Midbrain
Lesions

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1. VASCULAR LESIONS - TEGMENTUM OF THE MIDBRAIN (BENEDICKT'S SYNDROME)

A Sample Case: An usually alert 80 year old man was observed to have an episode of loss of consciousness. Upon awakening he complained of double vision and there was a notable tremor in his left arm.

The general physical exam was normal for a man his age. There was a mildly elevated blood pressure. Mental status was good and speech was articulate and appropriate.

With the eyelids retracted the right eye had a dilated pupil and a lateral strabismus. On attempted lateral gaze to the left the right eye would not proceed across the midline. Although vision was normal the right pupil did not constrict in either the direct or consensual pupillary light reflexes. Hearing was normal as were cranial nerves 5 through 12. Pain and temperature sensation from the face and body was normal bilaterally. Proprioception and vibratory sense on the right side of the body was normal but was diminished on the left.

Muscle strength on the left was slightly diminished and deep tendon reflexes were slightly increased. There was no Babinski sign. The finger-to-nose test was normal on the right but the left arm showed an intention tremor and dysmetria. There were occasional involuntary movements of the left arm.

Explanation -

Sudden momentary loss of consciousness is generally due to a transient ischemia but in this patient there are residual signs as well. In the right eye, 3rd nerve injury is indicated by lateral strabismus, inability to adduct on attempted lateral gaze, dilated pupil, and loss of pupillary constriction. Other cranial nerves showed no signs of involvement.

Diminished proprioception and vibratory sense on the left side of the body indicates a tegmental injury of the right medial lemniscus but without involvement of nearby spinothalamic tracts since pain and temperature sensation were normal.

There were incoordinate movements and intention tremor consistent with a cerebellar ataxia. Usually we associate left sided cerebellar signs with injury to the left cerebellar hemisphere; here injury on the right side of the midbrain has injured the cerebellothalamic fibers after they crossed from the left side. Involuntary movements are usually associated with injury to the basal ganglia and/or its associated nuclei, in this case the substantia nigra. Lesions to the right basal ganglia apparatus produce signs contralaterally through the corticospinal tract. Here, motor findings also included mild weakness and hyperreflexia on the left involving the corticospinal tract on the right side

The major indicators of Benedikt's syndrome are injury to the 3rd nerve and contralateral involvement of the cerebellothalamic fibers. Near the dentatothalamic fibers are the fibers of the medial lemniscus. Pressure from such a lesion can involve the nearby substantia nigra and sometimes the corticospinal tract. The blood supply to this area is from the distal portions of the lateral and paramedian branches of the basilar and posterior cerebral arteries.