Establishment of Normal Values for Objective Evaluation of Ocular Movements Using Videonystagmography

Abstract

This study aims to establish a set of normal values for the objective evaluation of ocular movements using videonystagmography (VNG). Thirty individuals aged between 18 and 50 years, with no history of vestibular symptoms, recurrent headaches, central nervous system (CNS) symptoms, or cochlear symptoms, were selected. The assessment of five types of ocular movements (saccades, pursuit, optokinetic tests, spontaneous nystagmus, and gaze tests) were conducted using VNG. Normal ranges were established for different parameters of saccades, including latency, precision, and velocity in both horizontal and vertical directions for each eye. Pursuit gain was calculated for each eye in all four directions. Optokinetic gain was determined when the stimulation screen moved in all four directions. Symmetric responses were observed in all subjects. No spontaneous nystagmus was observed with fixation, although two participants exhibited nystagmus upon removal of fixation. Gaze tests revealed no abnormalities.

Introduction

Videonystagmography (VNG) has emerged as a cornerstone diagnostic modality in the evaluation of balance disorders, offering detailed insights into the integrity of ocular motor function. However, the interpretation of VNG findings presents a considerable challenge, primarily attributable to the absence of established normative values for various parameters of ocular movements. Consequently, clinicians often encounter difficulties in distinguishing between physiological variations and pathological abnormalities in VNG tracings.

This study aims to address the dearth of normative data by conducting VNG testing in a cohort of healthy individuals devoid of vestibular symptoms or known risk factors for vestibular dysfunction. By elucidating the range of values for parameters for saccades, pursuit, gaze, and optokinetic systems in this asymptomatic population, we seek to provide clinicians with reference points for differentiating between normal and abnormal VNG findings.

Materials and Methods

Sample Selection

A total of 30 normal individuals (6 Males and 24 Females) were selected for this pilot study .

Participants

Employees of 3 different hospitals in the age range 18–50 years were recruited for the study. Permission was obtained from the hospital administration for conduct of the study. Written informed consent was taken from all those willing to participate in the study. The ratio of female employees was higher than that of male employees in all the 3 hospitals and hence majority of the study participants were females. .Exclusion criteria included any history of vestibular symptoms, headaches, other central nervous system symptoms, cochlear symptoms, or medication usage.

Materials Used

The study utilized a Cyclops Balance-eye Videonystagmography instrument, along with a 32-inch television for stimulation screen and a high-end laptop for data processing.

Data collection and methods

All 30 individuals underwent videonystagmography testing, which included assessments of five different ocular movements: saccades, smooth pursuit, optokinetic tests, spontaneous nystagmus in light, and gaze tests.

• Saccades: Testing involved 30 degree horizontal and vertical saccades at 0.3 Hz frequency for 30 s each. Parameters measured included latency, precision, and velocity for each eye separately.

• Smooth Pursuit: Smooth pursuit was assessed at 0.2 Hz for 30 s, with participants tracking a moving target 30 degree across the screen in horizontal and vertical directions.

• Optokinetic Test: Participants underwent optokinetic testing at 10º speed, with the screen moving in right to left, left to right, top to bottom and bottom to top directions for 30 s each.

• Spontaneous Nystagmus: Tests were conducted both in light and dark conditions for 30 s each.

• Gaze Tests: Gaze tests were performed with and without fixation, involving the movement of a target across the screen in all four directions.

Results and Discussion

Saccades

The study revealed consistent findings regarding saccadic movements. Both horizontal and vertical saccades exhibited similar patterns across the parameters studied. Latency, precision, and velocity were measured for both eyes.

1. Latency: This refers to the time delay between the appearance of a target stimulus and the initiation of the corresponding eye movement. It’s a measure of how quickly the eyes respond to a visual stimulus.

2. Precision: Precision in eye movements refers to the accuracy or consistency of the eye movements. It’s often measured as the percentage of the movement that occurs in a single step or fixation.

3. Velocity: Velocity measures how quickly the eyes move from one point to another, typically expressed in degrees of visual angle per second (°/s). It’s a measure of the speed of the eye movement. Our observations are noted in the table (See Table 1a–f).

Table 1.

Parametres of horizontal and vertical saccades

For horizontal saccades

1. Latency: The latency of saccades in both eyes falls within a similar range, with the right eye having a slightly narrower IQR (208–270 ms) compared to the left eye (204–276 ms). The median latency in the right eye is 247 ms, and in the left eye, it’s 250 ms.

