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D. G. Szymanowicz, P. Thiagarajan, W. Green, K. J. Ciuffreda, D. Ludlam; Vergence Dynamics in Traumatic Brain Injury. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2891.
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© ARVO (1962-2015); The Authors (2016-present)
Recent retrospective studies have found abnormalities of static clinical vergence parameters in traumatic brain injury (TBI) patients. The purpose of the present study was to investigate the first-order symmetric vergence dynamics in TBI.
Non-strabismic TBI patients (n=10, mean age 30.2±5.01years) with near vision symptoms, and asymptomatic visually-normal (non-TBI) subjects (n=10, mean age 25.2±0.91years), were tested. Binocular horizontal position of the eyes was recorded objectively and continuously using the Power Refractor II, based on infra-red video photorefraction, with a sampling rate of 12.5Hz and resolution of < =0.5 degrees. The stimulus amplitude was 6.5º for both convergence and divergence. Targets comprised the red fixation LED located on the measuring head of the power refractor at 1m and a white LED placed at 0.3m, both aligned along the midline. Subjects altered bifixation between the targets every 3-4 seconds upon command for one minute. Three convergence and 3 divergence responses were obtained for analysis from the eye position traces for each subject. An exponential decay function was fit to the traces, and the response amplitudes and time constants of convergence and divergence were obtained. The peak velocities were derived from first-order differentiation of the exponential equation. The mean amplitude, time constant, and the peak velocity of the responses of the TBI and normal groups were compared.
Mean convergence response amplitude of the TBI group (6.4±0.3°) and the normal group (6.2±0.2°) was not significantly different (p=0.65). Similarly, mean divergence response amplitude of the TBI group (6.4±0.2°) and the normal group (6.5±0.2°) was not significantly different (p=0.77). However, there were significantly reduced (p<0.01) mean peak velocities and increased time constants for both convergence (15.5±0.94°/sec; 432.4±31ms) and divergence (15.6±0.96°/sec; 448.8±39.5ms) in the TBI group when compared to convergence (28.7±1.2°/sec; 221±9.6ms) and divergence (24.8±1.2°/sec; 273.4±19.1ms) in the visually-normal group, respectively.
The TBI group demonstrated significantly slowed dynamic vergence responses. Since the neural control for vergence involves numerous motor and premotor areas, it appears that multiple axonal pathways are susceptible to diffuse axonal injury from the TBI, resulting in slowed vergence responsivity. More specifically, we speculate it may compromise the firing rate of vergence burst cells that are related to vergence velocity. However, the similar response amplitudes in both groups suggest unaffected activity of vergence tonic cells related to vergence angle.
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