EXTRAOCULAR MOVEMENTS

This section deals with the neural apparatus that controls eye movements and causes them to move simultaneously, up or down and side to side, as well as in convergence or divergence.

The neural control of eye movements is ultimately effected by alterations in activity in the nuclei and nerve fibers of the oculomotor, trochlear and abducens nerves. These are referred to as the nuclear and infranuclear pathways. Coordination of eye movements requires connections between these ocular motor nuclei; the internuclear pathways. The supranuclear pathways are responsible for generation of the commands necessary for the execution of the appropriate movement, whether it be voluntary or involuntary.

Classification & Examination of Eye Movements

Eye movements are either fast or slow. Fast eye movements include voluntary or involuntary refixation movements (saccades) and the fast phases of vestibular and optokinetic nystagmus (see below). The fast eye movement system is tested by command refixation movements and by the fast phase of vestibular and optokinetic nystagmus.

Slow eye movements include pursuit movements, which track a slowly moving target once the saccadic system has placed the target on the fovea, and which are tested by asking a patient to follow a slow, smoothly moving target, the slow phase movements generated by vestibular stimuli, the slow phase of optokinetic nystagmus, and vergence movements which-unlike all the other forms of eye movements-involve dysconjugate movements of the two eyes.

Under physiologic conditions, vestibular stimulation occurs from head movements. The resulting slow eye movements, known as the vestibulo-ocular responses (VOR), compensate for the head motion such that the position of the eyes in space remains static and steady visual fixation can be maintained. The doll's head maneuver is a clinical method of testing the vestibulo-ocular response. The patient is asked to fixate on a target while the examiner moves the head in a horizontal or vertical plane. If the vestibulo-ocular response is deficient, the compensatory eye movements are insufficient and must be supplemented by saccadic movements to maintain fixation. The head motion must be rapid-otherwise, pursuit mechanisms dominate the ocular motor response. In the unconscious patient, the doll's head maneuver is used to assess brainstem function. Since the pursuit and saccadic systems are not operative, the head movements can be slow. Absence of the vestibulo-ocular response leads to failure of the eyes to move within the orbit. Other methods of vestibular stimulation are whole body rotation and caloric testing (see below).

Generation of Eye Movements

A. Physiology:

1. Fast eye movements-

Understanding of the control of eye movements is most complete in the case of saccadic movements. Similar mechanisms are thought to apply to the fast phases of nystagmus. The generation of a saccade involves a pulse of increased innervation to move the eye in the required direction and a step increase in tonic innervation to maintain the new position in the orbit by counteracting the visco-elastic forces working to return the eye to the primary position. The pulse is produced by the burst cells of the saccadic generator. The step change in tonic innervation is produced by the tonic cells of the neural integrator, so-called because it effectively integrates the pulse to produce the step. Saccades are ballistic movements-ie, once initiated, their trajectory can not be altered-and there is a close relationship between the amplitude of movement and its peak velocity, larger movements having greater peak velocities. Loss of the saccadic generator function leads to slowing of saccades. Loss of the neural integrator function leads to a failure of maintenance of the desired final position, ie, a failure of gaze holding. Clinically, this usually manifests as a gaze-evoked nystagmus, with a drift of the eyes toward the primary position followed by a corrective saccade back to the desired position of gaze.

2. Slow eye movements-

The slow phase movements generated by vestibular stimuli are a direct response to the detection of movement by the semicircular canals. The canals are acceleration detectors, but their output is integrated to produce a velocity signal which is then conveyed to the ocular motor nuclei. The generation of pursuit movements is less well understood. The slow phase of optokinetic nystagmus is in part a pursuit movement, but there is also an additional specific optokinetic movement generated by the perception of movement of the background of the visual scene. This optokinetic movement appears to be generated by the pathways involved in generating slow phase vestibular movements but with an input from the retina, either via cortical centers or directly via a subcortical pathway. Vergence eye movements are generated in response to retinal disparity, ie, stimulation of noncorresponding retinal loci by the object of regard. Electromyography has established divergence as an active process, not a relaxation of convergence.