2. Precision: Precision varies slightly between the eyes. The right eye has an IQR of 75–90 and a median of 86, while the left eye has an IQR of 80–91 and a median of 87. Both eyes exhibit relatively consistent precision.

3. Velocity: Velocity measurements for both eyes are in degrees per second (°/s). The right eye has an IQR of 324–400 °/s and a median of 356 °/s, while the left eye has an IQR of 336–400 °/s and a median of 362 °/s. The velocity of saccades in the left eye appears slightly higher on average.

For vertical saccades

1. Latency: The latency of vertical saccades in both eyes is similar, with the right eye having an IQR of 195–280 ms and a median of 240 ms, and the left eye having an IQR of 200–281 ms and a median of 251 ms.

2. Precision: The precision of vertical saccades is consistent between the eyes. Both eyes have an IQR of 75–90 and a median of 86.

3. Velocity: The velocity measurements for vertical saccades also show consistency. The right eye has an IQR of 258–331 °/s and a median of 299 °/s, while the left eye has an IQR of 254–327 °/s and a median of 290 °/s.

Pursuit Tracking

The gain in pursuit tracking is a measure of how accurately the eye movement follows the target movement. It reflects the ratio of the actual eye movement to the target movement during pursuit tracking.

A gain of 1 indicates perfect tracking, meaning the eye movement matches the target movement exactly. If the gain is less than 1, it suggests that the eye movement is slower than the target movement, indicating a lag or deficiency in tracking. Conversely, if the gain is greater than 1, it suggests that the eye movement is faster than the target movement, possibly indicating overshooting or overcompensation.

In our study horizontal pursuit demonstrated gains ranging from 0.96 to 0.98, while vertical pursuit ranged from 0.805 to 0.905 (See Table 2a–h). These findings suggest proficient tracking abilities in healthy subjects.

Table 2.

Pursuit gain

Optokinetic Tests

Optokinetic gain, reflecting the eye’s ability to match the movement of a screen, was evaluated. Horizontal optokinetic gain median ranged from 0.89 to 0 0.965, and vertical optokinetic gain median ranged from 0.815 to 0.905. The optokinetic responses were symmetric in all subjects (See Table 3a–h).

Table 3.

Optokinetic gain

Spontaneous nystagmus

Spontaneous nystagmus is an involuntary eye movement without an external stimulus. No nystagmus was detected during fixation, while spontaneous nystagmus without fixation was observed in two individuals, one displaying vertical and the other horizontal nystagmus.

Gaze Tests

Gaze tests revealed no abnormalities in any of the participants, whether with fixation or without fixation.

Literature Review A review of existing literature reveals observations on various parameters such as saccadic movements, smooth pursuit, and optokinetic responses. Herein, we present a synthesized overview of these findings for comprehensive understanding.

Saccadic Movements

Studies have consistently reported mean peak velocities of saccades, varying with target positions.

Mean peak velocity of saccades was 213°/s (± 29°/s), 352°/s (± 50°/s) and 455°/s (± 67°/s) to a target position 5°, 15°and 30° horizontally, respectively, and 208°/s (± 36°/s), 303°/s (± 50°/s) and 391°/s (± 71°/s) to a target position 5°, 10° and 20° vertically. The latency ranged from 160 to 190 ms [1].

Mean peak velocity of saccades in deg/sec was 332 ± 20,445 ± 41 and 507 ± 55 to a target position 5°, 15°and 35° horizontally [2].

Peak velocity for 30 deg horizontal saccades ranged from 325.9 to 485.2, latency 153.1 to 279.0 and accuracy 63.7 to 86.4 [3].

Peak velocity determinations distributed normally and ranged from 281 to 541 (mean and SD = 393 +/- 50) deg/sec for 20 deg horizontal saccades [4].

Peak velocity for 20 deg horizontal saccades ranged from 283 to 581 deg/sec and latency 129to 255(mean ± SD 192 ± 32). Optokinetic nystagmus(36 deg/sec) had gain ranging from 0.52 to 1.15(mean ± SD 0.84 ± 0.16). Smooth pursuit gain ranging from 0.65 to 1.07(mean ± SD 0.86 ± 0.10) [5].

