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Cranial Nerve Palsies Jonathan D. Trobe, MD Professor of Ophthalmology and Neurology Departments of Ophthalmology (Kellogg Eye Center) and Neurology University of Michigan Medical System Ann Arbor, Michigan, USA Email: [email protected] Telephone: 734 7639147 Fax: 734 2328181 Kellogg Eye Center 1000 Wall Street Ann Arbor, Michigan 48105 Introduction The ocular motor cranial nerves. The position and movement of the eyes is controlled by three cranial nerves—oculomotor (cranial nerve III), trochlear (cranial nerve IV), and abducens (cranial nerve VI). The most complicated of the three ocular motor nerves, the oculomotor nerve controls adduction, supraduction, and infraduction via the medial, superior, inferior, and inferior oblique muscles. It also controls pupil constriction via the iris sphincter muscle and upper lid elevation via the levator palpebrae superioris muscle. The trochlear nerve controls infraduction and intorsion via the superior oblique muscle. The abducens nerve controls abduction via the lateral rectus muscle. Clinical effects of ocular motor cranial nerve malfunction. When the oculomotor nerve malfunctions, the patient develops some combination of adduction, supraduction, and infraduction deficits, possibly together with ptosis and a dilated, poorly constricting pupil. When the patient looks straight ahead, the affected eye will usually be deviated outward (exotropia) and sometimes also upward (hypertropia) or downward (hypotropia). When the trochlear nerve malfunctions, the patient develops a deficit in infraduction-inadduction. When the patient looks straight ahead, the affected eye will usually be deviated upward (hypertropia) and extorted on its anterior-posterior axis (excyclodeviation). When the abducens nerve malfunctions, the patient develops a deficit in abduction. When the patient looks straight ahead, the affected eye will usually be deviated inward. Accompanying these ductional deficits is a misalignment of the eyes in one or more gaze positions. The misalignment is typically “incomitant,” that is, it varies in degree with gaze 2 position. The misalignment will be greatest in the field of action of the palsied extraocular muscles. For example, an oculomotor palsy typically causes exotropia in straight ahead gaze that increases on contralateral gaze, as the weak medial rectus is activated. There will be an increasing hypotropia on upgaze and an increasing hypertropia on downgaze, as the superior and inferior rectus muscles become activated, respectfully. A trochlear palsy typically displays a hypertropia on the affected side in straight ahead gaze that increases on contralateral and downward gaze. An abducens palsy will cause an esotropia in straight ahead gaze that increases on ipsilateral gaze. Ocular motor cranial nerve lesions may be severe or mild. When the lesion is severe, the ductional deficit will generally be marked and the diagnosis relatively easy. But when the lesion is mild, the ductional deficit may be imperceptible, making diagnosis difficult. In such cases, measurement of ocular alignment is critical to reaching a correct diagnosis. (1) Also important in this setting is the assessment of the functions of the lid, pupils, and trigeminal nerve, as well as an appreciation of ocular surface and orbital abnormalities (conjunctival congestion, intraocular pressure, resistance to retropulsion of the eye, proptosis). Conditions That Mimic Ocular Motor Cranial Nerve Malfunction. It is unwise to presume that deficits in ocular movement and alignment are always due to ocular motor cranial nerve palsies. Such deficits may also be caused by extraocular muscle disorders (inflammation, trauma, neoplasms, dystrophies), failure of neuromuscular transmission (myasthenia gravis), and disruption of input to the ocular motor cranial nerve nuclei in the brain stem (internuclear ophthalmoplegia and skew deviation). (2) Distinguishing these conditions from ocular motor palsies requires experience and skill. Reading this chapter ought to be a helpful starter! 