Negative relative accommodation is an assessment of the highest capacity to reduce accommodation while keeping a sharp, distinct binocular vision. Accommodation is the process by which the eye varies its optical power to maintain focus or a clear image of a target as the object's distance changes. Usually, accommodation works like a reflex, including as a component of the accommodative- vergence reflex. However, an individual can also consciously control it. Vergence is the synchronous movement of the eyes in opposite sides to maintain or attain the single binocular vision. The vergence movements are connected to accommodation. When a person changes the focus of their eyes to look at something at a different distance, they automatically cause vergence and accommodation. It is also called the accommodative- convergence reflex.
Therefore, accommodation and vergence systems influence each other. It is the interplay between two systems that enable an individual to enjoy a clear single binocular sight. Accommodation plays a critical role in binocular vision through formations such as over accommodation, convergence accommodation, over convergence and accommodative convergence. The accommodation and vergence systems also affect refractive aberration and binocular vision disorders.
Also known as
The negative relative accommodation test can be done at the doctor's office or eye clinic by:
- An orthoptist
- An optometrist
- An ophthalmologist
The identification of functional ocular abnormalities relies on the clinical examination of accommodation and vergence and their interrelationship. NRA is a standard subjective procedure the doctor can use to evaluate a patient's ability to decrease accommodation relative to a near point vergence stipulation of 40 centimeters. It helps the doctor to detect accommodative and vergence anomalies.
Accommodative disorders interfere with the eyes' ability to focus on objects at varying distances leading to the lack of sharp retinal images. Accommodative disorders include:
- Accommodative infacility
- Spasm of accommodation
- Paralysis of accommodation
- Accommodative insufficiency
- Ill-sustained accommodation
Vergence abnormalities can cause disjunctive eye movements and affect the ability of the eyes to fixate and stabilize a retinal image. Vergence abnormalities include:
- Vertical phoria
- Basic exophoria
- Basic esophoria
- Divergence excess
- Convergence excess
- Vergence dysfunctions
- Divergence insufficiency
- Convergence insufficiency
- Fusional vergence dysfunction
Many factors can lead to the development of dysfunctional accommodative and vergence systems, including:
- Closed head trauma
- Slow accommodation
- Accommodative fatigue
- Accommodative adaptation
- Alteration in the visual environment
- Need to sustain increased accommodation for viewing targets at near
- Various drugs and systemic conditions such as diabetes, Graves's disease, Parkinson disease, myasthenia gravis, etc
The analysis of the impact of accommodation facility and accommodative response on relative accommodation while considering refractive aberrations offers doctors and researchers detailed information on the status of a patient's ocular system and binocular vision.
Preparation & Expectation
The negative relative accommodation test doesn't require any preparation.
The patient will sit facing a phoropter or refractor, which is a measuring instrument equipped with lenses of different power. The patient will look through the machine and focus on a Snellen chart at a distance of 20 feet. The doctor will instruct the patient to read aloud the smallest row of symbols or letters that they can see. He/she will shift the lenses on the machine until the patient can see clearly. The test is done on both eyes, one eye at a time. It will determine the patient's refraction error. The doctor will then ask the patient to look at a chart placed at about 40 centimeters from their eyes. He/she will add lenses in small increments (+0.25) until the patient reports that the vision has become blurry. The NRA value is the aggregate power of the lenses the doctor adds to reach that point when the patient’s vision is blurred.
High NRA values may indicate anomalies in the accommodation and vergence system. For example, an amount above +2.50 shows an over minus, which is latent hyperopia or uncorrected hyperopia. The patient is unable to focus on objects at a distance.
Where the results show abnormalities, the doctor may recommend treatment. Treatment and management may involve:
- Vision therapy
- Accommodative treatment to increase the speed, ease amplitude, and accuracy, of accommodative response
- Vergence therapy to improve sensorimotor fusion
- Prism therapy – horizontal prisms to do away with the symptoms of asthenopia and decrease fusional vergence pressure of vergence impairment. Vertical crystals to remove any vertical imbalance
- Lens therapy – plus lenses may help to decrease the motor pressure on either the accommodative or vergence systems
- Surgery – may be recommended to reduce the extent of a deviation
Risks & Complications
The negative relative accommodation test has no risks.