Corticopontine Tract

Brain Stem Cranial Nucleus

Simple motor tic

(blinking of right eye, left eye normal)

Clinical localization of brain stem lesions depends on knowledge of the tiered arrangement of cranial nerve nuclei, the intramedullary course of cranial nerve fibers, and their spatial relationship to tracts passing up and down the brain stem (see also p. 26). Lesions can be localized to the midbrain, pons, or medulla, and further classified in terms of their location in a cross-sectional plane as anterior, posterior, medial, or lateral. The "classic" brain stem syndromes are rarely seen in actual experience, as the patterns of damage tend to overlap rather ^ than occupy discrete areas of tissue. Brain stem H lesions that affect decussating neural pathways O proximal to their decussation produce crossed ■O deficits (p. 46); thus, some lesions produce ipsi-lateral cranial nerve palsies and contralateral ^ hemiparesis of the limbs and trunk.

^ Midbrain Syndromes (Table 4, p.358)

2 Lesions of the mid brain may involve its anterior 00 portion (cerebral peduncle, Weber syndrome); its medial portion (mid brain tegmentum, Benedict syndrome), or its dorsal portion (midbrain tectum, Parinaud syndrome). Occlusion of the basilar artery at midbrain level causes"top of the basilar syndrome".

Pontine Syndromes

The syndromes produced by anterior and posterior pontine lesions are summarized in Table 5.

■ Paramedian Lesions

Cause. Multiple lacunar infarcts are the most common cause.

Symptoms and signs. Unilateral lesions (mediolateral or mediocentral) cause contralateral paralysis, especially in the distal limb muscles; dysarthria; and unilateral or bilateral ataxia; and, sometimes, contralateral facial and abdu-cens palsies. Bilateral lesions cause pseudobul-bar palsy and bilateral sensorimotor deficits.

■ Lateral Pontomedullary Syndrome

Cause. Infarction or hemorrhage in the territory 70 of the posterior inferior cerebellar artery or aberrant branch of the vertebral artery.

Lesion. As in Wallenberg syndrome (p. 361) with additional involvement of the facial nerve nucleus, vestibular nerve nucleus, and inferior cerebellar peduncle.

Symptoms and signs. As in Wallenberg syndrome with additional ipsilateral findings: facial palsy (nuclear), rotatory vertigo, tinnitus, hearing loss, nystagmus, cerebellar ataxia.

Medullary Syndromes

Lesions usually involve the medial or the lateral portion of the medulla; the lateral medullary syndrome is called Wallenberg syndrome and may be associated with various oculomotor and visual disturbances (p. 86 ff). Ocular deviation. Vertical deviation (skew deviation) in which the ipsilateral eye is lower. Skew deviation may be accompanied by the ocular tilt reaction: ipsilateral head tilt, marked extorsion of the ipsilateral eye, and mild intorsion of the contralateral eye.

Nystagmus. Positional nystagmus may be horizontal, torsional, or mixed. See-saw nystagmus is characterized by intorsion and elevation of one eye and extorsion and depression of the other. Conjugate deviation to the side of the lesion. Abnormal saccades. Ocular dysmetria with overreaching (hypermetria) when looking to the side of lesion and underreaching (hypometria) when looking to the opposite side. Attempted vertical eye movements are executed with diagonal motion.

Cerebral peduncle (corticospinal and corticopontine tracts)

Medial lemniscus Substantia nigra Red nucleus CN III (fibers) Site of lesion

Aqueduct

Site of lesion

Cerebral peduncle (corticospinal and corticopontine tracts)

Aqueduct

Site of lesion

Fourth Cranial Nerve Nuclei Midbrain

Medial lesion

Aqueduct

Brain stem with cranial nerves

(at level of midbrain)

Medial lesion

Aqueduct

Mesencephalic nucleus of V Fourth ventricle Cerebellum

Brain stem with cranial nerves

(at level of midbrain)

Pyramidal tract

Spinothalamic tract

Medial pontine a. Site of lesion Sensory root of V

Principal sensory nucleus of V /—" Medial lemniscus

Pyramidal tract

Spinothalamic tract

Medial pontine a. Site of lesion Sensory root of V

Principal sensory nucleus of V /—" Medial lemniscus

Pons Nuclei Basilar Artery

Nucleus VI

Site of lesion

VII/

Proprio-ceptive fibers (VII)

Superior salivatory nucleus

Cochlear nuclei

Brain stem with cranial nerves

(at level of pons)

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