B. Anatomy:

1. Brainstem centers for fast eye movements-

The saccadic generator for horizontal eye movements lies in the paramedian pontine reticular formation. The output from this structure is channeled through the abducens nucleus, which contains both the motor neurons for the abducens nerve and the cell bodies of interneurons which pass via the medial longitudinal fasciculus to innervate the motor neurons in the contralateral medial rectus subnucleus of the oculomotor nerve. The neural integrator for horizontal eye movements appears to be located close to the paramedian pontine reticular formation in the nucleus prepositus hypoglossi.

The saccadic generator for vertical movements is in the rostral interstitial nucleus of the medial longitudinal fasciculus in the rostral midbrain. The pathway to the ocular motor nuclei for upward movements involves the posterior commissure, dorsal to the cerebral aqueduct, and its nucleus. The corresponding pathway for downward eye movements is less well defined. Neural integration for vertical eye movements seems to take place in both the interstitial nucleus of Cajal, close to the rostral interstitial nucleus of the medial longitudinal fasciculus in the midbrain and in the vestibular nuclei in the medulla.

2. Cortical centers for fast eye move-ments-

Voluntary saccades are initiated in the frontal lobe (frontal eye field area 8). The pathway descends through the basal ganglia and the anterior limb of the internal capsule into the brainstem, terminating in the midbrain pretectal area for vertical movements and crossing to the paramedian pontine reticular formation in the opposite side of the pons for horizontal movements. The generation of involuntary (reflexive) saccades, in response to a target appearing in the peripheral field of vision, depends upon activity within the superior colliculus, which receives information from the occipital cortex and also directly from the retina in a purely subcortical pathway.

3. Brainstem centers for slow eye movements-

The processing of information from the semicircular canals occurs in the vestibular nuclei, which then connect directly to the ocular motor nuclei. These pathways from the vestibular nuclei in the medulla to the pons and midbrain pass in a number of fiber tracts, including the medial longitudinal fasciculus.

4. Cortical centers for slow eye movements-

Pursuit movements originate in the occipital cortex. The pathway descends through the posterior limb of the internal capsule to the midbrain and ipsilateral paramedian pontine reticular formation. The slow phase of optokinetic nystagmus is likely to be generated at least in part in area V5 (or MT) at the junction of the occipital and temporal lobes, which is involved in motion detection. The descending pathway probably accompanies the pathway for pursuit movements. Vergence eye movements are generated in the occipital cortex, and the pathway also probably descends via the posterior limb of the internal capsule, together with the pathway for pursuit movements, to terminate in the rostral midbrain near or in the oculomotor nucleus. Impulses then pass directly to each medial rectus subnucleus and via the medial longitudinal fasciculus to the abducens nuclei. It is not clear whether convergence and divergence are controlled by the same or separate brainstem centers.

ABNORMALITIES OF EYE MOVEMENTS

Owing to the multiplicity of pathways involved in the supranuclear control of eye movements, with origins in different areas of the brain and an anatomic separation in the brainstem of the horizontal and vertical eye movement systems, disorders of the supra-nuclear pathways characteristically produce a disso-ciation of effect upon the various types of eye movements. Thus, the clinical clues to a supranuclear lesion are a differential effect on horizontal and vertical eye movements or upon saccadic, pursuit, and vestibular eye movements. In diffuse brainstem disease, such features may not be apparent, and differentiation from disease at the neuromuscular junction or within the extraocular muscles on clinical grounds can be difficult.

Disease of the internuclear pathways results in a disruption of the conjugacy of eye movements. In infranuclear disease, the pattern of eye movement disturbance usually complies with that expected of a lesion involving one or more cranial nerves or their nuclei.

1. LESIONS OF THE SUPRANUCLEAR PATHWAYS

Frontal Lobe

A seizure focus in the frontal lobe may cause involuntary turning of the eyes to the opposite side. Destructive lesions cause transient deviation to the same side, and the eyes cannot be turned quickly and voluntarily (saccadic movement) to the opposite side. This is called frontal gaze palsy, and recovery occurs when the opposite frontal eye field substitutes. Ocular pursuit to the opposite side is retained. There is no diplopia. Phenytoin can significantly affect saccades.

