7–10 days. An aneurysm should be suspected even with a seemingly benign pupil-sparing third nerve palsy if the patient is within the high-risk age for developing an aneurysm (between 20 and 50 years) and does not have diabetes mellitus or other vascular risk factors.
Pupil dysfunction or a progressive loss of function does not always indicate the presence of an aneurysm or other serious problem. The vasculopathic form of oculomotor
nerve palsy may produce some efferent pupillary defect in up to 20% of cases, although the pupillary involvement is generally mild (typically ≤1 mm anisocoria). Discovery of an elevation in fasting blood sugar, hemoglobin A1c, serum lipids, or blood pressure would increase the probability that microvascular ischemia is the cause of the third nerve palsy, but patients with these risk factors may also harbor aneurysms. Thus, pupillary involvement or progression should prompt neuroimaging in search of an aneurysm.
Conversely, a patient who presents only with efferent pupillary dysfunction (ie, the pupil is dilated and responds poorly to light) and who has normal eyelid and extraocular muscle function almost always has a benign disorder. Such isolated pupillary involvement is not a form of third nerve palsy but rather represents either a tonic (Adie) pupil, a pharmacologically dilated pupil, or a pupil that is mechanically damaged (as may occur with posterior synechiae). The clinician must exclude minor degrees of incomitant strabismus (by careful alternate cover testing or by Maddox rod testing in all positions of gaze) to exclude subtle findings of a third nerve palsy before concluding that the problem is limited to the pupil. Tentorial herniation is not a plausible explanation for an isolated, fixed, and dilated pupil in the absence of an altered mental status or other neurologic abnormalities.
The presence of head and periorbital pain is not helpful in establishing the cause of the third nerve palsy. Although most third nerve palsies caused by aneurysms present with pain, many vasculopathic palsies also produce pain that, in some cases, may be intense. In older adults, vasculitis (eg, giant cell arteritis) must also be considered.
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An acute, isolated, pupil-sparing but otherwise complete third nerve palsy in a patient over 50 years with appropriate vascular risk factors does not necessarily require neuroimaging but should prompt a general medical evaluation, with attention to serum glucose levels, systemic blood pressure, serum lipids, and a sedimentation rate. If and when progression occurs, if other cranial neuropathies develop, or if the expected recovery does not ensue within 3 months, then neuroimaging should be obtained to search for a mass or infiltrative lesion at the base of the skull or within the cavernous sinus. Occasionally, scans need to be repeated to discover a mass, especially if it is contained within the cavernous sinus. Lumbar puncture may be needed to detect carcinomatous meningitis, inflammation, or infection.
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