From the Guidelines
The accommodation reflex is a complex process that enables the eye to focus on near objects, involving the coordination of pupillary constriction, convergence of the eyes, and lens thickening, with its mechanism and effectiveness influenced by various factors, including age and lens design. When considering the accommodation reflex, it is essential to understand its components and how they work together to achieve near vision. The reflex involves the contraction of the ciliary muscles, which relaxes tension on the zonular fibers attached to the lens, allowing the elastic lens to assume a more convex shape and increase its refractive power 1. Some key points to consider about the accommodation reflex include:
- The reflex is controlled by the parasympathetic nervous system via the oculomotor nerve (cranial nerve III)
- The accommodation reflex diminishes with age due to hardening of the lens, resulting in presbyopia
- Disorders affecting this reflex can include Adie's tonic pupil, where the pupillary component is impaired, or accommodative insufficiency, where the ability to focus on near objects is reduced
- Accommodative lenses have been designed to change position in the eye with near-focusing effort, offering an alternative to allow patients to see well at distance with a modest improvement in near and intermediate vision when compared with monofocal lenses 1. The mechanism of improved distance and intermediate vision with accommodative lenses may involve pseudoaccommodation (increased depth of focus) and possibly a small degree of lens-position shift, as noted in biometric studies of IOL shift in response to accommodative effort 1. Overall, understanding the accommodation reflex and its various components is crucial for developing effective treatments and interventions for disorders affecting near vision.
From the Research
Accommodation Reflex
The accommodation reflex is a complex process that involves the coordination of multiple components, including ciliary muscle constriction, convergence of the eyes, and pupil constriction, to focus on near objects 2.
Mechanism of Accommodation
The primary rule for near vision is ciliary muscle constriction, synchronised convergence of both eyes, and pupil constriction 2. Any weaknesses in these components could result in an accommodative spasm.
Factors Affecting Accommodation
Several factors can affect the accommodation reflex, including:
- Excessive close work
- Emotional distress
- Head injury
- Strabismus 2
- Age-related changes, such as loss of elasticity of the ciliary muscle posterior attachments, which may contribute to presbyopia 3
Measurement and Restoration of Accommodation
A clear understanding of the anatomy of accommodative structures, the mechanism of accommodation, and the causes of presbyopia is necessary to understand whether accommodation may be restored to the presbyopic eye 4. Theoretically, it may be possible to restore accommodation to a presbyopic eye with an artificial accommodative intraocular lens.
Relationship between Accommodation and Myopia
The accommodative mechanism has been hypothesised to play a role in myopia progression, with a lag in accommodative response potentially leading to axial elongation and myopia development 5. Effective treatments for myopia control, such as orthokeratology, multifocal lenses, and atropine eyedrops, appear to induce changes in the dynamic accommodative response.
Clinical Implications
The accommodation reflex is an important aspect of vision, and its dysfunction can lead to various ocular afflictions, such as presbyopia and myopia. Understanding the mechanism of accommodation and its relationship with other ocular functions is crucial for the development of effective treatments and management strategies 2, 3, 4, 5.