Occipital Lobe

Smooth ocular pursuit may be lost with posterior lesions of the hemispheres. The patient is unable to follow a slowly moving object in the direction of the gaze palsy. The command (fast) eye movement is not lost, so pursuit is "saccadic." Sedative agents and carbamazepine can alter smooth pursuit eye movements.

Midbrain

Lesions of the posterior commissure cause impairment of conjugate upgaze. Lesions dorsal and medial to the red nuclei produce a downgaze paresis (trauma, infarcts).

Parinaud's syndrome (pretectal syndrome) is characterized by loss of voluntary upward gaze and convergence-retraction nystagmus and (usually) loss of the pupillary light response with retention of miosis in response to the near reflex. Convergence-retraction movements of the globe on attempted upward gaze is due to simultaneous firing of the rectus muscles due to loss of supranuclear control. There may also be an apparent accommodative spasm, a loss of conjugate voluntary downward gaze associated with loss of convergence and accommodation, ptosis or lid retraction, papilledema, or third nerve palsy. Surrounding structures may also be involved depending on the size and location of the lesion. Conjugate horizontal ocular movements are usually not affected. The syndrome results from tectal or pretectal lesions affecting the periaqueductal area. Pinealomas, infiltrating gliomas, vascular lesions (arteriovenous malformations), demyelinating disease, and trauma may produce this picture.

Pons

Lesions of the paramedian pontine reticular formation produce an ipsilateral horizontal gaze palsy affecting saccadic and pursuit movements. Vestibular slow phase movements are preserved owing to the direct pathway from the vestibular nuclei to the abducens and oculomotor nuclei.

Lesions of the brain stem that cause gaze palsies include vascular accidents, arteriovenous malformations, multiple sclerosis, tumors (pontine gliomas, cerebellopontine angle tumors), and encephalitis.

2. SUPRANUCLEAR SYNDROMES INVOLVING DISJUNCTIVE OCULAR MOVEMENTS

Spasm of the Near Reflex

The near reflex consists of three components: convergence, accommodation, and constriction of the pupil. Spasm of the near reflex is usually caused by hysteria, though encephalitis, tabes dorsalis, and meningitis may cause spasm by irritation of the supranuclear pathway. It is characterized by convergent strabismus with diplopia, miotic pupils, and spasm of accommodation (induced myopia).

If hysteria is the cause, atropine 1%, 2 drops in each eye twice daily, or minus (concave) lenses may give temporary relief. Psychiatric consultation is indicated for treatment of an underlying mental cause.

Convergence Paralysis

Convergence paralysis is characterized by a sudden onset of diplopia for near vision, with absence of any individual extraocular muscle palsy. It is caused by hysteria or destructive lesions of the supranuclear pathway for convergence. The combination of motor convergence failure and pupillary miosis confirms patient effort and an organic lesion. Multiple sclerosis, myasthenia gravis, head trauma, encephalitis, tabes dorsalis, tumors, aneurysms, minor cerebrovascular accidents, and Parkinson's disease are the most common organic causes.

