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A Sample Case: A 48 year old man suffered a sudden weakness of his left arm and leg which caused him to fall while shaving. He was helped to his feet but his left arm and leg felt stiff. In addition, he complained of seeing "double".

The neurologist found that the patient was alert with normal mental status. There was no evidence of increased intracranial pressure though his blood pressure was 200/95. There was a spastic paresis with a positive Babinski sign in the left extremities and loss of vibratory and positional sense on the left. The patient walked with an ataxic gait. Pain and temperature sensations were normal. There was diplopia when the patient looked toward the right side. At rest the right eye deviated toward the nose (internal strabismus or squint) while the left eye looked straight ahead. There was a paralysis of conjugate gaze toward the right, i.e, the right eye did not move laterally toward the right though the left eye did. Ocular convergence was normal.

Explanation -

The sudden onset suggests a lesion of vascular origin; the high blood pressure suggests the etiology. Though spastic paresis indicates involvement of the pyramidal tracts from the cerebrum on down, in this case, because of the 6th nerve injury at the level of the pons is indicated. In the pons the pyramidal tracts are in the basis pontis, and in this case the side opposite the weakness, i.e., the right side.

Ataxic gait, vibratory and positional deficits on the left suggest injury to the medial lemniscus, which lies near the midline in the ventral tegmentum, on the right. The ataxia could also have a cerebellar component due to injury of the basis pontis and the pontine nuclei. Normal pain and temperature perception indicate that the lesion was more limited to the midline rather than lateral where the spinothalamic and 5th nerve components lie.

Gaze paralysis to the right and internal strabismus of the right eye indicate weakness of the right lateral rectus and injury to the fibers of the right abducens nerve. If the 6th nucleus had been involved the medial rectus of the left eye would have shown signs as well, due to involvement of the nearby PPRF. Since convergence was preserved and only the lateral rectus of the right eye was paralyzed this was a lesion involving only the fibers of the 6th nerve.

This constellation of symptoms is consistent with the midline distribution of the paramedian branches of the basilar artery and occlusion of its branches in the caudal pons.