Brainstem and Cranial Nerves

Brainstem Dysfunction

Midbrain

Isolated cranial nerve (CN) III or CN IV palsy is rarely due to a midbrain lesion, despite the location of the nuclei. Midbrain lesions can produce partial or complete palsies of these cranial nerves plus hemiparesis from involvement of the cerebral peduncle or ataxia from damage to the red nucleus. In addition to these direct effects of midbrain lesions, many such lesions can obstruct the aqueduct, producing hydrocephalus affecting the lateral and third ventricles but sparing the fourth ventricle.

Important lesions of the midbrain include infarctions, intrinsic tumors, and extrinsic compression by tumors in the pineal region. Some of these are discussed in the following sections. Basilar thrombosis and top-of-the-basilar syndrome affect the midbrain, but they affect other structures as well, so they are discussed at the end of this chapter.

Parinaud Syndrome

Parinaud syndrome is due to dysfunction of the rostral aspect of the dorsal midbrain. The most common cause is a tumor in the pineal region, but infarction or hemorrhage can also cause the syndrome. Hydrocephalus can produce downward displacement of tissue in the midbrain region, also producing Parinaud syndrome. The lesion affects the pretectal region of the midbrain, the posterior commissure, and the interstitial nucleus.

Patients present with supranuclear difficulty with vertical gaze; there is defective voluntary gaze but preserved vertical vestibuloocular reflexes. Pupillary response is impaired to light but relatively preserved to accommodation (light-near dissociation). Other classic findings are convergence-retraction nystagmus, lid retraction, and lid lag. Convergence-retraction nystagmus consists of rhythmic convergence and divergence movements induced by upward gaze.

Partial lesions are often seen, making identification of the entire clinical syndrome uncommon. In practice, defective vertical gaze with light-near dissociation, often with unequal pupils, suggests the lesion. Because many patients with this syndrome have tumors, extension into the hypothalamic region can produce diabetes insipidus. Lesion of the caudal midbrain at the level of the inferior colliculus can produce defective downward gaze with preservation of pupillary responses.

Weber Syndrome

Weber syndrome is contralateral hemiparesis with ipsilateral oculomotor palsy that does not spare the pupil. It is due to damage to the cerebral peduncle and oculomotor nerve. The most common cause is posterior circulation ischemia, although tumors can produce the symptoms.

Syndrome of Benedikt

The syndrome of Benedikt consists of contralateral hemiparesis, ipsilateral oculomotor palsy, and contralateral cerebellar ataxia. The ataxia is a combination of corticospinal dysfunction and damage to the red nucleus. Case reports of this have been reported with strokes, tumors, or infection (tuberculoma) affecting the midbrain.

Pons

Pontine lesions can produce varied lesions depending on the rostrocaudal and dorsal-ventral extent of the damage. Important symptoms suggesting a pontine lesion include lateral gaze palsy from paramedian pontine reticular formation (PPRF) dysfunction, facial or abducens nerve palsy, and signs of parenchymal damage, such as corticospinal tract dysfunction. Lesions of the pons that produce predominantly ocular motor abnormalities were discussed earlier in the section on cranial nerves.

Millard-Gubler Syndrome

The Millard-Gubler syndrome is due to a lesion in the pons affecting the sixth nerve nucleus, corticospinal tract, and intra-axial portion of the seventh nerve. Clinical findings include ipsilateral lateral rectus palsy, contralateral hemiplegia, and ipsilateral facial weakness. The seventh nerve palsy is typical of lower motor neuron seventh nerve palsies.

Foville Syndrome

A lesion in the pons can affect the PPRF, corticospinal tract, and intraaxial portion of the facial nerve. Patients present with an ipsilateral facial palsy, contralateral hemiparesis, and gaze palsy to the side of the lesion.

Locked-In Syndrome

Locked-in syndrome is most commonly due to basilar artery thrombosis, although it can also be due to pontine hemorrhage or infarction or to central pontine myelinolysis. It is also called osmotic demyelination syndrome, related to rapid correction of hyponatremia.

Patients present with quadriplegia and facial weakness. Often, the only voluntary motor power is of vertical eye movements and eye closure. Patients are often misdiagnosed as being in a coma because of the absence of response to verbal command; but if the examiner is careful to test by eye blink or eye movement, the patient is found to be awake and able to take in sensory information. Patients quickly learn to communicate by a code with these minimal movements if given the opportunity.

The differential diagnosis of locked-in syndrome includes other causes of quadriparesis including Guillain-Barré syndrome and other neuropathies, botulism, myasthenia gravis, and profound metabolic derangement. The differential diagnosis also includes true coma, persistent vegetative state, and akinetic mutism.

