Abducens nerve palsy

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Abducens nerve palsy
Classification and external resources
DiseasesDB 2868


Disease Entity

Strabismus/ocular misalignment

Disease

Abducens nerve palsy is the most common ocular motor paralysis. The abducens (sixth) cranial nerve controls the lateral rectus muscle, which abducts the eye. Abducens nerve palsy causes an esotropia due to the unopposed action of the antagonistic medial rectus muscle. The affected eye turns in toward the nose and is unable to abduct properly. The deviation is constant and is usually greater at distance fixation than at near. The esotropia is also worse when the patient is looking toward the affected side.

Etiology

In pediatrics

  • Congenital.
    • Palsies can occur, albeit rare. Can sometimes be associated with birth trauma and with neurological conditions such as hydrocephalus and cerebral palsy.
  • Acquired.
    • Acquired abducens nerve palsies in childhood can be due to neoplasm, trauma, inflammation, and idiopathic etiologies. Nontraumatic acquired sixth nerve palsies may be due to benign recurrent sixth nerve palsy, elevated intracranial pressure, or pontine gliomas. Benign isolated abducens nerve palsy can occur in childhood following ear or throat infections or viral illness.
  • Trauma, secondary to open or closed head injuries
    • The trauma causes indirect pressure on the nerve, which is very susceptible to trauma as it passes over the apex of the petrous temporal bone to the cavernous sinus . In addition, traumatic abducens nerve palsy can occur secondary to bruising and/or hemorrhage causing pressure on the nerve.Closed head trauma may cause elevated intracranial pressure and secondarily produce a sixth nerve palsy. In addition, the instance of sixth nerve palsy after apparently trivial head trauma raises suspicion about a possible underlying intracranial tumor.
  • Pontine glioma.
    • Brainstem gliomas are particularly common in the pediatric population, and more than 80% arise from the pons with the peak age of onset between 5-8 years of age. Presenting symptoms include ataxia, disturbance of gait, and unilateral or bilateral abducens nerve palsy.
  • Elevated intracranial pressure
    • Elevated intracranial pressure can result in downward displacement of the brainstem, causing stretching of the sixth cranial nerves which are tethered in Dorello’s canal. Elevated intracranial pressure in children can occur secondary to a variety of different causes., including shunt failure, pseudotumor cerebri, posterior fossa tumors, neurosurgical trauma, venous sinus thrombosis, meningitis, or Lyme disease.
  • Intracranial tumor
    • Posterior fossa tumors, such as pontine glioma, medulloblastoma, ependymoma, cystic cerebellar astrocytoma can produce unilateral or bilateral abducens nerve palsies in children. Skull base tumors (meningioma, chordoma, nasopharyngeal carcinoma, metastasis) predominate in the adult population. Abducens nerve palsy can also present as a postoperative complication after resection of posterior fossa tumors in the pediatric population.
  • Meningitis
    • Hanna and others found abducens nerve palsy in 16.5% of patients with acute bacterial meningitis. Cranial nerve palsies in this setting tend to be multiple and bilateral.

In adults

  • Microvascular ischemia.
  • Trauma.
  • Idiopathic.
  • Less likely:
    • Multiple sclerosis.
    • Neoplasm.
    • Stroke.
    • Sarcoidosis/vasculitis.
    • Increased intracranial pressure.
    • Giant cell arteritis.
  • Lesions. Lesions causing abducens nerve palsy are usually classified by the location of the lesion.
    • Fascicular.
      • Demyelination, vascular disease and metastatic tumors are likely causes of fascicular damage. Lesions in this area can cause Foville’s syndrome. Foville’s syndrome (damage to the pontine tegmentum) is classified by partial sixth nerve palsy, ipsilateral facial weakness, loss of taste in the anterior portion of the tongue, ipsilateral Horner’s syndrome, ipsilateral facial sensory loss and ipsilateral peripheral deafness. Lesions in the fascicular area can also cause Millard-Gubler syndrome, which is a result of damage to the ventral pons, is characterized by sixth nerve palsy and contralateral hemiplegia, and may or may not also have ipsilateral facial paralysis.
    • Peripheral.
      • Causes of peripheral nerve damage include closed head injury, compression, and bacterial infection of the inner ear. Localized compression can be caused by a primary pituitary tumor, craniopharyngioma or meningioma. Metastatic tumors and aneurysms involving the basilar artery can also cause an abducens nerve palsy.

