The accommodation system is the mechanism that enables the eye to adjust its focus from near to distant objects and vice versa. It is the eye's focusing mechanism, i.e., the ability of the eye to automatically change its focal power by altering the shape of its lens. It enables the individual to see an object clearly as its distance changes or when one shifts their eyes to look at another object. For example, reading an article requires the eyes to accommodate the print while looking out of the window requires the eyes to adjust focus on a distant object.
The accommodation system is composed of the ciliary muscles, zonular fibrils and the lens substance and capsule. The ciliary muscle on the zonular fibers is the focusing muscle and it is responsible for changing the shape of the lens and its accommodative power. When the ciliary muscles contract during accommodation, the zonular fibrils attached to the center of the lens relax, leading to a change in the thickness and shape of the lens. The material that makes the eye lens is malleable in childhood and up to the early adulthood years. The lens gradually loses its flexibility, and by the age of 40, the rigidity of its nucleus decreases accommodation. Studies indicate that the stiffness or hardness of the lens increases more than 1000 times throughout one’s life.
Accommodative abnormalities can occur in children and young adults, significantly impacting the quality of life. The patients often have difficulty sustaining a prolonged near focus. It can cause problems in school or office work and also in physical activities like sports and games. At times, it may even be misidentified as a learning disability or another disorder. The accommodation dysfunction is separate from visual error or the need for glasses. However, an untreated visual error can influence the system. This emphasizes the need for parents to ensure their children have regular eye examinations.
Also Known As
- Eye focusing system
Problems & Disorders
The ciliary muscle and the lens can be affected by a myriad of factors leading to an accommodation dysfunction, also known as accommodation insufficiency. It is a disorder where the patient has problems with speed or ease of focus that are unrelated to natural changes especially ageing.
The symptoms of accommodative dysfunction include:
- Eye fatigue
- Blurred vision at near
- General fatigue, especially later in the day
- Difficulty adjusting focus from near and far
- Headaches - especially over the temples or brow
- Intermittent blurred vision at a distance after reading
The symptoms are often worse when one is tired, later in the day or during tasks.
There are several types of accommodative dysfunction, including:
It is the most common form of accommodative dysfunction. The patient has a weak accommodative sustaining ability and particularly a difficulty sustaining focus at near. The extra effort needed to maintain clear vision at near may affect performance on close tasks.
It is a seizure of the focusing muscles, which can prevent them from relaxing completely. It can occur due to the overstimulation of the parasympathetic nervous system. Generally, it is associated with fatigue and causes blurry vision both near and far. The disorder may also arise due to other causes such as brain tumor, trauma, myasthenia gravis, or the use of topical or systemic cholinergic drugs.
Paralysis of Accommodation
It is a sporadic disorder in which the system is unable to respond to stimulus. The condition may occur due to systemic/ocular disease, trauma, toxicity, poisoning, or the use of cycloplegic drugs. The disorder may affect one or both eyes and can be linked with a fixed, dilated pupil.
The patient has difficulty changing focus from near to far and vice versa. Often, the patient has blurred distance vision immediately after sustained near work. The Infacility may be a precursor to myopia.
The diagnosis of the accommodation system may involve a series of tests. The doctor may begin the evaluation with a review of the patient's history before conducting the eye examination. Usually, all components of the accommodation system occur in synchrony and are synergistic. The doctor will seek to isolate these individual components by holding one function constant to measure the flexibility or plasticity of the system.
The evaluation of the accommodation system may involve:
- Refraction test
- Stereopsis test
- Visual acuity test
- Cycloplegic refraction test
- Near point of convergence test
- Ocular motility and alignment test
- Accommodative amplitude and facility
- Relative accommodation measurements
- Ocular Health assessment and systemic health screening
The steps involved will vary depending on the eye test. For example, in a visual acuity test, the patient will sit or stand and cover one eye. With the uncovered eye, the patient will read aloud the letters or identify symbols they can see on a Snellen chart placed at a distance away. The patient will also do the near acuity test by reading the Jaeger cards held by the doctor.
The refraction test involves the patient sitting in front of a phoropter or refractor, which is a measuring instrument fitted with lenses of different power. The patient looks into the device and focuses on an eye chart 20 feet away. The doctor will change the lenses on the instrument until the glass is clear. It evaluates the patient's refractive status.
In ocular motility and alignment cover tests, the doctor will evaluate the patient's visual-motor function and their near and distant visions. The patient will sit, holding their head up and staring straight ahead. The doctor may begin by visually checking the eyes for signs of strabismus, cranial nerves palsy or dysfunction of the extra ocular muscle. Then, he/she will introduce a small target before the patient's eye and move it in various directions and ask the patient to trail it with their eyes. The cover/uncover test will assess the patient's distant vision.
There are three methods the doctor can use to treat eye focusing problems including vision therapy, lenses and surgery.
Vision therapy is the most recommended treatment option for young patients. It involves a specialized treatment program similar to physical therapy to train the patient's eyes and brain to work together. The treatment may combine office therapy with home exercises. It may employ a variety of training tools like lenses, special games and computer programs.
The doctor may recommend reading glasses to address visual complaints linked to focusing problems. However, eyeglasses alone are often insufficient to correct accommodative dysfunction. They may offer some relief, but the underlying problem and symptoms will persist.
In rare cases, the doctor may suggest eye muscle surgery to correct a focusing problem. The procedure involves permanently adjusting the eye muscles.
Visual hygiene is also vital. One should keep a reasonable working distance and take regular breaks from near tasks, such as reading and writing.