3 Diplopia. Ocular misalignment created by malfunction of the ocular motor nerves or its mimickers usually evokes the symptom of diplopia, or seeing the same object in two different locations. The fixating eye will focus the object of regard on its fovea, creating a clear image. The deviating eye will focus the object of regard on an extrafoveal part of the retina. Lacking the resolving power of the fovea, this extraretinal focus creates an unclear image that is displaced from the clear image generated by the fixating eye according to the direction of misalignment of the eyes. In some patients, ocular misalignment also creates the symptom of “visual confusion”, in which the patient perceives two different but superimposed images. This sensation results from the fact that the brain is recording images from the misaligned foveas of the eyes. However, visual confusion is such a disturbing sensation that the brain usually suppresses the image coming from the fovea of the deviating eye. The diplopia caused by ocular misalignment is always binocular. That is, the diplopia disappears when the vision of either eye is blocked (by closing the eye, applying an occluder). Patients often report diplopia that is perceived when only one eye is viewing. This “monocular diplopia,” in which the second (“ghost”) image overlaps the true image, is virtually always caused by disordered optics within the eye—uncorrected refractive error, corneal surface irregularity, iris hole, cataract, or displaced lens. The disordered optics can be confirmed by introducing a pinhole in front of the symptomatic eye. Doing so should immediately eliminate the ghost image. If not, the diplopia is either of psychogenic origin or the patient miscommunicated the symptom. Ocular misalignment without diplopia. Misalignment of the eyes does not always cause diplopia. Patients with poor vision in one eye or both, and those with attentional or other cognitive deficits may not appreciate diplopia. If the ocular misalignment is so minimal that it 4 creates a narrow separation of images, patients may report the sensation as “blurred vision” rather than double vision. If the misalignment develops within the first decade of life, the second image is usually promptly suppressed. Such suppression not only eliminates the sensation of diplopia, it causes subnormal acuity in the eye whose image is being suppressed, a phenomenon called “amblyopia.” Amblyopia is associated with neuronal drop-out in the lateral geniculate nucleus and ocular dominance columns of the visual cortex served by the affected eye. Amblyopia and its accompanying neurophysiologic effects develop only during the first decade of life when visual pathways are capable of modification. It can be reversed by preventing the unaffected eye from fixating and by forcing the affected eye to fixate, a task achieved by patching the unaffected eye. The younger the patient, the more likely the amblyopia will be reversed. Reversal of amblyopia has been documented up to age 12 years. (3) Oculomotor palsy Manifestations. (Figure 1) In its most extreme form, oculomotor palsy produces complete ptosis, a fixed, dilated pupil, and total compromise of adduction, supraduction, and infraduction. Incomplete palsies produce subtotal involvement of each of these components, or sparing of one or more of them. Lesions usually lie within the extra-axial course of the nerve, although brain stem lesions may sometimes be responsible. Causes.(4-11) (Tables 1, 2) Oculomotor palsy is uncommon before age 40, except after severe head trauma. It may rarely be present at birth. In youth, it may be acquired by inflammation, but life-threatening lesions, including aneurysms and neoplasms, must be scrupulously excluded even during this early phase of life. In middle and late adulthood, it 5 results most often from ischemia of the subarachnoid or extradural portions of the nerve owing to small vessel occlusive disease associated with diabetes mellitus, hypertension, and arteriosclerosis. Less commonly, inflammation, neoplasm, and trauma may be causative during this epoch. The causes and evaluation of oculomotor palsy (and other ocular motor palsies, see below) are determined by whether the palsy is “non-isolated” (one of several pertinent neurological findings) or “isolated” (the only pertinent neurologic finding). Non-isolated oculomotor palsy. Oculomotor palsy that is embedded among other pertinent findings is usually caused by trauma, mass lesions, or meningeal inflammations and neoplasms. For example, oculomotor palsy may be a sign of a cerebral hemispheric mass lesion that has induced herniation of the temporal lobe uncus over the tentorium cerebelli, compressing the oculomotor nerve against the rigid posterior clinoid process (“tentorial herniation syndrome”). Injury to