Bilateral cranial nerve 6 palsy in a patient with multiple sclerosis and vitamin D-dependent rickets

The development of multiple sclerosis (MS) is multifactorial. Elevated levels of vitamin D may lower the risk and reduce relapses by immunomodulatory mechanisms. Conversely, vitamin D-dependent rickets (VDDR), an inheritable form of rickets secondary to impairment in vitamin D synthesis or action, may increase MS risk. This has been described in three patients with VDDR type 1A. Here, we present a patient with VDDR type 2 - unclear if type 2A or 2B based on historical genetic testing - who subsequently developed MS. She presented with 8 weeks of binocular horizontal diplopia and was found to have 8 prism dioptres of esotropia in primary gaze and a mild limitation of abduction in both eyes. Radiological workup was consistent with MS demyelination. She was started on solumedrol infusions, with full resolution of the esotropia and abduction deficits. She has since been transitioned to ocrelizumab with vitamin D supplementation and has not had a relapse to date. It is important to consider MS in patients genetically predisposed to low vitamin D levels or functional impairment, as with VDDR. Vitamin D supplementation can achieve remission in some forms of VDDR, and its role in MS prevention in these patients should be considered. In patients with type 2A or 2B VDDR, who have impairment in receptor function, additional treatment modalities require investigation. Lastly, demyelination is a rare cause of bilateral cranial nerve 6 palsy. This case illustrates the importance of considering MS in cranial nerve palsies, particularly in patients with vitamin D deficiencies or functional impairment.