Leigh and Zee stated that normal persons frequently show undershooting when performing saccades. The degree of dysmetria is usually relatively small, i.e. 10% for un-predictable visual targets [6].

The study of Baloh and Honrubia reported an average accuracy of 88%, with a SD of 6% when performing saccades. [7] Saccadic velocity ranges from 30 to 700 deg/sec for amplitude ranging from 0.5 to 40 degree, latency 150–250 ms and dysmetria up to 10% of amplitude [8].

Abel et al. (4) reported a latency for 20° saccades of 230 ms with a SD of 63 ms [9].

The reliability was fairly good for the amplitude/peak velocity relationship, was good for the precision, and was excellent for the amplitude/duration, the target when moving horizontally at constant angular velocity (Young,1971) [10].

Our results were consistent with the above studies. For horizontal saccades at target 30 deg latency was 208–270 ms ,204-276ms, precision 75–90, 80–91% and velocity 324–400 ,336–400 deg per sec for the right and left eye respectively .For vertical saccades latency was 195–280,200–281 and velocity 258–331,254–327 for the right and left eye. The precision was 75–90 for both eyes.

Pursuit tracking.

When the frequency is 0.3 Hz, 0.45 Hz, 0.60 Hz, the left and right horizontal gain is 0.92 ± 0.07/0.93 ± 0.07, 0.87 ± 0.08/0.88 ± 0.11, 0.79 ± 0.11/0.78 ± 0.13, respectively and the up and down vertical gain is 0.82 ± 0.16/0.80 ± 0.16, 0.78 ± 0.17/0.72 ± 0.15, 0.68 ± 0.20/0.61 ± 0.15 respectively in smooth tracking [11].

Smooth pursuit gain was 1.06 ± 0.18 ,1.04 ± 0.16,1.03 ± 0.16,0.99 ± 0.16 in the R eye and L eye for horizontal at 16 deg/s ,33 deg/s and 0.97 ± 0.24 ,0.97 ± 0.22 for vertical 8 deg/s respectively [12].

The maximum velocity gain of smooth pursuit was, on average, 0.98 − 0.75, gradually diminishing with increasing target velocities of 10–60 degrees s-1 [13].

The horizontal smooth pursuit gain ranged from 0.89 to0.98 and vertical from 0.52 to0.91 [14].

Smooth pursuit gain ranged from 0.7 to 1 in frequency range 1.6 to 0.2hz [15].

Our observation was right and left horizontal gain 0.96(SD 0.06),0.96(SD 0.05) for right eye and 0.96(SD 0.1),0.96(SD 0.14) for left eye.The up and down vertical gain was 0.88(SD 0.15),0.81(SD 0.14) for right eye and 0.9(SD0.16),0.8(SD 0.17) for left eye.

Optokinetic gain.

Statistical significance in the gain between upward and downward OKN was noted for stimulus velocities of 30–60 degrees/s (p < 0.01), but not for higher velocities, because vertical OKN saturated around 40–50 degrees/s [16].

There is strong evidence that horizontal OKN is symmetrical in normal healthy adults. Vertical on the other hand is less well understood, although there is a belief that vertical OKN is asymmetrical with an upward preference [17].

The vertical optokinetic responses, in particular to upward-moving stimuli, were less well sustained at velocities above 30 degrees s-1 than the horizontal responses [18].

The best OK responses were obtained using stripes with lower spatial frequencies and lower stripe speeds (0.4 cyc/deg at 10 deg/s) [19].

In our study at 10 deg/s the gain values were less in vertical compared to horizontal and all the responses were symmetrical.

Limitations and Future Directions

While this study provides valuable information about oculomotor function in healthy individuals, the sample size was relatively small, and further studies with larger cohorts may provide additional insights. Additionally, longitudinal studies could explore age-related changes in oculomotor function. Future research may also investigate oculomotor responses in diverse populations and under various experimental conditions.

Conclusion

This study is an attempt to provide insights into the nuanced characteristics of oculomotor behaviour in healthy individuals. It will be helpful in assessing oculomotor function and identifying deviations indicative of pathological conditions. Continued research in this field will enable to refine our understanding of oculomotor mechanisms and enhance diagnostic approaches for oculomotor disorders.

Declarations

Conflict of Interest

The authors declare no conflicts of interest related to this study.