the peripherally-situated pupillomotor fibers (12) comes first, so that a fixed, dilated pupil may be the initial sign. However, hemiparesis and reduced consciousness soon follow. Oculomotor palsy that results from midbrain lesions (infarcts, masses, demyelination) almost always causes tremor, ataxia, hemiparesis, or gaze palsies. (2) Cavernous sinus or superior orbital fissure lesions often cause other ocular motor palsies, Horner syndrome, and trigeminal sensory dysfunction. Orbital lesions rarely cause oculomotor palsy. Instead, the impaired ocular movements and ptosis are caused by compression/infiltration of extraocular muscles and the levator palpebrae 6 superioris. Orbital lesions can be clinically suspected because they cause conjunctival congestion, elevated intraocular pressure, proptosis, or resistance to retropulsion of the eye. Isolated oculomotor palsy. Such a palsy, in which there are no other pertinent neurologic findings, has a different set of causes than does a non-isolated palsy. Oculomotor palsy noted at birth (13) is often blamed on birth trauma. When there is no evidence of trauma, dysplasia may be a better explanation. Aberrant regeneration (see below) is a common accompaniment, indicating considerable structural damage. Amblyopia is a concern and can be difficult to treat because of ptosis. In childhood, acquired isolated oculomotor palsy is caused by head trauma, inflammation (including infection), or neoplasm.(13) It must be investigated thoroughly. In children, recurrent oculomotor palsy, previously labeled “ophthalmoplegic migraine,” is now understood to be an idiopathic inflammatory mononeuropathy affecting the nerve root. MRI reliably shows enhancement of its peduncular segment. (14) In adulthood, extra-axial nerve ischemia accounts for at least 75% of cases. (2) Most patients have a family or personal history of conditions associated with small caliber vasculopathy, including diabetes, hypertension, cigarette smoking, and hyperlipidemia. (15, 16) The ischemia typically spares the peripherally-situated iris sphincter fibers, (17-21) so that anisocoria rarely exceeds 1mm.(21) Progression of the deficit up to 14 days after onset should not deter a diagnosis of ischemia, as such progression has been amply documented.(22) Full recovery occurs in nearly 100% of patients within three months.(23) If not, the ischemia occurred within the brain stem (24-26), although in most such brain stem infarcts, other neurological 7 manifestations are present (27) or ischemia was not the cause! The chance of recurrence of an ischemic oculomotor (or abducens) palsy is 15%.(22) The most important cause of isolated oculomotor palsy is cerebral berry aneurysm, usually located at the junction of the carotid and posterior communicating arteries, and less commonly at the apex of the basilar artery. (28-31) Patients are vulnerable over a wide age span, extending from age 20 to 80 years! The palsy results more often from expansion than rupture into the substance of the oculomotor nerve. It is therefore a “sentinel sign” of impending rupture, which may be imminent and fatal in 50% of cases. Not all patients report severe headache. Most cases will have an ipsilaterally dilated and weakly reactive pupil, but pupils may be normal especially if the extraocular muscle palsy is incomplete, as is often the case. (30, 31) Early diagnosis is essential, not only to preclude rupture and death, but because treatment of an unruptured aneurysm is far more successful than treatment of a ruptured aneurysm. Other causes of isolated oculomotor palsy are cranial base meningeal or cavernous sinus inflammations, neoplasms (especially pituitary adenomas), brain radiation, head trauma, carotid cavernous fistulas, intracavernous aneurysms, and nerve sheath tumors. (2) In the elderly, oculomotor palsy may rarely be a manifestation of giant cell (temporal) arteritis.(32) The damage is actually more likely to be in the extraocular muscles than in the nerve. Even so, consider this diagnosis even if constitutional symptoms are absent. Sedimentation rate or C-reactive protein elevation prompts pre-emptive corticosteroid treatment and temporal artery biopsy for definitive diagnosis. 8 Lesions that produce disruption of the nerve cause its regenerating axons to sprout to the wrong destinations. This phenomenon, known as “aberrant regeneration,” causes the upper lid to elevate on adduction and sometimes on depression of the affected eye.