Clinical Characteristics

Clinical characteristics

A 50 year-old Caucasian man was referred for the evaluation of dizziness and

“intermittent jumping” of the visual surround. His symptoms began approximately four year previously and included recurrent high-grade fevers (approximately 104° F), chills, diffuse joint pain, and maculopapular rash. These symptoms flared approximately three to four times a year. A year after the onset, he became dizzy with one of his typical flares. The patient described the sensation “as if someone was shaking a picture in front of me”. Since then he regularly experienced such a sensation every few seconds without a trigger. Each episode would last a second or two. He would also feel disoriented, lose his balance, and could fall. He also had high-pitched, constant tinnitus in both ears, but hearing was normal. For the last two years he occasionally had weakness of his right hand -- “It would occasionally twitch, or not work, when trying to write or to grab a spoon”. His family members noted forgetfulness and intermittent confusion.

When we evaluated the patient he was taking gabapentin 300 mg four times a day, clonazepam 1 mg at night, acetaminophen/hydrocodone 750/7.5 mg one to four times a day (as needed for pain), and acetaminophen/tramadol 37.5/2.25 mg as needed for pain. The patient had taken prednisone but stopped due to weight gain. He noticed that the tinnitus improved with gabapentin and the jumping vision improved with clonazepam. Methods

The patient was evaluated at The Johns Hopkins Hospital and agreed to participate in an investigation of the nature of his visual disturbance. Written informed consent was obtained. The study was approved by The Johns Hopkins Institutional

Review Board.

Experimental setup

Eye movements were recorded with the magnetic-field system [Robinson, 1963] using dual search coils (Skalar Medical, Delft, The Netherlands). During the experiment the patient’s head was stabilized with a bite-bar. The patient fixed his gaze upon a target that was projected straight ahead and 20 and 30 degrees to the right, left, up, and down. Eye movements were recorded both when looking at a target and in a completely dark room with no visual target. Raw search coil signals were sampled at

1000 Hz with 12 bit resolution and filtered in hardware (90-Hz low-pass Butterworth).

The data were further processed with custom written software written in the Matlab® environment (see Bergamin, et al., 2001 for details). Eye positions were represented as three dimensional rotation vectors in a head-fixed coordinate system. Rotation vectors were smoothed and eye positions and velocities were computed.

Analysis of saccadic oscillations

Data from each axis were processed separately. Saccadic oscillations were best characterized on eye velocity trace. Oscillations were separated interactively. Cycle- by-cycle analysis was performed in the following steps. The de-trended data were normalized with the mean velocity (i.e., normalized velocity = actual velocity – mean velocity). Normalization allowed the velocity to align on the abscissa with the peaks of the cycles remaining positive and the troughs negative. The point of intersection of the velocity data trace, moving from the negative value to the positive value with the abscissa was recorded. The ‘x’ co-ordinate of these values were called a positive zero crossing. The cycle duration was calculated from the distance between the two positive zero crossings. The inverse of the cycle duration yields the cycle frequency. Average of the peak and trough values of the cycle was considered as the peak velocity of the given cycle. Mean, median, standard deviation and 95% confidence intervals of the frequencies and peak velocities of all cycles are reported.

Analysis of eye drifts

For analysis of the lower-frequency torsional and vertical oscillations and of the torsional drifts, signals were differentiated, and smoothed using a sampling rate

(1000Hz) to compute eye velocity. A Savitzky-Golay filter (polynomial order: 3 and frame size: 5) smoothed the eye velocity signals.

Analysis of saccades

Saccades were interactively selected in the eye position trace. Saccadic amplitude, velocity, latency, and gain (actual/desired eye movement) were computed.

References

1. Robinson DA (1963) A method of measuring eye movement using a sclera

search coil in a magnetic field. IEEE Trans Biomed Eng 10: 137-145. 2. Bergamin O, Zee DS, Roberts DC, Landau K, Lasker AG, Straumann D (2001)

Three-dimensional Hess screen test with binocular dual search coils in a three-

field magnetic system. Invest Ophthalmol Vis Sci. 42:660-7.