Risk Factors

Inflammatory and microvascular conditions are risk factors for abducens nerve palsy. Some risk factors include multiple sclerosis, encephalitis, meningitis, cavernous sinus thrombosis, hypertension, hypercholesterolemia, aneurysm, diabetes, arteriosclerosis, and birth trauma.


Diagnosis

History

The history of the pattern of onset and associated symptoms can be very important in determining the etiology of an abducens nerve palsy. Sudden onset of abducens nerve palsy suggests a microvascular etiology. Conversely, slowly progressive onset suggests a compressive etiology. Subacute onset suggests a demyelinating process as a possible etiology. Associated pain suggests a microvascular etiology. If a patient has a lesion causing the abducens nerve palsy which affects other structures in the brain, other neurologic signs may be present (see Symptoms section).

Physical examination

All patients with presumed abducens nerve palsy need a complete ophthalmologic examination, including visual acuity, binocular function and stereopsis, motility evaluation, strabismus measurements at near, distance, and in the cardinal positions of gaze, measurement of fusional amplitudes, cycloplegic refraction, and evaluation of ocular structures in the anterior and posterior segments. Precise assessment of ductions and versions, as well as precise orthoptic measurements in lateral gazes are helpful in determining incomitance associated with abducens nerve palsy. Slow saccadic velocity in side gaze may be present and is helpful with the diagnosis.

Signs

Because the greatest motility deficit occurs on attempt to abduct the palsied eye, palpebral fissure widening upon abduction may be seen with maximal abduction effort. The patient may also present with a head turn toward the affected eye, to keep their eye opposite the field of gaze of the affected lateral rectus muscle and thereby avoid or minimize diplopia. 

Symptoms

Diplopia is the most common symptom. Patients will have horizontal uncrossed diplopia which is greater at distance than at near. The diplopia is also worse in the direction of the palsied muscle and gets better in the contralateral gaze (incomitant). In recent onset palsies, the deviation measures greater when the paretic eye is fixating and smaller when the nonparetic eye is fixing (primary and secondary deviations).

In cases of abducens nerve palsy due to raised intracranial pressure, patients may experience associated symptoms of headache, pain around the eyes, nausea, or vomiting. Associated pain suggests a microvascular etiology. If a patient has a lesion causing the abducens nerve palsy which affects other structures in the brain, other neurologic signs may be observed. If the etiology of the abducens nerve palsy is a brainstem lesion affecting the sixth cranial nerve fasciculus, there may be associated ipsilateral facial weakness, contralateral hemiparesis, or sensory abnormalities. If the abducens verve palsy presents together with other ipsilateral cranial nerve palsies, etiology could be a lesion involving the meninges, superior orbital fissure, orbital apex, or cavernous sinus. Presence of orbital signs or pulse synchronous tinnitus can suggest an arteriovenous fistula.

Clinical diagnosis

There is a limitation to abduction in the affected eye. Abducens nerve palsy causes an esotropia due to the unopposed action of the antagonistic medial rectus muscle. The affected eye turns in toward the nose and is unable to abduct properly. The deviation is constant and is usually greater at distance fixation than at near, and also greater when the patient is looking toward the affected side.

Diagnostic procedures

There is no universal consensus about diagnostic relevance of MRI in abducens nerve palsy. In general, if unilateral abducens nerve palsy presents acutely, MRI should routinely be performed, especially if there is no evidence of microvascular etology. Some clinicians prefer to routinely perform MRI in all patients with abducens nerve palsy, even with evidence of vasculopathy (Bendszus et al. 2001). Another study by Murchison and others in 2011 analyzed cost-effectiveness of MRI imaging as it related to lesions found and affected courses of treatment, and determined that it may not be medically necessary to perform MRI on every patient with an isolated cranial nerve palsy.