(33, 34) The significance of aberrant regeneration is that its cause is never ischemia, almost never inflammation, and almost always compression by neoplasm, cyst, or aneurysm. Mimickers. (Table 3) Imitators of oculomotor palsy are extraocular myopathy, myasthenia gravis, trochlear nerve palsy, internuclear ophthalmoplegia, and skew deviation. Extraocular myopathy is most commonly due to inflammation (Graves disease, idiopathic orbital inflammation, connective tissue diseases). Signs of orbital congestion (proptosis, soft tissue swelling, conjunctival hyperemia) will usually be present, although there are indolent forms without such features. Myasthenia gravis may fortuitously affect only those muscles served by the oculomotor nerve, but more commonly it also affects other extraocular muscles and causes ptosis and weakness in forced lid closure, mouth closure, neck and shoulder extension, and hip flexion. Difficulty breathing, chewing, and swallowing (“bulbar manifestations”) may also be present. In trochlear nerve palsy (see below), the patient typically reports seeing a torted image with one eye. In internuclear ophthalmoplegia, the eyes are often aligned in straight ahead gaze, with exotropia developing on gaze to the side contralateral to the palsy. Also, adduction of the affected eye is slow and often incomplete, and abduction of the unaffected eye produces nystagmus. Often the patient achieves much better adduction of the affected eye on convergence than on versions. In skew deviation, vertical misalignment is present without horizontal or torsional misalignment; saccadic pursuit, nystagmus, and limb or gait ataxia are frequent accompaniments.(35) 9 Evaluation. (Flow Chart #1) Although the degree of pupil sparing has traditionally been used to determine whether a compressive lesion such as aneurysm might be the cause of an isolated oculomotor palsy, pupil signs are not adequate to make this distinction, (31) particularly if the palsy is incomplete, as it usually is. Therefore, all patients who have isolated (and nontraumatic) oculomotor palsy should undergo brain imaging, principally to exclude aneurysm. CT angiography is more sensitive in detecting aneurysm, but is less sensitive in detecting and defining non-vascular lesions. Also, it has the drawback of delivering radiation and a dye load. But because it is so sensitive, widely available, and fast, it is the procedure of choice for non-pregnant adults. Others should undergo MRI and MR angiography. (36, 37) Because giant cell arteritis can give rise to manifestations that represent, or appear to represent, an oculomotor palsy, this diagnosis should always be considered at outset. If imaging studies are negative, giant cell arteritis is not a consideration, and the patient has ample risk factors for arteriosclerosis, no further testing is necessary except as part of risk factor abatement. If arteriosclerotic risk factors are not evident, then further diagnostic studies should be aimed at excluding inflammatory and neoplastic meningeal causes and mimickers of oculomotor palsy. If the vascular imaging is inadequate or equivocal in excluding aneurysm even after expert review, cerebral angiography may be necessary. (36, 37) A critical intermediate step is having the non-invasive studies reviewed by an experienced interventional neuroradiologist, as there is evidence that the studies may actually show the aneurysm, but that it is overlooked by inexperienced interpreters. (38) If an aneurysm is discovered as the cause of the oculomotor palsy, clipping may be allow improved recovery of nerve function relative to coiling. (39) 10 Among those who are initially suspected of having an ischemic oculomotor palsy, lack of resolution within 3 months or progression after 3 weeks should prompt reinvestigation. Non-isolated oculomotor palsies must undergo an evaluation contingent on the associated findings. Ophthalmic Treatment. Patients who have troublesome diplopia may sometimes be palliated with press-on (Fresnel) prisms, spectacle occluders, or eye patches. If diplopia remains after a period of at least six months, patients may be candidates for surgical realignment of the eyes. Surgery involves weakening the lateral rectus muscle, strengthening the other rectus muscles, and transposing the superior oblique to the insertion of the medial rectus muscle. Unfortunately, these procedures rarely restore a useful field of single