INTERNUCLEAR OPHTHALMOPLEGIA

The medial longitudinal fasciculus is an important fiber tract extending from the rostral midbrain to the spinal cord. It contains many pathways connecting nuclei within the brainstem, particularly those concerned with extraocular movements. The most common manifestation of damage to the medial longitudinal fasciculus is an internuclear ophthalmoplegia, in which conjugate horizontal eye movements are disrupted owing to failure of coordination between the abducens nerve nucleus in the pons and the oculomotor nerve nucleus in the midbrain. The lesion in the brainstem is ipsilateral to the eye with the adduction failure or opposite to the direction of horizontal gaze that is abnormal. In the mildest form of internuclear ophthalmoplegia, the clinical abnormality is restricted to a slowing of saccades in the adducting eye. In the most severe form, there is a complete loss of adduction on horizontal gaze (  Figure 14-12). Convergence is characteristically preserved in internuclear ophthalmoplegia except when the lesion is in the midbrain, when the convergence mechanisms may also be affected. Another feature of internuclear ophthalmoplegia is nystagmus in the abducting eye on attempted horizontal gaze, which is at least in part a result of compensation for the failure of adduction in the other eye. In bilateral internuclear ophthalmoplegia, there may also be an upbeating nystagmus on upgaze due to failure of control of gaze holding in the upward direction, and the eyes may be divergent; this is known as the wall-eyed bilateral internuclear ophthalmoplegia (WEBINO) syndrome.

Internuclear ophthalmoplegia may be due to multiple sclerosis (particularly in young adults), brainstem infarction (particularly in older patients), tumors, arteriovenous malformations, Wernicke's encephalopathy, and encephalitis. Bilateral internuclear ophthalmoplegia is most commonly due to multiple sclerosis.

A horizontal gaze palsy combined with an internuclear ophthalmoplegia, due to a lesion of the abducens nucleus or paramedian pontine reticular formation extending into the ipsilateral medial longitudinal fasciculus, affects all horizontal eye movements in the ipsilateral eye and adduction in the contralateral eye. This is known as a "one-and-a-half syndrome," or paralytic pontine exotropia.

NUCLEAR & INFRANUCLEAR CONNECTIONS

Oculomotor Nerve (III)

The motor fibers arise from a group of nuclei in the central gray matter ventral to the cerebral aqueduct at the level of the superior colliculus. The midline central caudal nucleus innervates both levator palpebrae superioris muscles. The paired superior rectus subnuclei innervate the contralateral superior rectus. The efferent fibers decussate immediately and pass through the opposite superior rectus subnucleus. The subnuclei for the medial rectus, inferior rectus, and inferior oblique muscles are also paired structures but innervate the ipsilateral muscles. The fascicle of the oculomotor nerve courses through the red nucleus and the inner side of the substantia nigra to emerge on the medial side of the cerebral peduncles. The nerve runs alongside the sella turcica, in the outer wall of the cavernous sinus, and through the superior orbital fissure to enter the orbit.

The parasympathetics arise from the Edinger-Westphal nucleus just rostral to the motor nucleus of the third nerve and pass via the inferior division of the third nerve to the ciliary ganglion. From there the short ciliary nerves are distributed to the sphincter muscle of the iris and to the ciliary muscle.

A. Oculomotor Paralysis:

Lesions of the third nerve nucleus affect the ipsilateral medial and inferior rectus and inferior oblique muscles, both levator muscles, and both superior rectus muscles. There will be bilateral ptosis and bilateral limitation of elevation as well as limitation of adduction and depression ipsilaterally. From the fascicle of the nerve in the midbrain to its eventual termination in the orbit, all other lesions produce purely ipsilateral results. Just before entering the orbit, the nerve divides into a superior and inferior branch; the former innervates the levator palpebrae and superior rectus muscles and the latter all other muscles and the sphincter.

If the lesion involves the third nerve anywhere from the nucleus (midbrain) to the peripheral branches in the orbit, the eye is turned out by the intact lateral rectus muscle and slightly depressed by the intact superior oblique muscle. (Incyclotorsion from the action of the intact superior oblique muscle can be observed by watching a small blood vessel on the medial conjunctiva as depression of the eye is attempted.) There can be a dilated fixed pupil, absent accommodation, and ptosis of the upper lid, often severe enough to cover the pupil. The eye may only be moved laterally. Trauma, aneurysm, viral infections, and vascular disease are the most common causes. Aneurysm usually arises from the junction of the internal carotid and posterior communicating arteries. Vascular disease includes diabetes mellitus, migraine, hypertension, and the collagenoses. The common location for vascular palsies is in the cavernous sinus region, where the pupillary fibers are peripheral and nourished better by the vasa vasorum. Compressive lesions such as aneurysms involve the external pupillary fibers early and produce pupillary dilation. Thus, aneurysm and vascular disease can be differentiated clinically, since in vascular lesions the pupillary responses are usually spared, whereas aneurysmal compression causes a completely fixed and dilated pupil. Less than 5% of vascular third nerve palsies are associated with complete pupillary palsy, and in only 15% is there partial pupillary palsy.