Laboratory test

  • Complete blood cell (CBC) count
  • Glucose levels
  • Glycosylated hemoglobin (HbA1C)
  • Erythrocyte sedimentation rate and/or C-reactive protein
  • Fluorescent treponemal antibody-absorption test, VDRL or RPR
  • Lyme titer
  • Glucose tolerance test
  • Antinuclear antibody test
  • Rheumatoid Factor test
  • MRI is indicated for the following:
    • Patients younger than 45 years
    • Associated pain or other neurologic abnormality
    • History of cancer
    • Bilateral sixth nerve palsy
    • Papilledema
    • Patients younger than 55 years with no vasculopathic history
    • In the event no marked improvement is seen or other nerves become involved
  • An LP can be considered if MRI results are negative.

Differential diagnosis

Differential diagnosis for abducens cranial nerve palsy includes Duane’s retraction syndrome (Types 1 and 3), congenital esotropia, thyroid eye disease, spasm of the near reflex, longstanding esotropia with medial rectus contracture, ocular neuromyotonia, and lateral rectus myositis. An abduction deficit can be produced by orbital disease, such as a blowout fracture with entrapment of the medial rectus muscle, or by neuromuscular disease such as ocular myasthenia gravis or Miller-Fisher syndrome. Clinical assessment for orbital, neuromuscular, and brainstem disease is the first step in evaluation for this condition, and after these are ruled out, an abducens nerve palsy can be diagnosed.

Management

General treatment

Treatment depends on etiology of the abducens nerve palsy. In general, underlying or systemic conditions are treated primarily. Most patients with a microvascular abducens nerve palsy are simply observed and usually recover within 3-6 months. Treatment for the diplopia associated with abducens nerve palsy can be managed with prisms, occlusion, botulinum toxin, or surgery. Occlusion using Bangerter filter or pirate patch can eliminate diplopia and confusion, prevent amblyopia or suppression in younger patients, and decrease the possibility of ipsilateral medial rectus contracture. Base-out Fresnel prisms can be used to help the patient maintain binocular single vision in the primary position, but are not usually useful due to the incomitance of the deviation. Botulinum toxin injections to the medial rectus of the affected eye is sometimes used to prevent secondary contraction of the medial rectus, or during transposition procedures to weaken the muscle not operated on. In general, surgical intervention is reserved for patients who have had stable orthoptic measurements for at least 3-6 months. 


Surgery

Strabismus surgery can be performed for persistent abducens nerve palsies that demonstrate stable measurements over 6 months period. Forced duction test is performed in the office or in operating room in order to assist with surgical planning. Often, a resection of the affected lateral rectus and recession of the ipsilateral medial rectus (recess/resect or “R and R” procedure) is performed. Alternatively, a resection of the affected lateral rectus with a recession of the contralateral medial rectus may be performed.

Various forms of transposition surgeries can be considered. Jensen, Hummelsheim, Augmented Hummelsheim with resections +/- Foster modifications, Knapp's procedure are some of the examples. Botulism toxin can also be used as a temporizing treatment.

Surgical follow up

Patients must be managed closely postoperatively, and any residual diplopia can be managed with prisms.

Complications

The most likely complication following surgical correction of abducens nerve palsy is the risk of over- or under-correction, which can be managed postoperatively with prisms.

Prognosis

The prognosis for sixth nerve palsy depends on the underlying etiology. Rush and Younge reported a recovery rate of 49.6% in 419 nonselected sixth nerve palsy cases, and a higher rate of 71% in 419 patients with diabetes mellitus, hypertension, or atherosclerosis.

Additional Resources

References

  1. Bendszus M, Beck A, Koltzenburg M, et al. MRI in isolated sixth nerve palsies. Neuroradiology 2001;43(9):742-745.
  2. Murchison AP, Gilbert ME, Savino PJ. Neuroimaging and Acute Ocular Motor Mononeuropathies. Arch Ophthalmol. 2011;129(3):301-305.
  3. Thurtell MJ, Tomsak RJ, Daroff RB. What do I do now? Neuro-ophthalmology.  Oxford, New York;  2011.
  4. Von Noorden GK. Binocular Vision and Ocular Motility: Theory and Management of Strabismus, Third ed. 
  5. Rush JA, Younge BR. Paralysis of cranial nerves III, IV, and VI. Arch Ophthalmol 1981;99:76–79