binocular vision. For those who cannot be helped with eye muscle surgery, diplopia can be eliminated with an opaque (black paint in pupil space) contact lens. (40) Trochlear palsy Manifestations. (Figure 2) The least common of the three ocular motor cranial nerve palsies, trochlear palsy is also the most difficult to diagnose. Incomplete forms typically display full—or apparently full--ductions. The distinctive feature is that damage to this muscle causes torsional misalignment. The patient may not spontaneously report seeing a tilted image with one eye, but this symptom can often be brought out on examination. The patient may affect a head tilt to eliminate diplopia. 11 The eyes will be vertically misaligned, the higher eye on the lesioned side (Figure 165.2). The degree of misalignment will increase as gaze is directed away from the lesioned side and with the head tilted toward the lesioned side. These phenomena are noted in the “three-step test.” (1) Step one measures the hypertropia in straight-ahead gaze. Step two compares the hypertropia in right and left gaze. Step three compares the hypertropia in right and left head tilt. A right trochlear nerve palsy produces a “right-left-right” pattern of misalignment: right hypertropia in straight-ahead gaze; greater hypertropia on left than right gaze; and greater hypertropia on right than left head tilt. A left trochlear palsy produces a “left-right-left” pattern. The examiner should also note whether the patient has pathologically high vertical fusional reserves (4 prism-diopters or greater), and whether the vertical misalignment is greater in upgaze than downgaze, indications of a long-standing (and possibly decompensated congenital) lesion. A final diagnostic step is the double Maddox Rod test for torsional misalignment. Maddox Rod lenses are placed in the trial spectacle frames of both eyes with their axes aligned vertically. As patients view a fixation light with both eyes, they will see two horizontal lines. They are asked if the lines appear tilted with respect to each other. If so, they are instructed to adjust the knob that controls the orientation of the Maddox Rod lens in the right spectacle frame so as to make the two lines parallel. If the eyes are extorted with respect to each other, they will move the Maddox Rod lens to a counterclockwise position. The torsional deviation is then read in degrees from the trial frame. Excyclodeviation of more than 3 degrees is common in 12 acquired trochlear nerve palsies. Greater than 10 degrees indicates a bilateral trochlear nerve palsy. Causes. (41-46) (Tables 1, 2) As with oculomotor palsy, the causes of trochlear palsy can be divided into whether the palsy is non-isolated or isolated. Non-isolated trochlear nerve palsies. Like non-isolated oculomotor nerve palsies, nonisolated trochlear nerve palsies are usually caused by traumatic, neoplastic, or inflammatory lesions of the brainstem or cranial base. The most common lesion lies in the dorsal midbrain, where brain stem or pineal region masses may cause damage. Some combination of vertical gaze palsies, light:near dissociated pupils, upper lid retraction, and ataxia is likely to be present. Lesions of the cavernous sinus cause some combination of oculomotor, abducens, and trigeminal dysfunction as well as Horner syndrome to the trochlear palsy. However, because the trochlear nerve is the most resistant of the ocular motor nerves to any type of cavernous sinus lesion, it is often spared. Orbital lesions do not cause trochlear nerve palsy except surgery that peels the periorbita off the medial orbital wall and trauma that fractures the trochlea. Isolated trochlear nerve palsies. The three major causes of isolated trochlear nerve palsies are closed head and surgical trauma, breakdown of a congenital weakness in the nerve, and extra-axial ischemia. Nerve sheath tumors, extra-axial compression by tumor, and inflammation are less common. 