Pure Motor Hemiparesis

Lacunar infarction of the ventral aspect of the pons, the basis pontis, can especially damage the corticospinal tract fibers, leaving more dorsally located cranial nuclei and nerves spared. This condition results in contralateral hemiparesis with few or no associated signs.

The pons derives arterial supply from the basilar artery, which in turn sends circumferential arteries around the pons. These arteries send penetrating branches into the pons. Occlusion of one of these penetrating vessels is thought to be responsible for brainstem lacunar syndromes. The differential diagnosis of pure motor hemiparesis includes infarction in the ventral pons or the internal capsule.

Clumsy Hand-Dysarthria Syndrome

Lacunar infarction of the basis pontis may involve not only the corticospinal tract, as with pure motor hemiparesis, but also the facial nerve. Patients present with dysarthria, dysphagia, clumsiness, and corticospinal tract signs. Differential diagnosis includes lesions of the internal capsule.

Medulla

Lateral Medullary Syndrome

Lateral medullary (Wallenberg) syndrome is usually due to occlusion of the posterior inferior cerebellar artery (PICA), which is a branch of the vertebral artery or the vertebral artery itself. Findings include ipsilateral ataxia due to damage to the inferior cerebellar peduncle and cerebellum, ipsilateral Horner syndrome from involvement of the intrinsic sympathetic axons that descend into the cervical cord, and vertigo with nausea with or without vomiting from damage to the vestibular nuclei. Ipsilateral pharyngeal and laryngeal dysfunction due to damage to the nucleus ambiguus produces dysarthria and dysphagia. Ipsilateral trigeminal involvement produces sensory loss on the face. Damage to the ascending spinothalamic tract produces decreased pain and temperature sensation contralateral to the lesion.

Medial Medullary Syndrome

The medial medullary syndrome is less common than the lateral medullary syndrome. Infarction of the medial medulla is due to occlusion of branches of the vertebral artery, which project to form the anterior spinal artery. These penetrating branches can be occluded, infarcting the pyramids, medial lemniscus, and hypoglossal nerve. Patients present with contralateral hemiparesis, contralateral loss of position and vibratory sensation, and ipsilateral paresis of the tongue.

Cranial Nerve Dysfunction

Cranial nerves are discussed individually except for the oculomotor, trochlear, and abducens, which together control ocular movement. Lesions affecting one cranial nerve solely or predominantly are discussed under the respective nerve. Brainstem lesions producing cranial nerve dysfunction in addition to other neurologic dysfunctions are discussed at the end of the chapter.

Olfactory nerve

Anosmia (lack of smell) is produced by peripheral or central processes. Head injury is a common cause of anosmia due to disruption of the small olfactory nerves that penetrate the cribriform plate. Tumors of the olfactory grove compress the inferior frontal region, producing few neurologic signs. Patients may present with mood alteration. Although patients rarely complain of olfactory abnormalities, examination can reveal anosmia. Other important causes of anosmia include increased intracranial pressure and some neurodegenerative diseases, including Parkinson disease and Alzheimer disease.

Sinusitis can impair flow of air to the sensory regions and is a more common etiology than any of the others discussed here.

Optic nerve

Optic nerve lesions usually produce monocular visual loss, and subacute deficits may not be initially noticed by the patient. Formal testing may reveal reduced visual acuity that cannot be improved by correcting for refractive error. Color vision is often impaired. If a focal portion of the optic nerve is compressed or otherwise damaged, there may be a monocular abnormality in visual fields. Fig. 1.3.1 (in Chapter 1.3 ) shows changes in visual fields with lesions of the optic nerves, chiasm, and tracts.

Important causes of optic neuropathy include the following:

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Optic neuritis
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Multiple sclerosis (MS)
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Neuromyelitis optica (NMO) spectrum disorder
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Anti-myelin oligodendrocyte glycoprotein Anti-MOG disease
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Ischemic optic neuropathy (arteritic and nonarteritic)
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Tumors
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Malnutrition (alcohol tobacco amblyopia)
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Trauma

Optic neuritis is characterized by inflammation and demyelination of the optic nerve. This finding is usually idiopathic, although approximately 50% of patients with optic neuritis subsequently develop MS. Clinical features cannot clearly differentiate isolated optic neuritis from MS-associated optic neuritis.

For the diagnosis of MS, history or examination has to indicate evidence of other neurologic lesions that could be due to central demyelination. Magnetic resonance imaging (MRI) is helpful to rule out optic nerve compression and also to look for signs of white matter changes in the brain. See Chapter 5.9 for a discussion of the clinical criteria for MS.

Tumors can affect the optic nerves, and the three most important are gliomas, meningiomas, and pituitary tumors. Optic nerve gliomas can occur as isolated entities or in association with neurofibromatosis. Patients present with progressive visual loss. The tumor location can be identified based on details of the visual fields. Proptosis (forward protrusion of the eye) suggests involvement of the nerve in the orbit, and pituitary tumors produce deformation of the optic chiasm, resulting in junctional scotomata.

