Accommodation is the process through which the eyes change focus in order to adjust for an object’s distance from the viewer. This is done through a combination of pupil size adjustment and lens shape change. Visual accommodation helps ensure that we can maintain clear vision when looking at objects from varying distances. It occurs reflexively most of the time, without our thinking about it, but can also be performed consciously and deliberately.
Visual accommodation is used a near-infinite number of times throughout our day-to-day activities, such as when we look down at our phone (something we all probably ought to do less of!), then back up at whatever we’re supposed to be paying attention to, or when we look ahead on the road to figure out why traffic has slowed, then back to the fast-approaching bumper in front of us. Visual accommodation typically occurs so quickly (approximately 0.2 seconds) that we are not even aware of it, and is part of the accommodation reflex [1].
The accommodation reflex, also known as the accommodation-convergence reflex, the near reflex, or the near triad, is a three-part reflex that is responsible for allowing us to focus on objects up close. The reflex involves three main responses:
The accommodation reflex begins when an object appears out of focus on the retina, or when we are intentionally shifting our visual fixation to a near object. This information from the visual system travels along the optic nerve to the optic chiasm, through the optic tract to the lateral geniculate nucleus in the thalamus of the brain, where it is then relayed to the primary visual cortex via the optic radiation [2]. When this information arrives in the primary visual cortex, it is processed by the brain, which then initiates a response. Nerve impulses are sent from the primary visual cortex to the visual association cortex, then to the superior colliculus in the midbrain.
From here, the signal is relayed to the oculomotor and Edinger-Westphal nuclei of the oculomotor nerve (our third cranial nerve), in the midbrain. [2]. The oculomotor nerve then facilitates contraction of the medial rectus muscle, resulting in convergence of the eyes. At the same time, tThe Edinger-Westphal nucleus communicates with the ciliary ganglion to produce a contraction of the sphincter pupillae muscles leading to constriction of the pupil, and a contraction of the ciliary muscles resulting in a rounding and thickening of the lens [2].
Concussions and other forms of brain injury can lead to an alteration in our accommodative function, impairing our ability to focus our vision at different distances. This is referred to as accommodative dysfunction. It is estimated that accommodation dysfunction occurs in around 50% of patients after concussion injuries, making it all the more important to understand how to properly test for it [3]. Patients with accommodative dysfunction will commonly report difficulty focusing their vision at close distances, or shifting their focus between near objects and distant objects. They will also likely experience difficulty with reading including blurred vision, and report symptoms such as headache, eye strain, dizziness, nausea or fatigue with tasks that require near vision.
You can assess visual accommodation in your clinical setting using a relatively simple test that does not require any sophisticated, high-tech equipment. Having a convergence rule will make measuring accommodation much easier and allow you to assess it with a higher degree of accuracy, which can be particularly important when taking subsequent measurements to compare to your original ones. A convergence rule is a ruler-like stick with a card that can be slid along it, such that the card can be held in front of the patient’s face at varying distances from their eyes. Accommodation is measured in each eye separately, one eye at a time.
Normal accommodation is considered to be 10 cm for persons aged 30 or younger. After this, there is a gradual decline in accommodation with age. This occurs primarily because the lens of the eye becomes less flexible and thus harder to change shape, though some studies have suggested that possible reduced performance of the ciliary muscle may also play a role [4] [5]. By the age of 60, accommodation is typically measured somewhere around 1 meter, and does not generally decrease much further. Understanding this normal decline in the focusing power of the human eye is important, as your patient’s accommodation may be poorer than 10 cm but still normal for their age group.
It’s also important to ensure during testing that your patient is wearing any corrective eyewear such as glasses or contact lenses that they normally use, as otherwise their accommodation may appear impaired when it is in fact no different from their pre-injury baseline. You should also ask your patient if they have progressive or bifocal lenses, as they may need to change which part of the lens they are looking through depending on the target’s distance.
When accommodation is found to be abnormal, it is called accommodative dysfunction. There are several forms that accommodative dysfunction may take.
You may not be able to differentiate between all of these variations of accommodative dysfunction in your clinical practice, but exercises to work on rehabbing visual accommodation can nonetheless help to improve accommodative function [3].
Accommodative dysfunction can at times be treated in a clinical rehab setting, using different exercises to work on retraining the neurological pathway associated with visual accommodation. It’s important to note that any exercises aimed at addressing dysfunction of the visual system following a concussion are not doing so by “strengthening the eye muscles”, but rather retraining the brain and nervous system to use the eyes more efficiently.
Examples of exercises to address deficits in accommodation include ramp and step exercises. These types of exercises involve having your patient work on shifting or maintaining their focus on an object or objects as the distance of the object changes. In ramp exercises, the distance of the target object changes progressively while the patient attempts to maintain their focus on it. In step exercises, the patient is shifting their focus between more than one target, each at varying distances.
In cases where accommodation does not appear to improve with rehabilitative exercises, referral to a neuro-optometrist for a more specialized assessment of their visual system may be needed. Several studies have indicated that a more formalized, in-office vision rehabilitation program, such as those facilitated by a neuro-optometrist, produce superior outcomes than at-home exercises when compared to placebo [3].
Due to visual accommodation’s close relationship with convergence as part of the accommodation reflex, it is not uncommon for convergence insufficiency to be present alongside accommodative dysfunction. In fact, one study of 100 adolescents evaluated as part of a concussion program, identified that 46% of patients had more than one vision diagnosis, with convergence and accommodation being the two most common. Patients presenting with both convergence and accommodative dysfunctions will benefit from performing rehab activities that integrate both of these two functions, such as Brock String exercises [3].
Visual accommodation is an important and often reflexive function of the visual system that involves adjusting the eye’s focus based on the distance of the target from the viewer. It occurs when the ciliary muscles contract in each eye to change the shape of the eye’s lens, thickening and rounding the lens to focus on close objects, and flattening the lens to focus on far objects.
Accommodative function is commonly disrupted following concussion injuries, with patients typically experiencing blurred vision, difficulty with near work (particularly for sustained periods), or difficulty changing focus between objects at different distances. When performing activities involving accommodation, patients with accommodative dysfunction may experience symptoms of headache, dizziness, nausea, fatigue, or eye strain, among others.
Accommodative dysfunction can be treated with rehabilitative exercises in your clinical practice, though cases that do not respond to your treatment may require additional assessment and intervention from a neuro-optometrist. Understanding how to accurately test visual accommodation is critical to ensuring any deficits are identified and that your patient receives appropriate care to correct them.
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