13 Closed head injury is a very common cause. (47) The incisural course of the nerve makes it particularly vulnerable to being impaled on the rigid tentorium as the brain is jostled. (48) Neurosurgical manipulation in the tentorial region is another frequent traumatic cause. (40) Breakdown of a congenital trochlear nerve palsy is common. Patients report that they could originally eliminate diplopia by focusing their eyes, changing gaze position, or tilting the head. For unexplained reasons, they eventually become unable to contain the tendency toward misalignment and develop intermittent diplopia. Three examination features help identify the palsy as congenital: the misalignment is worse in upgaze, fusional vergences are above normal, and Double Maddox Rod testing fails to disclose the perception of a tilted image. MRI may disclose a small superior oblique muscle. Adult-onset trochlear nerve palsy may also be caused by extra-axial ischemia, although less often than oculomotor and abducens palsies. Patients have ample arteriosclerotic risk factors. The palsy disappears within three months. Involvement of the peripheral segment of the nerve by inflammation, meningeal neoplasm, or nerve sheath tumor is a rare but important cause of trochlear nerve palsy.(2) Unlike oculomotor palsy, trochlear nerve palsy is not caused by berry aneurysm. Mimickers. (Table 3) Four conditions imitate trochlear nerve palsy: 1) partial oculomotor nerve palsy; 2) extraocular muscle disorder; 3) myasthenia gravis; and 4) skew deviation (see 14 Differential diagnosis of oculomotor palsy, above). These conditions do not conform to the three-step test and do not show excyclodeviation on double Maddox Rod testing. For distinctive features of these mimickers, refer to the section on mimickers of oculomotor palsy. Evaluation. Traumatic trochlear nerve palsies require no brain imaging or other diagnostic studies unless the trauma is considered too trivial or remote to have been the cause. Nontraumatic and non-isolated trochlear nerve palsies (Flow Chart #2) should be evaluated according to the additional findings (see Oculomotor Palsy, above). Patients with isolated trochlear nerve palsies who have features suggestive of a decompensated congenital lesion do not need to undergo further evaluation. Nor do adults who have enough arteriosclerotic risk factors to allow a presumptive diagnosis of ischemia as the cause. They may be observed for recovery, which should occur within three months. All others should undergo MRI, and if negative, lumbar puncture. Vascular imaging is unnecessary because cerebral aneurysm is not a diagnostic consideration Ophthalmic Treatment. Among patients who have troublesome diplopia from persistent trochlear nerve palsies where the cause is known, eye muscle surgery is usually successful in restoring single binocular vision over a wide gaze range.(40) Surgery consists either of strengthening the superior oblique muscle or weakening its yoke muscle, the contralateral inferior rectus muscle. 15 Sixth cranial nerve palsy Manifestations. (Figure 3) Abducens nerve palsy produces an esodeviation that is greatest on gaze toward the affected side, with or without an abduction deficit. Underdiagnosis occurs when there is no obvious abduction deficit. Overdiagnosis occurs when the examiner fails to consider that an obvious abduction deficit may also be caused by a restrictive extraocular (shortened) medial rectus muscle, spasm of convergence, or myasthenia gravis (see Mimickers, below). Causes.(2, 40, 49, 50) (Tables 1, 2) As with oculomotor and trochlear nerve palsies, the causes of abducens palsy depend on whether the palsy is non-isolated or isolated. Nonisolated abducens palsies. Mobius syndrome consists of esotropia with bilateral horizontal gaze paresis, together with atrophic weakness of facial or tongue muscles and other malformations. Multiple cranial nerve nuclear aplasias are found pathologically.(51) Acquired non-isolated abducens palsies may also be caused by brainstem lesions that damage the abducens fascicles.(2) Although brain stem fascicular lesions may rarely cause an isolated abduction deficit,(52, 53) lesions in this region (inflammation, infarction, mass lesions, Wernicke encephalopathy) typically also damage other pathways, including the medial longitudinal fasciculus, cerebellar peduncles, trigeminal, facial, and acousticovestibular nerves, or the corticospinal tracts. If the adjacent medial longitudinal fasciculus is affected, the patient will develop a “one-and-one-half syndrome,” consisting of an ipsilateral gaze palsy and an ipsilateral adduction deficit.(54) Involvement of the other structures causes ataxia, nystagmus, facial palsy, or contralateral hemiparesis. 