A 72-year-old woman experienced oblique diplopia on awakening from a nap. She went back to sleep, and when she awoke the next morning, her right eyelid was markedly droopy. When she lifted her eyelid, she still had diplopia. She had some dizziness and nausea but no other focal neurologic deficits, no head or eye pain, and no recent constitutional symptoms. Her medical history was positive for hypertension, hypothyroidism, and an intermittent cardiac arrhythmia (not atrial fibrillation). She had been prescribed antihypertensive and thyroid hormone replacement medications.
Examination in an emergency room disclosed no clinical signs except for ptosis and abnormal ocular motility of the left eye. A complete blood count, erythrocyte sedimentation rate, C-reactive protein, and electrolytes were normal. Electrocardiography showed normal sinus rhythm with no acute changes, and the chest X-ray was normal. Brain computed tomography (CT) and CT angiography were normal. Four days later, brain MRI revealed an area of hyperintensity on diffusion-weighted imaging (DWI), FLAIR, and T2 images in the right paramedian midbrain (Fig. 1). Small abnormal signal foci were also noted in the thalamus bilaterally. The diagnosis was acute stroke, and the patient was prescribed antiplatelet therapy.
One week later, she underwent neuro-ophthalmic evaluation. Visual acuity was 20/25 in the right eye and 20/20 in the left eye with normal color vision. Visual fields were full to confrontation, and ophthalmoscopy was normal. Pupils measured 4 mm in dim illumination and constricted briskly and equally to bright light. There was complete right upper lid ptosis. In primary position, alternate cover testing showed an exotropia of 6 prism-diopter and left hypertropia of 5 prism-diopter. There was moderate reduction in adduction and supraduction of the right eye (Fig. 2). Adducting and supraducting saccades in the right eye were slow. All eye movements in the left eye were normal. The diagnosis of partial right third palsy secondary to midbrain infarction was made.
While the third nerve may be damaged anywhere along its course, a third nerve palsy due to midbrain infarction is uncommon. Either the oculomotor nucleus or the fascicles may be affected (1). The topographic anatomy of the oculomotor nucleus has been well described and correlates with typical clinical findings (2). Because lesions affecting the fascicular portion of the third nerve are unusual, there is still debate concerning the exact location of fascicles subserving individual extraocular muscles. Reports of third nerve palsy due to brainstem stroke with clinicoradiographic correlation have helped to clarify our understanding of the topographic distribution of third nerve fibers within the midbrain tegmentum. Because the fascicles fan out as they course through the midbrain, it is possible to impair certain muscles and spare others. Patterns of third nerve weakness range from involvement of a single muscle (3) to weakness of all muscles (2,4). A pattern in which all muscles except the pupil are involved has received particular attention because it mimics the pattern seen in palsy secondary to ischemia involving the extra-axial segment of the third cranial nerve (5-7). A single case has been reported of midbrain infarction in which only the pupil was involved (8).
Based on reports of fascicular palsy with specific patterns of third nerve weakness (3,9), topographic models for the distribution of third nerve fibers in the midbrain have been proposed. Castro et al (3) described a 2-dimensional model in which fibers, from lateral to medial, innervate the inferior oblique, superior rectus, medial rectus, levator palpebrae superioris, inferior rectus, and pupil, respectively. This model was further modified by Ksiazek et al (9) to include a rostral-caudal dimension. The 2 cases in their report demonstrated impairment only of pupil and inferior rectus muscle function. Schwartz et al (10) subsequently documented a complementary case in which pupil and inferior rectus functions were selectively spared. While our case is similar to that of Schwartz et al (10) and provides further support for this model (Fig. 3), we also include DWI to demonstrate the location of the lesion and acute infarction as its cause.
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