MRI in isolated sixth nerve palsies

In previous studies the origin of the majority of isolated sixth nerve pals ies was not clear or was ascribed to vascular disease. Our purpose was dete rmine how frequently a causative lesion was demonstrated on MRI in patients with an acute unilateral sixth nerve palsy. We performed a prospective stu dy of 43 patients using a standardised protocol. In 27 patients (63 %) a le sion was identified on the initial MRI relevant to the sixth nerve palsy; 2 1 (49 %) were found to have a tumour or tumour-like lesion; the frequency o f presumed vasculopathy in this group was 15 %. There were 16 patients (37 %) with an initially normal MRI, of whom 10 (62 %) had a history of vasculo pathy, a significantly different proportion from the group of patients with a visible causative lesion. MRI after 3-6 months was normal in all patient s with a normal initial MRI. We suggest that MRI should routinely be perfor med in patients presenting with an acute sixth nerve palsy, even those with evidence of a vasculopathy. If the symptoms regress spontaneously and ther e is a history of vasculopathy, follow-up MRI is not necessary.