16 Cavernous sinus lesions often damage the abducens nerve, but the proximity of other ocular motor, trigeminal, and sympathetic nerves favors their involvement as well. However, the abducens nerve differs from the other ocular motor and trigeminal nerves in lying within the sinus rather than on its outer dural wall, so that internal sinus lesions such as aneurysms, fistulas, and thrombosis are apt to affect it out of proportion to other nerves. Orbital lesions rarely cause an abducens nerve palsy. Abduction deficits are more likely to result from impaired function of the lateral rectus, which may be impeded by tumor, infiltrated, or scarred. Isolated palsies. Brainstem dysplasia accounts for most isolated congenital abducens palsies. Often accompanied by other dysplastic manifestations, the most common variant is Duane syndrome, in which neurons in the abducens nucleus whose axons are destined for the lateral rectus muscle fail to develop.(55) The result is reduced or absent ipsilateral abduction, often accompanied by palpebral fissure narrowing on attempted adduction. The fissure narrowing is a consequence of retraction of the globe produced by co-firing of the medial and lateral rectus muscles, the latter supplied by aberrant axons from the oculomotor nerve. The lack of diplopia (because the misalignment is congenital) and narrowing of the lid fissure are clues to the diagnosis. Acquired isolated abducens palsies are unusual in children. Causes include inflammation and neoplasm. 17 In adults, most isolated abducens palsies are caused by extra-axial microvascular ischemia.(2) Like oculomotor and trochlear nerve palsies, they should recover fully within three months. Isolated abducens nerve palsies may also be caused by basal meningeal, clival, sphenoid, and petrous inflammation and masses. Other considerations are increased (56) or decreased intracranial pressure in spontaneous intracranial hypotension,(57) following lumbar puncture,(58) spinal anesthesia,(59) or shunting (60) (“false-localizing abducens palsy”). The abducens nerve is especially vulnerable to head trauma because it is anchored at the petroclival junction, where it takes nearly a 90-degree turn to enter Dorello’s canal and the cavernous sinus. Accordingly, concussive downward or backward displacement of the brain is apt to shear the nerve as it goes through Dorello’s canal. Traumatic abducens palsy may be associated with fractures and dislocations at the craniocervical junction. The presence of a clival epidural hemorrhage on imaging should call attention to this possibility, which may require stabilization to prevent spinal cord injury. (61) As with oculomotor palsy, one should always consider giant cell arteritis as a cause of isolated abducens palsy in the appropriate setting. Intradural berry aneurysm is never the cause of an abducens palsy. Mimickers. (Table 3) Abducens palsy may be mimicked by three conditions: 1) myasthenia gravis; 2) extraocular muscle disorder; and 3) spasm of the near reflex. 18 Although myasthenia gravis often produces ptosis and reduced eye movements in several directions, it may present with an isolated abduction deficit in one eye. Inflammation, contusion, entrapment, or scarring of the medial rectus muscle may also cause an isolated abduction deficit. Diagnosis depends on clinical or imaging evidence of soft tissue abnormalities in the orbit. Spasm of the near reflex is a common and overlooked mimicker of abducens nerve palsy. It is a psychogenic disturbance that consists of excessive convergence, which will interrupt lateral gaze to produce a “quivering” movement of the eyes. The quivering results from intermittent convergence movements that interrupt horizontal gaze. Pupil constriction and inappropriate accommodation are accompaniments, but pupil constriction may be difficult to detect and accommodation will not be measurable in older adults. (62) Abduction usually becomes full when tested with the other eye occluded. This maneuver interrupts the visual cues that allow the patient to maintain an artificial state of hyperconvergence. Patients who have spasm of the near reflex have a somatoform disorder or are malingering. Evaluation. Traumatic abducens palsy requires no brain imaging or other diagnostic studies unless the trauma is considered too trivial or remote to have been the cause. Patients with non-isolated abducens nerve palsies (Flow Chart #3) should undergo brain imaging guided by the constellation of neurological findings. If an isolated and non-traumatic abducens palsy in an adult has features of a Duane syndrome, no diagnostic studies are necessary as the lesion is congenital. It no such features are present, and the palsy can be attributed to an ischemic microvascular event, no diagnostic studies are necessary unless the condition evolves or fails to resolve within three months. 