Positive relative accommodation is an evaluation of the highest capability to activate accommodation while keeping a clear, distinct binocular vision. Accommodation is the operation by which the eye shifts its optical power to keep focus or a sharp image of an object at all distances. It denotes the change in the eye lens' refractive power. Positive accommodation allows a person to see near objects clearly by reducing the focal length of the lens. Relaxation of accommodation or negative accommodation facilitates the viewing of distant objects by increasing the focal length. The vergence angle/vergence is the angle formed by the lines of sight of the two eyes in the parallel axis. Convergence eye movement is an expansion in a vergence angle and happens when looking at a near object with both eyes. Divergence eye movement is a reduction in the vergence angle.
Under normal circumstances, the accommodation and vergence systems work simultaneously. Triggering the accommodation system elicits a reaction from the vergence system through accommodative-vergence network and stimulating the vergence system draws an accommodation reaction through the vergence-accommodation network. The networks are necessary to make sure the responses of the accommodation and vergence systems are synchronized. The interplay creates clear retinal images, synthesized in the brain to produce a sharp binocular picture. The two systems also influence refractive errors and binocular vision abnormalities.
Also Known As
The positive relative accommodation test can be conducted at the eye clinic or doctor's office by:
- An orthoptist
- An optometrist
- An ophthalmologist
The detection of functional ocular anomalies relies on the clinical evaluation of accommodation and vergence and their interdependence. PRA is a standard subjective evaluation the doctor uses to examine a patient's ability to decrease accommodation relative to a near point vergence. Analyzing the accommodation variations in a stable vergence system can help the specialist to better understand the accommodation facility and roles of these two systems. It can assist the doctor in identifying accommodative and vergence abnormalities.
Accommodative abnormalities can affect the ability of the eyes to capture clear images at all distances. Accommodative anomalies include:
- Accommodative insufficiency
- Paralysis of accommodation
- Ill-sustained accommodation
- Spasm of accommodation
- Accommodative infacility
The accommodative system can be affected by a range of factors, including:
- Certain drugs and systemic disorders like myasthenia gravis, diabetes, etc.
- Need to maintain an increased accommodation for looking targets at near
- Accommodative adaptation
- Accommodative fatigue
- Slow accommodation
Vergence disorders can lead to disjunctive eye movements and impact the ability of the eyes to focus and create a steady retinal image. Vergence disorders include:
- Fusional vergence dysfunction
- Convergence insufficiency
- Divergence insufficiency
- Vergence dysfunctions
- Convergence excess
- Divergence excess
- Basic exophoria
- Basic esophoria
- Vertical phoria
Many factors can lead to the impairment of the vergence systems, including:
- Closed head trauma
- Change in the visual environment
- Systemic conditions such as Alzheimer's disease, Graves's disease, Parkinson disease, myasthenia gravis, etc.
The examination of the effects of the accommodation facility and the amplitude of accommodation on relative accommodation while taking into account the refractive errors gives doctors critical information about the status of the visual system and binocular vision of a patient. PRA can also serve as an indirect analysis of fusional vergence.
Preparation & Expectation
The positive relative accommodation test is a simple exam that doesn't require any preparation.
The patient will sit in front of a refractor or phoropter. It is a machine fitted with lenses of varying power. The doctor will ask the patient to look through the instrument and focus on a Snellen chart placed about 20 feet away. He/she will require the patient to read aloud the smallest row of letters that they can see. The doctor will rotate the lenses on the device until the patient has a clear view. The test is conducted on both eyes, one eye at a time. It captures the patient's visual error. The doctor will then instruct the patient to look at a chart placed at about 40 centimeters from their eyes. He/she will increase the lenses in small increments (-0.25) until the patient's vision becomes blurry. The PRA value is the aggregate of the lenses increments up to the point where the patients' vision is blurred.
High PRA values are considered evidence of disorders in the accommodation and vergence system. For instance, PRA values over -3.50 D are indicative of accommodative excess, while values below -1.50 D show accommodative dysfunction.
Abnormal results will prompt the doctor to determine the causes and recommend treatment, which may include vision therapy or surgery to correct a deviation in extreme cases.
Vision treatment options include:
- Accommodative therapy to boost the speed, ease amplitude, and accuracy of the amplitude of accommodation
- Vergence therapy to stimulate sensorimotor fusion
- Prism therapy – vertical crystals to do away with any vertical imbalance. Horizontal prisms remove the symptoms of asthenopia and relieve fusional vergence pressure of vergence impairment.
- Lens therapy – plus lenses aid to decrease the motor pressure on either the accommodative or vergence systems
Risks & Complications
The positive relative accommodation test has no risks.