Magnetic Oculomotor Prosthetics for Acquired Nystagmus

Abstract

Purpose: Acquired nystagmus, a highly symptomatic consequence of damage to the substrates of oculomotor control, often is resistant to pharmacotherapy. Although heterogeneous in its neural cause, its expression is unified at the effector-the eye muscles themselves-where physical damping of the oscillation offers an alternative approach. Because direct surgical fixation would immobilize the globe, action at a distance is required to damp the oscillation at the point of fixation, allowing unhindered gaze shifts at other times. Implementing this idea magnetically, herein we describe the successful implantation of a novel magnetic oculomotor prosthesis in a patient.

Design: Case report of a pilot, experimental intervention.

Participant: A 49-year-old man with longstanding, medication-resistant, upbeat nystagmus resulting from a paraneoplastic syndrome caused by stage 2A, grade I, nodular sclerosing Hodgkin's lymphoma.

Methods: We designed a 2-part, titanium-encased, rare-earth magnet oculomotor prosthesis, powered to damp nystagmus without interfering with the larger forces involved in saccades. Its damping effects were confirmed when applied externally. We proceeded to implant the device in the patient, comparing visual functions and high-resolution oculography before and after implantation and monitoring the patient for more than 4 years after surgery.

Main outcome measures: We recorded Snellen visual acuity before and after intervention, as well as the amplitude, drift velocity, frequency, and intensity of the nystagmus in each eye.

Results: The patient reported a clinically significant improvement of 1 line of Snellen acuity (from 6/9 bilaterally to 6/6 on the left and 6/5-2 on the right), reflecting an objectively measured reduction in the amplitude, drift velocity, frequency, and intensity of the nystagmus. These improvements were maintained throughout a follow-up of 4 years and enabled him to return to paid employment.

Conclusions: This work opens a new field of implantable therapeutic devices-oculomotor prosthetics-designed to modify eye movements dynamically by physical means in cases where a purely neural approach is ineffective. Applied to acquired nystagmus refractory to all other interventions, it is shown successfully to damp pathologic eye oscillations while allowing normal saccadic shifts of gaze.

Figure 1

Photographs of right eye obtained during surgery showing: (A) the exposed inferior rectus muscle (long arrow) with a squint hook retracting the muscle at its insertion to the globe (short arrow) and (B) the positioned orbital magnet fixed to the orbital floor (long arrow) and the lower lid being retracted with a Desmarres retractor (short arrow).

Figure 2

A, Orbital computer tomographic image showing the position of the right implant in the coronal plane. Note that the space between the 2 components is not visible because of artefact. B, Lateral plain radiograph showing the position of the right implant in the sagittal plane in 3 positions of gaze: neutral, upgaze, and downgaze. Note the movement of the eye component relative to the orbital component.

Figure 3

Example oculomotor trace before (Pre) and after (Post) surgery. A sample of eye movement amplitude during attempted fixation, indexed along the vertical displacement and plotted before (black) and after (red) bilateral implantation. The trace is derived from the left eye in a position of moderate damping: 15° downgaze in the vertical midline. Note the obvious fall in amplitude.

Figure 4

Spider plots of the nystagmus parameters in the left eye before and after bilateral surgery. Each of the panels corresponding to the 9 positions of evaluated gaze shows the median amplitude, frequency, presaccadic velocity, and intensity of the nystagmus in the left eye at that position before (Pre; solid black line) and after (Post; solid red line) bilateral surgery. The dotted lines show ±1 standard error of the median. The significance (P) values at each corner are derived asymptotically from a 2-sample Kolmogorov-Smirnov test for a difference in the paired distributions. The axes ranges are set to the maximum across eyes and conditions. On each of the 4 axes, a smaller value corresponds to a decrease in the corresponding nystagmus parameter: where the resultant diamond shape is smaller after implantation, the nystagmus is improved objectively. Note that there is generally a marked improvement in the parameters, especially in the downgaze positions as observed clinically.

Figure 5

Spider plots of the nystagmus parameters in the right eye before and after bilateral surgery. Each of the panels corresponding to the 9 positions of evaluated gaze shows the median amplitude, frequency, presaccadic velocity, and intensity of the nystagmus in the right eye at that position before (solid black line) and after (solid red line) bilateral surgery. The dotted lines show ±1 standard error of the median. The significance (P) values at each corner are derived asymptotically from a 2-sample Kolmogorov-Smirnov test for a difference in the paired distributions. The axes ranges are set to the maximum across eyes and conditions. On each of the 4 axes, a smaller value corresponds to a decrease in the corresponding nystagmus parameter: where the resultant diamond shape is smaller after implantation, the nystagmus is improved objectively. Note that there is generally a marked improvement in the parameters, especially in the downgaze positions as observed clinically.

Figure 6

Visualization of the impact on fixation of the implantation. Plotted for each eye before and after surgery are fits of the spatial probability density functions of the point of gaze during fixation at the maximally damped position for each eye. The spatial scale is in degrees, identical in the x- and y-planes. The narrower the distribution is, the more stable the fixation is. Note the apparent improvement in the stability of fixation reflecting the change in nystagmus parameters shown in Figures 3 and 4.