19 Otherwise, MRI with attention to the cranial base is the appropriate study, followed by lumbar puncture as appropriate. Vascular imaging aimed at berry aneurysm is not necessary as such lesions do not cause abducens nerve palsy. However, if the diagnosis of an unremitting abducens palsy remains elusive, CTA or MRA, or even catheter angiography may be necessary to detect a posterior-draining carotid-cavernous fistula.(63) Ophthalmic Treatment. Among patients who have troublesome diplopia from persistent abducens palsy where the cause is known, eye muscle surgery is often successful in restoring single binocular vision in straight-ahead gaze. However, surgery provides a wide range of single binocular vision only when some lateral rectus function is present preoperatively.(40) When some abduction is present, the surgical options are weakening the medial rectus and strengthening the lateral rectus muscles of the affected eye or resection of the lateral rectus muscle in the affected eye and recession of the medial rectus in the unaffected eye. If no abduction is present, the only option is transposing the vertical rectus muscles to the insertion of the lateral rectus muscle, sometimes combined with chemodenervation of the medial rectus muscle with intramuscular injection of botulinum toxin.(64) Botulinum toxin denervation of the medial rectus has also been promoted as a means of promptly but temporarily restoring normal alignment in patients with acute abducens nerve palsy.(65, 66) However, it rarely provides a durable and wide zone of single binocular vision, and it requires anesthesia and an ophthalmologist expert in injecting the toxin into the extraocular muscle under electromyographic monitoring. A randomized prospective study failed to show that it provided better outcome than observation alone.(67) 20 References 1. Borchert MS. Principles and techniques of the examination of ocular motility and alignment. In: Miller NR, Newman NJ, eds. Walsh & Hoyt’s Clinical Neuro-Ophthalmology. Vol 1, 6th ed. 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Nontraumatic causes of isolated ocular motor nerve palsies in adults* Oculomotor palsy Trochlear palsy Abducens palsy Ischemia Breakdown of Ischemia Berry aneurysm Cranial base inflammation/mass Miscellaneous** congenital lesion Ischemia Increased or decreased intracranial pressure Tentorial Cranial base inflammation/mass Miscellaneous** inflammation/mass Miscellaneous** *In order of estimated frequency **Brainstem demyelination; giant cell arteritis, radiation injury, nerve sheath tumor, carotidcavernous fistula, cavernous sinus aneurysm, dental anesthesia, migraine, Wernicke encephalopathy (abducens palsy). 27 Table 3. Mimickers of ocular motor nerve palsies Oculomotor palsy Trochlear palsy Abducens palsy Extraocular myopathy Oculomotor palsy Myasthenia gravis Myasthenia gravis Extraocular myopathy Extraocular myopathy Trochlear palsy Myasthenia gravis Spasm of near reflex Internuclear ophthal- Skew deviation moplegia Skew deviation _______________________________________________________________ Figure Legends Figure 1 Left oculomotor nerve palsy. Upper panel shows left upper lid ptosis. Lower panel shows exodeviated left eye in primary gaze position with dilated left pupil (center), absent left adduction (left), reduced left supraduction (top), reduced left infraduction (bottom), intact left abduction (right). 28 Figure 2. Left trochlear nerve palsy. Upper panel shows slight left hypertropia in primary gaze position (center), slight left hypertropia in right gaze (left) and normal alignment in left gaze (right). Lower panel shows left hypertropia in left head tilt (right) and apparently normal alignment on right head tilt (left). Figure 3 Left abducens nerve palsy. Center panel shows inwardly deviated left eye (esotropia); right panel shows that left eye does not abduct; left panel shows normal ductions in both eyes. 29 Flow Chart #1. Evaluation of Isolated Non-traumatic Oculomotor Palsy. *Preferred for nonpregnant adults. **Clipping of aneurysm may provide relatively improved recovery of oculomotor palsy (Reference 39). 30 Flow Chart #2. Evaluation of Isolated Non-traumatic Trochlear Palsy. Flow Chart #3. Evaluation of Isolated Non-traumatic Abducens Palsy. 31