Trauma commonly damages the eye or optic nerves, and detailed evaluation must be performed as soon as possible after the injury. Periorbital edema may make ocular examination impossible by 24 to 48 hours after the injury.

Ocular Motor System

Detailed eye movement examination is warranted in every patient with subjective diplopia or who has signs of an ocular motor defect on neurologic examination.

The following should be done as part of the ocular motor examination:

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Movements of each eye are tested in all of the primary directions of gaze.
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Diplopia is assessed with different directions of gaze.
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Pupillary responses to light and accommodation are assessed.
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The swinging-flashlight test is done to test for an afferent pupil.

The eyes are tested together, although the examiner should concentrate on the specific responses of each eye. Nystagmus of one eye should prompt examination of the eyes individually. The examiner must use knowledge of neuroanatomy during the examination to determine whether the defects are best explained by a defect in cranial nerves, brainstem, brain, or extraocular muscles.

The precision of eye movement may limit the ability of the examiner to detect subtle divergence of gaze; therefore, the patient should be asked about diplopia evoked by directions of gaze.

Pupillary response is elicited by shining a bright light into each eye of the patient, one eye at a time. The diameter of each pupil before and during light exposure is noted. When the light is shone in one eye, both eyes should constrict equally.

The swinging-flashlight test examines pupillary responses from both eyes. The room must be dark enough that the eyes are not constricted, yet light enough that the diameter of the pupils can be determined. A bright light is shone first into one eye then quickly into the other. The normal response is constriction of both pupils when the light is initially shone into one eye. Quickly moving the light between the eyes should result in no alteration in diameter. If both pupils constrict more when light is shone into one eye than the other, the eye eliciting the lesser constriction is described as having an afferent defect, which points to an optic nerve or orbital lesion.

Optokinetic nystagmus (OKN) is elicited by the movement of a patterned background. The typical real-life stimulus is passing scenery as a car drives through the countryside. The eyes fix on an object of interest, and the image moves toward the rear of the vehicle. When fixation can no longer be maintained, gaze is redirected toward the front of the vehicle, and a new target is selected. The fast phase of the nystagmus is toward the front of the vehicle. In clinical practice, OKN is tested by passing a striped tape in front of the eyes at a constant velocity. The nystagmus is analogous to the eye movements elicited by the vehicle movement. OKN is normally symmetric with movement of the tape in either direction. Lesion of one hemisphere in the parietooccipital region produces an abnormal or absent OKN when the tape is moved toward the side of the lesion. An even more compelling stimulus is movement of a mirror held close in front of the patient. Compensatory movements are virtually impossible to suppress.

One of the most important uses of OKN is for diagnosis of psychogenic blindness. Patients who are blind will not develop OKN, whereas patients with psychogenic blindness often will.

Differential Diagnosis of Ocular Motor Abnormalities

Examination of the eyes is complex if more than one cranial nerve is affected; although multiple cranial neuropathies must be considered, central lesions and neuromuscular lesions should be considered as well. The deficits associated with single lesions are listed in Table 3.4.1 . Ocular motor defects and associated anatomic localizations are presented in Table 3.4.2 .

Table 3.4.1

Focal Lesions of the Ocular Motor System

Lesion	Findings
Left frontal lobe destructive lesion	Left gaze preference, often with right hemiparesis
Left frontal lobe irritative lesion (e.g., seizure)	Right gaze preference, sometimes with nystagmoid movements of eyes
Left parietooccipital region lesion	Defective smooth pursuit in both horizontal directions of gaze in the contralateral hemifield
CN III palsy	Ptosis and impaired upward gaze and adduction; extrinsic compression lesions affecting pupil early; nerve ischemia often spares pupil
CN IV palsy	Impaired depression of the ipsilateral eye while in adduction; head tilt to the side opposite the lesion due to external rotation of eye
CN VI palsy	Impaired abduction of eye
Medial longitudinal fasciculus	Internuclear ophthalmoplegia; impaired adduction of ipsilateral eye with gaze to side opposite lesion; nystagmus of the contralateral (abducting) eye
Pontine lesion (Millard-Gubler syndrome)	Ipsilateral CN VI palsy, facial palsy, and contralateral hemiparesis
Midbrain lesion (Parinaud syndrome)	Impaired upgaze with dilated and nonreactive pupils

