Accommodative Insufficiency

Excerpt

As an optical system, the eyes function in line with the basic principles of refraction. Visual clarity depends on the ability to bring incident light rays to a point focus at the fovea centralis. Schematically, the cornea and crystalline lens provide the requisite total dioptric power (about +60 diopters; the cornea makes up about + 40 to + 48 diopters).

Accommodation is the adaptative faculty that enables clear vision with subjective variation in the linear distance of visual targets. This is accomplished by the autonomic adjustment of crystalline lens dioptric powers to provide focus on the presented stimuli. The main stimulus is 'blur.'

The focusing powers of the crystalline lens are directly proportional to the increase or decrease in lens convexity. At the near point of accommodation, the ciliary muscles contract and relax zonular tension, thus enabling increased convexity and focusing power. For the far point of accommodation, the ciliary muscles relax and exert greater tension on the zonules (of Zinn), thus reducing lenticular convexity.

Under typical conditions, the healthy eye responds to near-point (linear) stimuli by activating the near triad, which includes accommodation, convergence, and miosis. These mechanisms are synergistically engaged to enable single, clear binocular vision.

Inhibition in one of these reflexes can most likely lead to dysfunction of accommodative response. Concerning the near reflexes, accommodation and convergence are most closely intertwined. This interrelationship can be reported as the 'accommodative convergence to accommodation (AC/A) ratio. Depending on the underlying accommodative dysfunction, the ac/a ratio may be either low, normal, or high.

The accommodative mechanisms can be subject to functional anomalies in maintaining visual clarity. Based on the Duke-Elder classification, these include:

1. Accommodative insufficiency (AI)

2. Accommodative spasm

3. Accommodative inertia

4. Ill-sustained accommodation

5. Accommodative infacility

AI is a functional vision anomaly characterized by the inability to sustain focus at near. With this condition, the accommodative response is often suboptimal compared to expected normal ranges.