The amplitude of accommodation is the highest potential increment in diopter power or optical power that an eye can attain by changing its focus. It is the difference between the optical power needed for seeing up close and focusing at a distance. It denotes the range of object distances at which the image on the retina is sharply in focus. AA is the level of adjustment in the refractive power of the eye that is created by accommodation. Accommodation is the system by which the eye controls its refractive power by changing or modifying the shape of its crystalline lens. In the accommodation process, the ciliary muscle (this is the focusing muscle) contracts, enabling the zonular fibers to relax. The relaxation results in the equatorial edge of the lens moving away from the sclera in an accommodative response, causing increased lens roundness (convexity). The increase in convexity takes place on the front surface of the lens.
The accommodative response diminishes with age and can also be affected by disease and specific medication. Generally, young adults have an accommodative response of 12-14 diopter (D); adults range between 4 and 8 D, and after the age of 50, AA declines to less than 2D. Researchers suspect that the loss of accommodation occurs primarily because of the hardening of the lens with age. The process is known as presbyopia. Researchers are still trying to establish the other actors in presbyopia. Potential contributing factors include the inelasticity of the eye lens capsule, the geometry of zonular attachment and changes in dimensions of the lens with age.
Accommodative insufficiency arises where AA is below the expected for the patient’s age and is not because of sclerosis of the lens. Accommodative insufficiency affects many children and adolescents. Most of the young people who experience difficulties with reading have dyslexia or learning disabilities because of accommodative problems. Even where eye disorders are not the primary cause of poor academic performance, the conditions can contribute significantly to a child’s challenges with school work and sports. Excellent binocular skills contribute to better educational and sports performance. Sports like tennis, baseball and basketball require accurate depth perception, which depends on excellent binocularity. It is advisable for parents to take children who have academic problems for a comprehensive eye examination.
Also Known As
- Accommodative response
Problems & Disorders
The use of computers in schools, workplaces and at home, has drawn attention to the effect of binocular vision dysfunction on the quality of life, and especially on comfort and performance. Ocular discomfort rises with the level of computer use, and a large proportion of symptomatic computer workers have binocular vision problems. This applies to other populations who undertake sustained near work like writers, students, lawyers and accountants.
The symptoms of accommodative dysfunction include:
- Eye fatigue
- General fatigue
- Blurred vision at near
- Difficulty changing focus from near and distance
- Intermittent blurred vision at a distance after reading
Usually, the symptoms are worse when the patient is tired, like during tasks or later in the day.
Accommodation disorders include:
nearsightedness, the inability to focus on distant objects
Farsightedness, where distant objects are clear while the near ones look blurry
Eye strain, fatigue or weakness of the eyes accompanied by dimmed vision
An age-related disorder in which the ability to focus on close objects progressively deteriorates
A temporary and intermittent change in the visual error of the eye towards nearsightedness
Latent squint, a disorder where the eyes’ directions are different from each other, during a rest position, i.e., when engaged in binocular vision.
The doctor can measure the accommodative response during a routine eye examination. There are many procedures the doctor can use to diagnose AA, including:
- Push-up test
- Minus lens test
- Push-down test
- Modified push-up test
- Dynamic retinoscopy exam
- Open-field autorefractor exam
The steps involved will depend on the methods the doctor will use. Open-field autorefractor and dynamic retinoscopy are objective procedures. However, in practice, subjective methods are more frequently used to measure AA.
The patient will sit in a room with normal illumination and look at the Snellen chart placed at a given distance. The tests may be done monocularly on the patient’s dominant eye. For the push-up test, the patient begins by looking at a target at a distance of about 40 cm. The target is then moved slowly towards the patient. The doctor will ask the patient to focus on the target and to report when it first becomes blurred.
In the push-down test, the accommodative target is slowly pushed towards the patient until they observe a blur. The target is then pushed away until the patient is able to read just the 20/20 row of letters on the chart. In the minus lens procedure, the target is positioned at a range of 40 cm. The doctor will ask the patient to focus on the 20/20 row of letters, and a minus power is added to subjective refraction (previously-determined) until the letters became and remain blurred.
Where the diagnosis indicates that the patient has low amplitude of accommodation, the doctor will consider the appropriate treatment and management strategies to address the condition. The doctor will seek to relieve ocular, psychological and physical symptoms associated with the disorder and to assist the patient in functioning efficiently at work, in school performance and in sports activities.
The treatment and management options include:
Accommodative therapy aims at increasing the speed, ease, accuracy and amplitude of the accommodative response. If the treatment is successful, the patient will be able to make quick accommodative responses without fatigue.
Lenses and prism therapy
The doctor can prescribe prism glasses or lenses in some cases, for example, to eliminate symptoms of asthenopia.
It is rarely used for nonstrabismic binocular vision abnormalities because the purpose of extraocular surgery is to reduce the degree of the deviation.