Effect of dexmedetomidine, an alpha2-adrenoceptor agonist, on human pupillary reflexes during general anaesthesia

Abstract

Aims: To test the hypothesis that the alpha2-adrenergic agonist, dexmedetomidine, dilates the pupil and does not alter the pupillary light reflex of anaesthetized patients.

Methods: Eight volunteers were administered general anaesthesia with propofol, nitrous oxide and alfentanil. One hour and 25 min after induction of anaesthesia, a 45 min infusion of dexmedetomidine was begun, targeting a plasma concentration of 0.6 ng x ml(-1). Pupil size, pupillary light reflex amplitude, light reflex recovery time, and reflex dilation were measured before and during dexmedetomidine infusion.

Results: Dexmedetomidine produced no change in pupil size and light reflex recovery time, increased the light reflex from 0.30 +/- 0.14 to 0.37 +/- 0.12 mm and significantly reduced pupillary reflex dilation by 72 +/- 62%.

Conclusions: These pupillary effects of dexmedetomidine in humans are difficult to reconcile with the findings obtained in cats and rats that have demonstrated a direct inhibitory effect of alpha2-adrenergic agonists on the pupilloconstrictor nucleus. The increase in the magnitude of the light reflex in response to dexmedetomidine does not necessarily involve an anxiolytic mechanism.

Figures

Figure 1

Pupil size (a) and light reflex amplitude (b) for all volunteers for the duration of the study. Dexmedetomidine infusion to target a plasma concentration of 0.6 ng ml−1 was started at time 0. Shaded areas show the standard deviations. Statistical analysis shown in Table 1.

Figure 2

(a) Averaged scans for all volunteers demonstrating the light reflex before (•) and after (○) dexmedetomidine. (b) The difference between the scans before and after dexmedetomidine. This difference represents the added light reflex brought about by dexmedetomidine. The increase in the light reflex is limited to the first 2 s and does not involve the recovery portion of the reflex. The light stimulus was presented during the first 0.5 s of each scan.

Figure 3

Reduction of pupillary reflex dilation by dexmedetomidine. Reflex dilation is shown before (top) and after (bottom) dexmedetomidine. The noxious stimulus was delivered for the first 3 s of each scan. To eliminate the effect of the light stimulus on the dilation produced by the noxious stimulus, the records were obtained by subtracting the scan during the noxious stimulus from the scan immediately preceding that scan. Scans are the averaged scans following the noxious stimulus for all eight volunteers. Statistical analysis shown in Table 1.