Sixth Nerve Palsies

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Etiology and Systemic Associations Acquired sixth nerve palsies, whether isolated or not, are usually caused by tumors (especially glioma and medulloblastoma) and trauma (47%-62%)3,24,191,269,287,405 A significant number of cases are also due to inflammatory causes such as meningitis (including from Lyme disease), Gradenigo's syndrome,117 cerebellitis, and postviral sixth nerve palsy. The clinician is also faced with numerous other possible etiologies (Table 5-4).

Clinical Features and Assessment As previously mentioned, a lesion affecting the sixth nerve nucleus produces an ipsilateral horizontal gaze palsy. Injury to the nerve at any other location along its course results in absent or poor abduction of the ipsilateral eye (Fig. 5-7).

Of course, poor abduction is not specific to sixth nerve palsies and may also be caused by disorders of the neuromus-cular junction (e.g., myasthenia gravis), restrictions (e.g., medial orbital wall fractures with tissue entrapment), and inflammation (e.g., orbital myositis). The examiner considers and excludes these possibilities before establishing the diagnosis of sixth nerve palsy. If a congenital anomaly of innervation, such as Duane's syndrome, is clearly identified as the cause of abduction deficit, no further investigation of the eye movement abnormality is necessary.

Acute comitant esotropia can also follow head trauma (usually minor), febrile illness, migraine, or occlusion of an eye or may not be related to any obvious inciting cause.75,170,385,460 This condition is distinguishable on examination from a bilateral sixth nerve palsy. However, although an acute comitant esodeviation without accompanying signs is usually benign, it may in some cases be the harbinger of an intracranial tumor such as cerebellar astrocytoma or pontine glioma29,526 or other pathology such as a Chiari 1 malformation.517 Absence of symptoms or signs such as headaches, papilledema, or nystagmus may not rule out the possibility of an intracranial pathology. Therefore, a thorough ophthalmic examination is performed. MRI is

TABLE 5-4. Etiology of Infranuclear Sixth Nerve Palsy.

Location/signs

Etiologies

Fascicle

Ipsilateral Vllth nerve palsy, facial analgesia, loss of taste from anterior two thirds of tongue; peripheral deafness; Horner's syndrome, contralateral hemiparesis

Subarachnoid space

Papilledema; other cranial nerve palsies

Petrous apex Ipsilateral seventh nerve palsy; pain in eye or face; otitis media, leakage of blood or cerebrospinal fluid from ear; mastoid ecchymosis; papilledema Cavernous sinus/superior orbital fissure Ipsilateral Horner's syndrome; ipsilateral IIIrd, IVth, Vth cranial nerve involvement; proptosis; disc edema; orbital pain; conjunctival injection Orbit

Ipsilateral IIIrd, IVth, Vth cranial nerve involvement; proptosis; disc edema; orbital pain; conjunctival injection

Uncertain

Tumor; demyelination; hemorrhage; infarction

Meningitis; meningeal carcinomatosis; trauma; increased intracranial pressure causing downward pressure on brainstem; after lumbar puncture, shunt for hydrocephalus, spinal anesthesia, or halopelvic cervical traction; clivus tumor; cerebellopontine angle tumor; berry aneurysm; abducens neurinoma

Mastoiditis; thrombosis of inferior petrosal sinus; trauma with transverse fracture of temporal bone; persistent trigeminal artery, aneurysm, or arteriovenous malformation

Cavernous sinus thrombosis; carotid-cavernous fistula; tumor; internal carotid aneurysm

Tumor; pseudotumor

Transient abducens palsy of newborn; after febrile illness or immunization; migraine; toxic; idiopathic indicated if the esotropia is unresponsive to correction of refractive error, there is no history of flu-like illness, or no improvement is seen over the course of 1 to 4 weeks.

Natural History and Clinical Workup Newborns may demonstrate a transient sixth nerve palsy that is frequently unilateral and occasionally accompanied by a temporary ipsilateral seventh nerve palsy.53,267,291,400 Simple observa-

Chinese Eyes
FIGURE 5-7. Right sixth cranial nerve palsy. These photos show the limitation of abduction on attempted right gaze typical of a sixth cranial nerve palsy. Forced duction testing of this patient's right eye showed no restriction to abduction.

tion is generally sufficient because resolution typically occurs within 4 to 10 weeks.

Older infants and children may develop transient isolated sixth nerve palsies 1 to 3 weeks after nonspecific febrile or respiratory illnesses,267,405 after a specific viral illness such as varicella,350 after immunization,522 before mononucleosis,273 or without any obvious precipitating factor.435 Some of these palsies may recur, and the recurrences have no serious implica-tions.2,60,65,399,474,476 Again, aggressive investigation is not warranted, but two simple studies are advised: (1) a complete blood count with differential, which may show lymphocytosis as evidence of a recent viral infection, and (2) examination of the ears for otitis media. The parents are warned to observe for any new signs or symptoms. Careful reexamination at regular intervals is essential; deterioration or improvement in lateral rectus function provide important evidence for or against a progressive mass lesion. Most children in this group recover abducens function within 10 weeks, although a prolonged (9 months) palsy may rarely occur.267

Persistence, without improvement, or deterioration of an isolated sixth nerve palsy in a child beyond about 3 months requires an intensive neurological, neuroradiologic, and otolaryngologic evaluation. In adults, a substantial number of isolated sixth nerve palsies that last beyond 6 months are caused by potentially treatable, often slow-growing, tumors.111,159,426 In a Mayo Clinic series of 133 children with acquired sixth nerve paresis, 15 presented with an isolated sixth nerve palsy due to tumor.405 Of these, 12

developed additional neurological signs within a few weeks, whereas 3 patients took 2 to 3 months to develop additional signs. An additional problem is that a physician may not always be able to confirm that the sixth nerve palsy in a child is isolated. Therefore, if close follow-up to resolution of the palsy or paresis is not possible, neuroimaging is recommended.24

Treatment

Amblyopia prevention is always key in children younger than 7 to 9 years of age. Providing full hyperopic correction also relieves the demand for accommodation and thus decreases the chance of worsening esotropia.

Treatment options include botulinus toxin injection and surgery. One approach is to inject botulinus toxin into the antagonist medial rectus muscle to prevent tightening of the unopposed medial rectus,442,444 sometimes allowing binocular vision in primary position, while the palsy is resolving.218 Reducing medial rectus contracture with botulinus toxin injection may also improve a surgical result.302

Prognosis

Spontaneous recovery of abduction in childhood sixth nerve palsy or paresis is much less common than in adults. The rate of residual strabismus was found to be 66% in one study of any sixth nerve palsy or paresis in patients 7 years of age and younger, likely a result of permanent structural deficits without complete recovery in the setting of tumor and hydrocephalus-shunt malfunction as the most frequent etiologies. The rate of amblyopia in this study was 20%, thus highlighting the need for parent education and close follow-up.

The highest rates of spontaneous recovery have been reported in idiopathic (67%24), infectious (50%24), inflammatory (90%191), and traumatic (33%-50%24-191) cases.

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