The vergence eye movement system is a critical component of extraocular movement and the visual system. The vergence system is unique in that it is the only visual response that causes the eyes to roll in opposite directions. Vergence eye is divided into two subgroups; convergence and divergence. When a person looks at an object, the eyes must rotate around a parallel or horizontal plane. It ensures that the image is projected to the right spot on the retina in both eyes. When a person looks at a distant object, the eyes roll away from each other. It is called a divergence eye movement. For a close object, the eyes roll towards each other. The eye movement is known as convergence. Vergence eye movements are primarily involuntary, although an individual can influence convergence through a voluntary effort.
The vergence eye movements are interlinked with the accommodation system, which controls the refractive power of the eye lens. When a person shifts their focus or attention from an object to another at a different distance, they automatically trigger vergence and accommodation. It is also called accommodation-convergence reflex.
The vergence system plays a vital role in the fixation of eye movements and depth perception. The vergence system works with the saccadic system to ensure that the eyes can change fixation and adapt rapidly and symmetrically. A sharp, distinct binocular image requires accurate alignment of the visual plane. This enables the eyes to change fixation from one distance to another to facilitate localization within space and depth perception. This function is dependent on an efficient vergence system.
Problems & Disorders
Vergence impairment can occur where the system is unable to undertake near vision tasks properly. It could be because the tasks need sustained and accurate vergence and accommodative functioning without fatigue, or they don’t have the stereoscopic signals needed for proper vergence responses.
The symptoms of vergence dysfunction include:
- Eye fatigue
- Blurred vision
- Double vision
- Motion sickness
- Systemic fatigue
- Loss of concentration
Vergence function may be impaired by:
- Head injury
- Change in the visual environment, eg., increased near work in situations like computer use, school, and office work
- Diseases such as Alzheimer's disease, Graves's disease, Parkinson disease, myasthenia gravis, etc
There are several types of vergence dysfunction, including:
- Divergence excess
- Convergence excess
- Vertical heterophoria
- Divergence insufficiency
- Convergence insufficiency
- Fusional vergence dysfunction
- Basic exophoria and esophoria
The patient can exert convergence voluntarily, and so convergence is vulnerable to persistent subjective disturbances. Often, vergence impairment relates to convergence. It can be displayed as a weakness or an excess, such as convergence paralysis or seizure. Generally, divergence dysfunction is evidenced by a diminished function like divergence weakness, while vertical and cyclovergence disorders are rare.
Vergence anomalies are various visual abnormalities, and most patients with vergence dysfunction have disorders in many areas of binocular sight. For instance, a patient with vergence insufficiency can have a lower-level accommodative dysfunction, or a patient with an accommodative impairment might have a lesser vergence disorder. Therefore, to diagnose vergence dysfunction, the doctor may need to conduct a series of tests.
The diagnosis of vergence dysfunction may involve:
- Stereopsis test
- Refraction test
- Visual acuity test
- Cycloplegic refraction test
- Near point of convergence test
- Ocular motility and alignment test
- Near fusional vergence amplitudes test
- Accommodative amplitude and facility test
- Relative accommodation measurements test
- Ocular health assessment and systemic health screening
- Supplemental tests, such as vergence facility, accommodative convergence/accommodation ratio and distance fusional vergence amplitude
The steps involved will depend on the type of test. For example, the refraction and relative accommodation measurement tests will include the patient sitting in front of a machine called a refractor or phoropter, which will measure their refractive error. In the visual acuity test, the doctor will use the Snellen and Jaeger eye charts to check the patient's distant and near vision. In the distant vision test, the table will be placed about 20 feet from the patient. The patient will close or block one eye and read aloud the smallest letters or symbols they can see with the open eye. The doctor will hold the Jaeger cards and lead the patient during the test.
In the ocular motility and alignment test, the patient stands or sits on a chair holding their head up and staring straight ahead. The doctor will check the 12 extraocular muscles and their effect on the movement of the patient’s eyes. He/she will hold a small object about 16 inches in front of the patient’s eyes. He/she will shift the object to different sides and instruct the patient to follow it with their eyes, without moving the head. The specialist can also conduct a cover/uncover test in which the patient looks at a distant object. He/she will cover one of the patient’s eyes and uncover it after several seconds. The test is done for both eyes.
Where the diagnosis confirms vergence disorders, the doctor may recommend treatment.
Treatment options include:
- Vergence therapy to activate sensorimotor fusion
- Lens therapy can help to relieve the motor pressure on the accommodative or vergence systems
- Prism therapy – Vertical crystals can help to correct any vertical imbalance. Horizontal prisms do away with the symptoms of asthenopia and relieve fusional vergence pressure of vergence impairment.
- Surgery - In rare cases to correct a deviation
Vision therapy is the most commonly used treatment for vergence dysfunctions.