Table 3.4.2

Localization of Ocular Motor Defects

Defect	Features	Localization and Type of Lesion
Downbeat nystagmus	Vertical nystagmus with fast phase downward; accentuated by downgaze and suppressed by upgaze	Craniocervical junction, e.g., tumor, malformation, cerebellar lesion
Horner syndrome	Miosis, ptosis, often with ipsilateral hypohydrosis	Ipsilateral brainstem, spinal cord, lower brachial plexus, or cervical sympathetic chain
Left gaze preference	Eye looks left but may be driven across midline by vestibuloocular reflex	Left frontal lobe if preference can be overcome with doll’s head maneuver; right pons if not
Millard-Gubler syndrome	Ipsilateral CN VI and VII palsy and contralateral hemiparesis	Ipsilateral pons affecting CN VI nucleus, intraaxial portion of CN VII, and corticospinal tract
Ocular bobbing	Jerking of eyes with a fast phase downward and slow return	Pons—though some diffuse brainstem lesions may also do this
Parinaud syndrome	Impaired upgaze with dilated and nonreactive pupils	Midbrain, usually compressive lesion, e.g., pineal region tumor
Skew deviation	Vertical diplopia with one eye persistently elevated	Brainstem or cerebellum; lower eye is on the side of the lesion
Internuclear ophthalmoplegia	Impaired adduction of the contralateral eye with gaze to one side	Medial longitudinal fasciculus on side of the eye that fails to adduct
Horizontal nystagmus	Fast deviation of eyes to one side with slower return to the opposite side	Brainstem of vestibular system, e.g., many brainstem tumors or stroke

Cerebral Lesions

The anatomy of cerebral control over eye movement was discussed earlier. Massive lesions of one hemisphere are associated with a contralateral hemiparesis plus gaze preference to the side of the lesion, that is, the side opposite to the hemiparesis. With oculovestibular reflexes, the eyes can usually be brought across the midline. More restricted lesions of the hemisphere will result in smaller deficits, usually due to frontal lobe damage.

Frontal Lobe Lesions

Frontal lobe lesions result in gaze preference to the side of the lesion. Because the hemisphere is important for pursuit to the side of the lesion, pursuit is impaired as well. Surviving neurons may compensate for the gaze preference within hours, if there are incomplete lesions; more typically, the gaze preference lasts days and then corrects.

Some patients are seen with eye deviation toward the side of the paralysis. These patients often have deep lesions involving the thalamus.

Frontal lobe irritative lesions affecting the frontal eye fields may cause paroxysms of gaze toward the side contralateral to the lesion. Seizures would be a typical example. Following the seizure, the gaze would likely be to the side ipsilateral to the lesion as a postictal phenomenon.

Parietal and Occipital Lobe Lesions

The parietooccipital cortex is involved in smooth pursuit (i.e., in directing attention to an object of interest) in addition to its role in visual perception. Optic radiations through the parietal and occipital lobes project to the striate cortex of the occipital lobe; the prestriate region of the cortex is involved in analysis of this input. Lesions in this parietooccipital region result in impaired pursuit movements in both directions of horizontal gaze in the contralateral visual field.

Brainstem Lesions

Brainstem lesions that predominantly affect eye movements are discussed here, whereas those that produce other deficits are discussed in the brainstem section. Some important entities affecting eye movements discussed elsewhere include Parinaud syndrome, Millard-Gubler syndrome, and Foville syndrome.

Horner Syndrome

Horner syndrome consists of miosis (pupil constriction) plus ptosis and is a manifestation of damage to sympathetic fibers. Associated findings may include ipsilateral anhidrosis (impaired sweating), enophthalmos, and, in congenital cases, hypopigmentation of the iris. Although the pupil is small, it reacts normally to light and accommodation. Causes of Horner syndrome include lesions at the following levels:

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Brainstem
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Spinal cord
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Brachial plexus
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Sympathetic chain

Brainstem lesions affect the sympathetic fibers as they descend into the spinal cord. The sympathetic pathway is adjacent to the spinothalamic tract, so Horner syndrome due to a brainstem lesion is often associated with contralateral loss of pain and temperature sensation. Symptoms associated with specific brainstem syndromes are discussed in the following paragraph; many of them are associated with Horner syndrome.

The sympathetic fibers descend in the lateral aspect of the cervical cord, and lesions in this region can produce Horner syndrome.

Central cord lesions may produce bilateral Horner syndrome, which is clinically difficult to recognize. Ptosis and miosis with no other ocular motor abnormality suggest this syndrome.

The sympathetic efferents exit the spinal column at T1; they briefly join the brachial plexus and then ascend in the cervical sympathetic chain. Lesions affecting the T1 root may include intervertebral disc lesions, avulsion injuries, cervical rib lesions, and tumors.

The cervical sympathetic chain ascends with the carotid artery into the skull, then through the cavernous sinus, and then into the orbit. Lesions of these axons can occur with neck and skull-base tumors and carotid surgery. Lesions can also be found in the cavernous sinus and posterior aspect of the orbit.

Horner syndrome may occur with cluster headache and occasionally migraine. Diagnosis is by typical history and exclusion of other causes.

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