Some apparently vascular oculomotor palsies with or without pupillary sparing can be seen on MRI to have focal mesencephalic infarcts without the usual rubral tremor or other local signs. In compressive lesions, the pupil may become constricted because of aberrant regeneration (see below), or a concomitant Horner's syndrome (sympathetic paresis) can produce a "frozen" pupil of 3-4 mm.

Bilateral nuclear third nerve palsies can also be associated with sparing of the lids. Bilateral peripheral third nerve palsies can occur secondary to interpeduncular lesions such as basilar artery aneurysm or a herniated hippocampus of the temporal lobe.

Monocular elevator paralysis or inability to elevate in both abduction (superior rectus) and adduction (inferior oblique) can occur as a congenital defect or as a complication of thyroid ophthalmopathy, orbital myositis, orbital floor fracture, myasthenia gravis, paresis of the superior division of the third nerve (tumor, sinusitis, postviral), or midbrain stroke.

Third nerve palsies in children may be congenital or may be due to ophthalmoplegic migraine, meningitis, or postviral.

B. Oculomotor Synkinesis (Aberrant Regeneration of the Third Nerve):

This phenomenon is characterized by (1) lid dyskinesias on horizontal gaze (ie, the levator palpebrae superioris fires when the medial rectus fires); (2) adduction on attempted upgaze (ie, the medial rectus fires when the superior rectus fires); (3) retraction on attempted upgaze (ie, co-firing of recti, which are retractors); (4) pseudo-Argyll Robertson pupil (ie, no light response, no near response in the primary position but a "near" response on adduction or adduction-depression-pupillary innervation from medial or inferior rectus); (5) pseudo-Graefe's sign (ie, no lid lag on downgaze but lid retraction due to lid innervation from the inferior rectus); and (6) a monocular vertical optokinetic nystagmus response (due to co-firing muscles fixing the involved eye, allowing only the normal eye to respond to the moving target). This oculomotor synkinesis probably occurs not only as a combination of misdirection of sprouting axons into the wrong sheaths and subsequent muscle co-firing but also as a consequence of ephaptic transmission or cross-talk between axons without covering myelin sheaths.

Oculomotor synkinesis can occur secondary to severe trauma or compression of the third nerve by a posterior communicator artery aneurysm, or primarily due to an internal carotid aneurysm or meningioma in the cavernous sinus. If compression lasts several weeks, strabismus surgery is often required to achieve binocular single vision.

C. Cyclic Oculomotor Palsy:

Cyclic oculomotor palsy can complicate a congenital third nerve palsy; it is a rare predominantly unilateral event with a typical third nerve paresis showing cyclic spasms every 10-30 seconds. During these intervals, ptosis improves and accommodation increases. This phenomenon continues unchanged throughout life but decreases with sleep and increases with greater arousal. It is probably a periodic discharge by damaged neurons of the oculomotor nucleus which sum- mate subthreshold stimuli until a discharge occurs

D. Marcus Gunn Phenomenon (Jaw-Winking Syndrome):

This rare congenital condition consists of elevation of a ptotic eyelid upon movement of the jaw. Acquired cases occur after damage to the oculomotor nerve with subsequent innervation of the lid (levator palpebrae superioris) by a branch of the fifth cranial nerve. Muscular palsies may be present.

Trochlear Nerve (IV)

Motor (entirely crossed) fibers arise from the trochlear nucleus just caudal to the third nerve at the level of the inferior colliculus; they then run posteriorly, decussate in the anterior medullary velum, and wind around the cerebral peduncles. The fourth nerve travels near the third nerve along the wall of the cavernous sinus to the orbit, where it supplies the superior oblique muscle. The fourth nerve is unique among the cranial nerves in arising from the dorsal brainstem.

