The ciliary muscle and Zinns zunules change the curvature of the lens. (principle of accommodation)
Accommodation (from Latin accomodatio = adaptation) is the eye ability to adapt for clear vision at various distances. It is provided for by coordinated operation of three elements: the ciliary muscle, the Zinns zonule and the crystalline lens. The normal state for the eye is the far accommodation, then muscles are relaxed. To watch an item near, the ciliary muscle is contracted while the Zinns zunules are relaxed, this results in the crystalline lens increases its circumflexion (becomes more convex. This increases the optic power by 12-13 diopter, the light rays are focused within retina and the image is clear. Without an accommodation stimulus the ciliary muscle gets relaxed, the refraction power is decreased and the eye is focused to infinite distance again and that is desaccommodation or the far-accommodation.
Accomodation and age-relation
A most important conditions for normal accommodation is the lens elasticity. Unfortunately it deteriorates with age. The crystalline lens has greatest accommodation properties in childhood. It declines with age and gradually (usually after 40-45) the ability to see well anear goes down, the so called presbyopia develops, which is an age-related hyperopia. In most cases visual accommodation is lost completely by 60-70.
The accommodation responsible for far-sight at twilights disappears. This is one of the reasons for bad eyesight (uncomfortable vision) in the evenings and at night. The mean accommodation value is 2,0 diopters, thus, under bad light hypermetropia by 2,0 diopters, while the eye without any refraction abnormality (emmetropic eye) becomes myopic, and myopia goes 2,0 diopters up.
Causes of accommodation problems
The main signal for the accommodation reflex to be triggered is misfocusing of an image on the retina under optimal light condition due, i.e. light rays from the near object are not focused on the retina, and this misfocusing is receipted by the brain and provides a pulse to activate accommodation. The nerve pulse running along the oculomotor nerve signals the ciliary muscle to contract. The muscle is activated causing reduction in Zinns zonules tension, and they change the curvature of the lens. This brings the focus to the retina. If the sight is shifted to far the focus comes back to the retina and there is no signal of misfocusing, nor a nerve pulse and the ciliary muscle is relaxing to bring back tension in the Zinns zonules causing reduction in the lens curvature, letting it go flat.
Accommodation spasm may be triggered by the factors below:
- visual overload (TV, computer, studies in the evening);
- bad lighting at the working place;
- bad day regimen (not enough fresh air walks, sports sleep);
- bad sizing of table/chair/child’s height;
- bad eyes-to-book distance (30-35 cm optimal);
- weakness of neck and back muscles;
- blood supply disturbance in cervical spine;
- irrational nutrition;
- vitamin deficiency
- lack of physical activity.
Eye accommodation ability is expressed in diopters or linear values
- Accommodation functional ground state means absence of accommodation impetus within the field of vision
- Accommodation interval is the distance from the farthest (far-sight) to the nearest (near-sight) spots of clear vision.
- Accommodation extent is the difference in eye refraction indexes (in diopters) for the nearest and the farthest spots of clear vision.
- Accommodation capacity (reserve) is the unused part of accommodation extent (in diopters) at the eye fixation spot.
Accommodation indexes for each eye separate are called “absolute”. And for the two eyes together – “relative” as they are evaluated under convergence of optic axes.
Accommodation is closely linked to convergence. With the same convergence angle of the optic axes, the accommodation efforts vary in patients with different visual acuity. E.g. children with uncorrected hypermetropia (far-sightedness) of moderate to high degree can develop accommodated converging strabismus.
Accommodation disorders types
- accommodation spasm
- accommodation paralyses
- age-related accommodation imparing (presbyopia).
is most common in patients with hyperopia and astigmatism due to absence or incorrect prescription of glasses. Such patients complain about fast fatigue of eyes when reading, printed fonts being not clear, reddening of eyes and eyelids edges, burning and itching sensation. Feeling of a foreign body in the eye (so called blepharoconjunctivitis), headache sometimes accompanied by vomiting. The main cause of this condition is accommodation extra strain for near sight when the reserves are limited. The therapy for this condition is the definitive glasses or contact correction of refraction anomaly.
is often in children and youngsters. It is disturbance of eye (ciliary) muscle functioning, resulting in loss of clear vision for near and far objects. Ophthalmology defines accommodation spasm as abundantly sturdy accommodation stress caused by such contraction of the ciliary muscle that does not disappear when accommodation is not required. Statistically every sixth school kid suffers from such disorder.
Accommodation paralysis and paresis
as a rule are neurogenic or can develop after an injury or toxication. Patients with normal eyesight or hyperopic experience deterioration of near-sight. Myopic patients are less affected as their eyesight does not drop significantly or stays intact. Paralysis restricts accommodation extent, the reserves are exhausted.
Age-related accommodation impairing (presbyopia)
is a physiology phenomenon linked to ageing of the crystalline lens, as it is getting firmer and loses its elasticity. The treatment method here is optimal correction selection for near sight considering age and initial refraction.
Manifestation of accommodation spasm
- fast fatigue when working with near located objects;
- burning or smarting sensation, reddening of eyes;
- image located near is not clear, the one far off is blurred, doubling sometimes;
- a child is tired at classes sooner, learning progress is slow;
- headaches, often mistaken for growing-up alteration.
This condition may continue for several months or years.
Preventive care and treatment
Today the accommodation spasm is considered to be a major cause of children’s myopia.
Continuous contraction of the ciliary muscle is accompanied by its impaired blood supply and nourishment. Blood circulation disturbance leads to accommodation weakness and development of chorioretinal dystrophy. Therefore the complex approach to accommodation spasm diagnostics is very important as it may explain its cause and work out proper treatment procedure.
Today the accommodation spasm can be treated, on top of eye drops dilating pupil and eye exercise, using special computer games decreasing visual strain and various types of laser, magnetic and electric stimulation. Very important to go for a general massage with an emphasis on the cervical area twice a year. The child nourishment should include food rich in vitamins and minerals.
The early prophylactic and treatment of accommodation spasm can prevent myopia development.