Effect of Phenylephrine on the Accommodative System

Abstract

Accommodation is controlled by the action of the ciliary muscle and mediated primarily by parasympathetic input through postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia. During accommodation the pupil constricts to increase the depth of focus of the eye and improve retinal image quality. Researchers have traditionally faced the challenge of measuring the accommodative properties of the eye through a small pupil and thus have relied on pharmacological agents to dilate the pupil. Achieving pupil dilation (mydriasis) without affecting the accommodative ability of the eye (cycloplegia) could be useful in many clinical and research contexts. Phenylephrine hydrochloride (PHCl) is a sympathomimetic agent that is used clinically to dilate the pupil. Nevertheless, first investigations suggested some loss of functional accommodation in the human eye after PHCl instillation. Subsequent studies, based on different measurement procedures, obtained contradictory conclusions, causing therefore an unexpected controversy that has been spread almost to the present days. This manuscript reviews and summarizes the main research studies that have been performed to analyze the effect of PHCl on the accommodative system and provides clear conclusions that could help clinicians know the real effects of PHCl on the accommodative system of the human eye.

Figure 1

Schematic diagram of both sympathetic and parasympathetic divisions from the autonomic nervous system, showing the main receptors and neurotransmitters involved in each case.

Figure 2

Anterior segment image of the human eye taken by Optical Coherence Tomography by means of the Visante Omni System (Carl Zeiss AG, Oberkochen, Germany). (a) Under myosis conditions (constriction of the pupil). (b) Under mydriatic conditions (dilation of the pupil).

Figure 3

Agonist versus antagonist behaviour. The substances naturally produced by our body act on cellular receptors. The agonist is not the natural substance, but it acts similarly occupying the receptors and activating them, imitating or even enhancing therefore the effect produced by the natural substance. On the contrary, an antagonist mimics the natural substance to take its place in the receptor, blocking thus the cellular activity.