A. Trochlear Paralysis:

Lesions of the fourth nerve are commonly vascular, traumatic, or idiopathic (congenital or developmental with later decompensation). However, cerebellar tumors can also present with a fourth nerve lesion as an early sign. The nerve is vulnerable to injury at the site of exit from the dorsal aspect of the brainstem. Both nerves may be damaged by severe trauma as they decussate in the anterior medullary velum, resulting in bilateral superior oblique palsies.

Superior oblique palsy results in upward deviation (hypertropia) of the eye. The hypertropia increases when the patient looks down and with adduction. In addition, there is excyclotropia; therefore, one of the diplopic images will be tilted with respect to the other. Torsional symptoms suggest an acquired late-onset superior oblique palsy: correspondingly, the lack of torsional symptoms suggest an early onset of the deviation. Tilting the head toward the involved side increases the deviation. Tilting the head away from the side of the involved eye may relieve the diplopia, and patients frequently present with a head tilt. Miscellaneous causes include multiple sclerosis, a brainstem arteriovenous malformation, orbital pseudotumor, and myasthenia gravis. Strabismus surgery is effective in patients who fail to improve with time.

B. Superior Oblique Myokymia:

A monocular microtremor of the superior oblique muscle can rarely occur. It is an acquired, haphazard, and episodic overaction of the superior oblique muscle characterized by rapid torsional movements of one eye. Patients notice oscillopsia when this occurs, and the symptoms can be improved by carbamazepine. The cause may be compression of the trochlear nerve by an aberrant artery.

Abducens Nerve (VI)

Motor (entirely uncrossed) fibers arise from the nucleus in the floor of the fourth ventricle in the lower portion of the pons near the internal genu of the facial nerve. Piercing the pons, the fibers emerge anteriorly, the nerve running a long course over the tip of the petrous portion of the temporal bone into the cavernous sinus. It enters the orbit with the third and fourth nerves to supply the lateral rectus muscle.

A. Abducens Nucleus Lesion:

The abducens nucleus contains the motor neurons to the ipsilateral lateral rectus and the cell bodies of interneurons innervating the motor neurons to the contralateral medial rectus. It is the final common relay point for all horizontal conjugate eye movements, and a lesion within the nucleus will produce an ipsilateral horizontal gaze palsy affecting all types of eye movement including vestibular movements. This contrasts with a lesion of the paramedian pontine reticular formation, in which vestibular movements are preserved

B. Abducens Nerve Paralysis:

(See also Chapter 12.) This is the most common single muscle palsy. Abduction of the eye is reduced or absent; esotropia is present in the primary position and increases upon gaze to the affected side. Movement of the eye to the opposite side is normal. Möbius' syndrome (congenital facial diplegia) can be associated with a sixth nerve or conjugate gaze palsy. Vascular disorders (arteriosclerosis, diabetes, migraine, and hypertension) are common causes. However, dural arteriovenous fistula, basilar artery disease, increased intracranial pressure, lumbar puncture, tumors at the base of the skull, meningitis, and trauma are other frequent causes. Arnold-Chiari malformation (congenital downward displacement of the cerebellar tonsils) can also produce brainstem traction and sixth nerve palsies. Lyme disease can produce an isolated sixth nerve palsy as well as those that occur secondary to meningeal involvement. A child with a sixth nerve palsy should be evaluated for a brainstem tumor (glioma) or inflammation if trauma was not present or if trauma was minimal. Pseudo-sixth nerve palsies can occur in Duane's retraction syndrome, spasm of the near reflex, thyroid eye disease, myasthenia, dorsal midbrain compression (Parinaud's syndrome), or long-standing strabismus and in medial rectus entrapment by an ethmoid fracture.

