Oculomotor schwannoma causing a progressive complete third-nerve palsy

Correspondence to Mr Ronak Ved, [email protected]

Background

Schwannomas make up ~8% of all intracranial tumours and are derived from peripheral nerve schwann cells. They are benign in nature; malignant change is unusual.¹ Of the cranial nerves, schwannomas arise in the vestibular nerve most commonly, followed by trigeminal, glossopharyngeal, vagal and facial nerves. Accessory, hypoglossal, oculomotor and abducens nerve schwannomas are rare.^{2 3} Schwannomas of the oculomotor nerve in particular are rarely isolated lesions; they are typically found alongside trigeminal nerve schwannomas as part of a hereditary tumour disease, such as neurofibromatosis type 2.^{2 3} There have only been 40 cases of oculomotor schwannoma described in the literature, and therefore their presenting features, and optimal treatment strategies, remain to be fully elucidated.³ These lesions can present with features of a complete oculomotor nerve palsy, which classically includes: a non-reactive, dilated pupil, a complete ptosis, and ophthalmoparesis of elevation and adduction. The ipsilateral eye may be observed to lie in abduction, slight depression and intorsion in primary gaze; this is often described as a ‘down and out’ position.

This constellation of signs is attributed to lesions which compress the oculomotor nerve; such pathology may compromise the parasympathetic fibres of the nerve, which supply the sphincter pupillae muscle. These parasympathetic fibres originate in the Edinger-Westphal nucleus of the midbrain and run on the superficial surface of the third nerve and are thus vulnerable to compressive lesions. As many of the underlying causes may be amenable to surgical treatment strategies, the presence of pupillary involvement in a third-nerve lesion is termed a ‘surgical third-nerve palsy’.³ The differential diagnosis for a surgical third-nerve palsy is wide; identifying features which may point to the possibility of individual diagnoses, such as schwannoma, is desirable. Table 1 outlines an anatomical schema for the differential diagnosis for a surgical third-nerve palsy.³ When presenting alongside a history of headache or pain, the presence of an acute complete third-nerve palsy is considered a surgical emergency, as a key differential diagnosis at this point is an enlarging intracerebral aneurysm until proven otherwise (table 1).

Table 1

A selected differential diagnosis for a complete third-nerve palsy, including clinical features which can help in differentiating the underlying pathologies⁵

Location on the course...