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Sixth Nerve Palsy

Definition

Cranial nerve six supplies the lateral rectus muscle allowing for outward (abduction) eye movement. A sixth nerve palsy, also known as abducens nerve palsy, is a neurological defect resulting from an impaired sixth nerve or the nucleus that controls it. This may result in horizontal double vision (diplopia) with in turning of the eye and decreased lateral movement.

Symptoms

For all intensive purposes causes of abducens nerve palsy can be classified as congenital or acquired. Isolated congenital sixth nerve palsy is quite uncommon. If congenital the usual presentation is accompanied by other cranial nerve deficits as seen with Duane's retraction or Moebius syndromes. Strabismus, commonly known as "lazy eye," may mimic the appearance of abducens nerve palsy and may go undetected until adulthood because of compensatory mechanisms allowing for alignment of the eyes when focusing. Abduction deficits may also result from myasthenia gravis, thyroid eye disease, inflammation and orbital fractures which imitate sixth nerve palsies.

A myriad of causes resulting in abducens nerve palsies have been reported. In order to better differentiate these one must take into account the patient's age and underlying illnesses. In children trauma and tumors were reported as the most common causes. Therefore if no trauma has occurred one must consider a tumor of the central nervous system in the pediatric population. Other causes include idiopathic intracranial hypertension, inflammation following viral illness or immunization, multiple sclerosis, fulminant ear infections, Arnold-Chiari malformations and meningitis.

New onset palsies in adults can stem from myasthenia gravis, diabetes, meningitis, microvascular disease (atherosclerotic vascular disease) or giant cell arteritis (arterial inflammation). Other causes include Lyme disease, syphilis, cancers, autoimmune disorders, central nervous system tumors, and vitamin deficiencies.

Children may be found to have head tilt or in-turning of the affected eye, with reduction of outward gaze. They will very rarely complain of double vision, while adults may describe two images, side by side (horizontal diplopia), which are furthest apart when looking towards the affected eye. Covering of one eye, no matter which one is covered, and gazing away from the affected eye will resolve their diplopia. Patients may also note muscle weakness, possibly heralding myasthenia gravis, or headache and jaw pain, raising the possibility of giant cell arteritis.

Optic nerve swelling or jumpy eye movements (nystagmus) may occur at any age and warrants immediate work-up for a central nervous system tumor.

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