Medial Midbrain Weber Syndrome

Medial midbrain (Weber) syndrome results from occlusion of branches of the posterior cerebral artery (Figures IV-5-18 and IV-5-19).

In medial midbrain syndrome, exiting fibers of CN III are affected, along with corticobulbar and corticospinal fibers in the medial aspect of the cerebral peduncle. Third nerve lesions result in a ptosis, mydriasis (dilated pupil), and an external strabismus. As with any brain stem lesion affecting the third cranial nerve, accommodation and convergence will also be affected. Corticospinal tract lesions produce contralateral spastic hemiparesis of both limbs. The involvement of the corticobulbar fibers results in a contralateral lower face weakness seen as a drooping of the corner of the mouth. The patient will be able to shut the eye (blink reflex is intact) and wrinkle the forehead.

Figure IV-5-18. Ventral Midbrain Syndrome (Weber Syndrome)

Table IV-5-8. Ventral Midbrain Syndrome

Structure

Sign

Corticobulbar tract Fibers of III

Contralateral spastic hemiparesis, mosdy upper limb Contralateral spastic hemiparesis of lower half of FACE Ipsilateral oculomotor palsy

1. Dilated pupil

2. Ptosis

3. Eye pointing down and out (lateral strabismus)

Nucleus III

Nerve III Corticospinal tract Corticobulbar tract

Clinical Correlate

Neurons in both the raphe and locus coeruleus degenerate in Alzheimer disease.

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