Accommodation-types, paralysis and accommodative spasm. Presbyopia
Accommodation is a dynamic process of increasing the refractive power of the eye. It is the ability of the eye to have clear image at different distances, shorter than the far point of clear vision. Accommodation depends mostly on the ability of the lens to change its refractive power. The exact mechanism of accommodation is not known but the principal fact is that accommodation is a feature of increase in the curvature of the lens.
Types of accommodation
- Tonic accommodation: it is due to tonus of ciliary muscle and is active in abscence of stimulus. The resting state of accommodation is not at infinity but rather at an intermediate distance
- Proximal accommodation: is induced by the awareness of the nearness of a target. This is independent of the actual dioptric stimulus
- Reflex accommodation: is an automatic adjustment response to blur which is made to maintain a clear and sharp retinal image
- Convergence accommodation: amount of accommodation stimulated or relaxed associated with convergence. The link between accommodation and convergence is known as accommodative convergence and is expressed clinically as AC/A ratio.
Assessment of accommodation:
- Dynamic retinoscopy
- Subjective measurement of accommodation amplitudes with for example RAF rule
- Facility of accommodation with lens flipper
Paralysis of accommodation: Cycloplegia refers to complete abscence of accommodation. Some of the causes are atropine, internal opthalmoplegia due to neuritis associated with diphtheria, syphilis, diabetes, alchoholism, cerebral or meningeal diseases. Complete third nerve paralysis due to intracranial or orbital causes. Systemic medications such as antihypertensives, and antidepressant. The clinical features are; blurred vision at near, photophobia, diplopia, micropsia (objects may appear smaller than they are due to a false sense of distance), enlarged pupils. Treatment; find cause,dark glasses are effective in reducing glare, convex lenses for near vision, if the paralysis is permanent.