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N. E. Collins, M. AlKalbani, P. Kavanagh, A. Mahmud, G. Boyle, D. Coakley; Effects of Caffeine on Fixational Eye Movements. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2882.
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To investigate the acute effect of caffeine on fixational eye movements (ocular microtremor and microsaccades). Ocular microtremor (OMT), a minute eye movement present at all times in normal subjects, is a measure of arousal in terms of reticular-formation activity, and has been demonstrated as a potential measure of brainstem integrity in multiple sclerosis, coma and brainstem death. Drugs which alter arousal and alertness may affect OMT, and confound its measurement in the clinical setting. We investigated whether the common stimulant caffeine affects OMT, and whether it is a source of variability in studies of fixational microsaccades.
The effect of a single oral dose of caffeine on ocular microtremor was examined in a randomised double-blind crossover study. Twenty healthy subjects were studied on two occasions at least a week apart, after ingestion of 350ml caffeinated (180mg) and decaffeinated (< 10mg) coffee, in randomized order. OMT recordings were made using the piezoelectric strain-gauge technique, at baseline, and at 30 minutes and 90 minutes after coffee ingestion.
Compared with baseline, there was a small but significant increase in OMT peak frequency at 30 minutes after caffeine ingestion from 85.9Hz (SD 3.4Hz) to 87.9Hz (SD 3.3) (95% CI for difference between means: 1.0Hz, 3.0Hz; p < 0.001). OMT frequency remained slightly elevated at 90 minutes after caffeine ingestion. No effect was seen with decaffeinated coffee. There was also a trend towards fewer microsaccades following caffeine ingestion, from 49.6 microsaccades/min at baseline to 36.2 microsaccades/min at 30 minutes, (95% CI for difference between means: -3.4, 30.2), although this did not reach significance (p = 0.111).
The results demonstrate that the frequency of OMT is increased in response to caffeine. The increase in OMT frequency was relatively small, and not clinically significant in the context of previous studies of OMT. In studies of fixational eye movements where small differences in parameters are considered significant, caffeine should be taken into account as a potential source of variability.
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