Intracranial dermoid cyst presenting as an isolated fourth nerve palsy

Abstract

Sirs: A 68-year-old man presented with 4 months of painless vertical diplopia on left gaze. Examination revealed a right fourth nerve palsy, which was confirmed by the Bielschowsky head tilt test and demonstrated on the Hess chart. There was a normal vertical fusion range, no corrective head posture or further neurological abnormality detected. Systemic examination found borderline hypertension (145/ 91 mmHg) and a normal fasting glucose. He denied any history of trauma. His only medical history was treatment for dyslipidemia. MRI scan revealed a 1–2 cm strongly hyperintense welldefined rounded mass on T1and T2-weighted images (Figs. 1, 2). The lipid-containing mass was applied to the dorsum of the tectum and left of the midline. Its appearance was hypointense on the fat-suppressed pulse sequence images (Fig. 3). These findings are characteristic of dermoid cysts. On consultation with the neurosurgical team, the decision was made to observe this intracranial dermoid cyst. At 16 months follow-up there has been no clinical progression. He remains under neuro-ophthalmic review. A space-occupying lesion is an uncommon cause of any isolated pupil sparing third, fourth or sixth cranial nerve palsy. A presumed microvascular palsy is a common aetiology and the majority of these resolve within 6 months. It has been recommended that sudden onset and isolated cranial nerve palsy can be observed, using neuroimaging if there is an insidious onset, progression (i.e. is no longer isolated) or non-resolution [1, 2]. In the case presented the onset of vertical diplopia was over the period of 4 months, and hence due to the insidious onset was scanned urgently. The radiological differential diagnosis of the hyperintense mass on T1-weighted images (Fig. 2) would include haematomas, aneurysms and lipid rich masses. High signal intensity on T1-weighted images and a hypointense T2 appearance, in this location, would be demonstrated by a dermoid cyst, a thrombosed berry aneurysm, an intracranial tetratoma or a quadrigeminal cistern lipoma. Fat suppression and inversion recovery sequences are therefore helpful to narrow the differential diagnosis and prove the mass is lipid containing (Fig. 3) [3]. Further image characteristics, such as chemical shift artefact, can differentiate between a dermoid cyst and a lipoma. Dermoids are typically well-circumscribed lesions that can contain lipid metabolites, hair and hair follicles, calcifications, sebaceous and sweat glands and decomposed epithelial cells containing cholesterol and keratin [4]. The composition determines their appearance: a typical high signal intensity on T1-weighted images, due to the high fat content; and a heterogeneous signal on T2-weighted sequences, due to the mixed content of the lesion [3]. Dermoid cysts are rare, congenital, non-neoplastic lesions that account for 0.7–1.8% of all intracranial tumours, with the posterior cranial fossa being the commonest location [4]. Supratentorial lesions have been reported presenting as an isolated third nerve palsy and other symptoms, with infratentorial dermoids causing sixth nerve palsy and cerebellar signs [5]. To our knowledge, an intracranial dermoid presenting as an isolated trochlear nerve palsy has not been described. R. Tailor S. P. Mollan (&) M. A. Burdon Birmingham and Midland Eye Centre, City Hospital, Dudley Road, Birmingham B18 7QH, UK e-mail: soozmollan@doctors.org.uk