Pendular, Gaze-Evoked and Abducting Nystagmus in MS

This is a 40-year-old woman with a history of multiple sclerosis who presented for oscillopsia. On examination, she had bilateral internuclear ophthalmoplegia (INO-adduction lag OU and abducting nystagmus OU), with a corresponding exotropia that increased in right and left gaze. She also had horizontal pendular nystagmus, most notable in primary gaze and with a brief pause with blinking and after saccades (typical findings with pendular nystagmus). There was evidence of bilateral optic neuropathy with dyschromatopsia, pallor, and 20/100 best corrected acuities OU, subnormal in part due to optic nerve disease and in part due to her nystagmus. While delay in visual inputs (due to optic nerve involvement) is one theory for pendular nystagmus in MS (could explain why nystagmus is usually more prominent in the worse-seeing eye), in most patients, instability of the neural integrators is likely to play a significant role in the genesis of pendular nystagmus. This patient also had gaze-evoked nystagmus (GEN) vertically and horizontally, which is related to dysfunction of the neural integrators (interstitial nucleus of Cajal [INC] for vertical/torsional movements; nucleus prepositus hypoglossi and medial vestibular nucleus for horizontal movements; cerebellum [especially flocculus/paraflocculus] or its connections when GEN is in all directions). Due to the superimposed horizontal pendular and abducting nystagmus, her horizontal GEN was best appreciated in the adducting eye. Vertical GEN (especially upbeat in upgaze) is common with bilateral INOs, either related to damage involving vertical gaze holding pathways that travel through the medial longitudinal fasciculi, or related to involvement of the adjacent paramedian tracts, which relay eye position signals from INC to cerebellar flocculus. Further reading: Nerrant and Tilikete. Ocular motor manifestations of multiple sclerosis. Journal of Neuro-Ophthalmology 2017;37:332-340.