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A Sample Case: A 60 year old man was suddenly stricken with paralysis of his right arm and leg. On reaching the hospital the major notable findings of the neurologic evaluation were spastic paresis of the right upper and lower extremities with increased deep tendon reflexes and a positive Babinski sign. The arm and leg on the left side performed in an ataxic manner on the finger-to-nose and heel-to-shin test but strength was normal. The muscles of the lips and cheeks were weak on the right side but the patient could tightly close both eyes and wrinkle his forehead symmetrically.

An internuclear ophthalmoplegia was observed in which the left eye would not adduct on attempted lateral gaze to the right although both eyes converged on near object. There was some loss of vibratory sense and discriminitive touch from the right arm.

Explanation -

The internuclear ophthalmoplegia helps to localize this lesion to the pons. On attempted lateral gaze to the right only the lateral rectus of the right eye functions; the medial rectus does not adduct the left eye. This would occur with lesions of the left medial longitudinal fasciculus (MLF) wherein the connection between the 6th and 3rd nerve for conjugate horizontal deviation is broken. Since both eyes converge it demonstrates that the 3rd nerves are intact (and that for convergence the pathway to the oculomotor nuclei does not involve the MLF).

The spastic paralysis of the arm, leg and only lower face on the right indicates injury to the upper motor neurons, i.e., corticospinal, corticobulbar tracts, on the left. On the left side there were ataxic cerebellar signs consistent with injury to the cerebellum on the right or, in this case the right basis pontis. Most of the upper motor neurons lie medially in the basis pontis.

In the area of the tegmentum, between the basis pontis containing the upper motor neurons and the MLF, lies the medial lemniscus. Injury to the medial aspect of the medial lemniscus would lead to loss of discriminative touch to the contralateral upper extremity.

The medial portion of the basis pontis and the MLF lie within the area of supply of the paramedian branches of the upper portion of the basilar artery.