C. Duane's Syndrome:

Duane's syndrome is uncommon (< 1% of cases of strabismus) and in almost all cases congenital. It is a stationary, nearly always unilateral condition consisting of deficient horizontal ocular motility characterized by complete or partial deficiency of abduction. Evidence based on pathologic studies has determined that Duane's syndrome can be due to congenital absence of the sixth nerve with coinnervation of the lateral rectus by a branch of the third nerve. Therefore, attempted adduction movements result in retraction of the globe and narrowing of the lid fissure. The visual handicap is seldom severe. Visual acuity can be normal, and the eye is otherwise normal. Unless the deviation is very large, strabismus surgery is best avoided.

Cochlear nucleus lesions producing sensorineural hearing loss occur in 6.8% of cases of Duane's syndrome. Congenital malformations may also include the facial and skeletal bones, the ribs, and the external ear. Ocular anomalies can include epibulbar dermoids. Acquired Duane's syndrome is a rare event occurring after a peripheral nerve palsy.

D. Gradenigo's Syndrome:

Gradenigo's syndrome is characterized by pain in the face (from irritation of the trigeminal nerve) and abducens palsy. The syndrome is produced by meningeal inflammation at the tip of the petrous bone and most often occurs as a rare complication of otitis media with mastoiditis or petrous bone tumors.

Symptoms and Signs of Extraocular Muscle Palsies

Diplopia occurs when the visual axes are not aligned. This is especially true when the onset of strabismus is after age 6 (suppression and abnormal retinal correspondence do not develop). Dizziness or dysequilibrium may be associated but disappears with monocular patching. Head tilt occurs, especially in paresis of the superior oblique muscle, when the tilt is to the opposite side to avoid diplopia by moving the eye out of the field of action of the paralyzed muscle. Vertical saccadic velocity can differentiate a superior oblique palsy from inferior rectus palsy. Horizontal saccadic velocity can differentiate a restricted globe with pseudo-sixth nerve from sixth nerve paresis. Forced duction tests should also be done, since a paresis could be simulated by a restricted yoke muscle.

Ptosis is caused by weakness or paralysis of the levator muscle. Any extraocular muscle palsy that occurs with minor head trauma (subconcussive injuries) should be investigated for a basal tumor. The minimally positive edrophonium test is unreliable because it can be nonspecific. Fascicular lesions involving the portion of a cranial nerve within the brainstem resemble peripheral nerve lesions but can be differentiated on the basis of other brainstem signs and their subsequent poor recovery. For vascular causes of cranial nerve palsies, recovery by 4 months is the rule. Palsies that persist longer than 6 months-especially those involving the sixth nerve-should be evaluated for an underlying structural compressive lesion (tumor, arteriovenous fistula, aneurysm).

Syndromes Affecting Cranial Nerves III, IV, & VI

A. Superior Orbital Fissure Syndrome:

All extraocular peripheral nerves pass through the superior orbital fissure and can be involved by trauma or by tumor encroaching on the fissure

B. Orbital Apex Syndrome:

This syndrome is similar to the superior orbital fissure syndrome with the addition of optic nerve signs and usually greater proptosis and less pain. It is caused by an orbital tumor, inflammation, or trauma that damages the optic and extraocular nerves

C. Complete Ophthalmoplegia (Sudden):

Complete ophthalmoplegia of sudden onset can be due to brainstem vascular disease, Wernicke's encephalopathy, pituitary apoplexy, Fisher's syndrome, myasthenia crisis, bulbar poliomyelitis, diphtheria, botulism, meningitis, and syphilitic or arteriosclerotic basilar aneurysm.

THE CEREBELLUM

The cerebellum has an important modulating influence on the function of the neural integrators. Thus, it is involved in gaze holding and the control of saccades, particularly the relationship between the pulse and the step of saccade generation. Cerebellar dysfunction produces gaze-evoked nystagmus, by its influence on gaze holding, and abnormalities of saccades, including saccadic dysmetria, in which the saccadic amplitude is inaccurate, and postsaccadic drift due to a mismatch between the pulse and step of the saccade.

The cerebellum is also important in the control of pursuit eye movements, and cerebellar dysfunction may thus result in broken (saccadic) pursuit.

10.1036/1535-8860.ch14

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