Comparisons between pharmacologically and Edinger-Westphal-stimulated accommodation in rhesus monkeys

Accommodation results in increased lens thickness and lens surface curvatures. Previous studies suggest that lens biometric accommodative changes are different with pharmacological and voluntary accommodation. In this study, refractive and biometric changes during Edinger-Westphal (EW) and pharmacologically stimulated accommodation in rhesus monkeys were compared.

Accommodation was stimulated by an indwelling permanent electrode in the EW nucleus of the midbrain in one eye each of four rhesus monkeys. Dynamic refractive changes were measured with infrared photorefraction, and lens biometric changes were measured with high-resolution, continuous A-scan ultrasonography for increasing stimulus current amplitudes, including supramaximal current amplitudes. Accommodation was then stimulated pharmacologically and biometry was measured continuously for 30 minutes.

During EW-stimulated accommodation, lens surfaces move linearly with refraction, with an increase in lens thickness of 0.06 mm/D, an anterior movement of the anterior lens surface of 0.04 mm/D, and a posterior movement of the posterior lens surface of 0.02 mm/D. Peak velocity of accommodation (diopters per second) and lens thickness (in millimeters per second) increased with supramaximal stimulus currents, but without further increase in amplitude or total lens thickness. After carbachol stimulation, there was initially an anterior movement of the anterior lens surface and a posterior movement of the posterior lens surface; but by 30 minutes, there was an overall anterior shift of the lens.

Ocular biometric changes differ with EW and pharmacological stimulation of accommodation. Pharmacological stimulation results in a greater increase in lens thickness, an overall forward movement of the lens and a greater change in dioptric power.