Subarachnoid space
As the VIth nerve passes through the subarachnoid space it lies adjacent to anterior inferior and posterior inferior cerebellar and basilar arteries and is therefore vulnerable to compression against the clivus. Typically palsies caused in this way will be associated with signs and symptoms of headache and/or a rise in ICP. Petrous apex
The nerve passes adjacent to the mastoid sinus and is vulnerable to mastoiditis, leading to inflammation of the meninges, which can give rise to Gradenigos syndrome. This condition results in a VIth nerve palsy with an associated reduction in hearing ipsilaterally, plus facial pain and paralysis, and photophobia. Similar symptoms can also occur secondary to petrous fractures or to nasopharyngeal tumours. Cavernous sinus/Superior orbital fissure
The nerve runs in the sinus body adjacent to the internal carotid artery and oculo-sympathetic fibres responsible for pupil control, thus, lesions here might be associated with pupillary dysfunctions such as Horners syndrome. In addition, III, IV, V1, and V2 involvement might also indicate a sinus lesion as all run toward the orbit in the sinus wall. Lesions in this area can arise as a result of vascular problems, inflammation, metastatic carcinomas and primary meningiomas. Orbit
The VIth nerves course is short and lesions in the orbit rarely give rise to isolated VIth nerve palsies, but more typically involve one or more of the other extraocular muscle groups. Diagnosis
Differential diagnoses
Differential diagnosis is rarely difficult in adults. Onset is typically sudden with symptoms of horizontal diplopia.
Management
The first aims of management should be to identify and treat the cause of the condition, where this is possible, and to relieve the patients symptoms, where present. In children, who rarely appreciate diplopia, the aim will be to maintain binocular vision and, thus, promote proper visual development.Thereafter, a period of observation of around 6 months is appropriate before any further intervention, as some palsies will recover without the need for surgery. Symptom relief and/or binocular vision maintenance
This is most commonly achieved through the use of fresnel prisms. These slim flexible plastic prisms can be attached to the patients glasses, or to plano glasses if the patient has no refractive error, and serve to compensate for the inward misalignment of the affected eye. Unfortunately, the prism only correct for a fixed degree of misalignment and, because the affected individuals degree of misalignment will vary depending upon their direction of gaze, they may still experience diplopia when looking to the affected side. The prisms are available in different strengths and the most appropriate one can be selected for each patient. However, in patients with large deviations, the thickness of the prism required may reduce vision so much that binocularity is not achievable. In such cases it may be more appropriate simply to occlude one eye temporarily.