Pharmacological and physiological manipulation of ocular accommodation

Abstract

The primary aim of this thesis was to investigate in-vivo changes to ocular morphology during ocular accommodation, with particular attention to ciliary muscle changes, using pharmacological agents and physiological stimuli. To determine the optimum target for accommodation studies, accommodative responses to different stimuli were investigated. Insignificant differences in accommodative responses to a letter target and Maltese cross were found. Anterior Segment Optical Coherence Tomography (AS-OCT) and semi-automated software were used for in-vivo investigation of ciliary muscle morphology. As expected, anti-muscarinic agents reduced the accommodative amplitude, the greatest effect evident with cyclopentolate hydrochloride 1%, which was, in turn, associated with a reduced forward movement of the ciliary muscle, possibly due to the restricted movement of radial and longitudinal fibres. Pilocarpine nitrate 2% induced a contractile shortening and anterior thickening of the ciliary muscle in both pre-presbyopic and presbyopic eyes. However, pilocarpine does not seem to be a super-stimulus for accommodation. Comparison between cyclopentolate and pilocarpine ciliary muscle morphology further supported the concept that anterior movement of ciliary muscle mass is essential to elicit higher accommodative amplitudes. Ciliary muscle asymmetry was identified between horizontal and vertical meridians. The findings confirmed a longer ciliary muscle in axial elongated eyes, however, only within the temporal, nasal, and superior muscle quadrants. Nevertheless, during accommodation, anterior thickening and contractile shortening of the muscle was evident for all four quadrants. The effect of accommodation on posterior eye conformation revealed a homogenous elongation of the posterior pole, with the greatest expansion occurring along the inferior-nasal quadrant, rather than the visual axis. The possibility of a vitreous role in accommodation is discussed. The studies presented herein suggest a forward movement of ciliary muscle mass is necessary for accommodation. Further research is required to establish whether a mechanical restriction occurs to the muscle with increasing age, resulting in presbyopia. Moreover, investigations are required to explore the variation in ciliary muscle morphology in eyes with ametropia, as well as the likely role of the vitreous in accommodation.