Accommodation and its Anomolies

Definition

Accommodation is the mechanism by which the eye changes refractive error by altering the shape of its crystalline lens.

Mechanism of accommodation

The essential feature of accommodation is an increase in the curvature of the lens which affects mainly the anterior surface. This alteration in shape inverses the convergent power of the eye, so that the focus can be altered as and when required.

Types of accommodation

- Physical accommodation
- Physiological accommodation

Physical accommodation

The ability of the lens to alter its shape is called physical accommodation. It is measured in diopters.

Physiological accommodation

The power of the ciliary muscle to contract is called physiological accommodation. Physiological accommodation is the cause and physical accommodation is the effect.

Characteristic features of accommodation

1. Range
2. Amplitude

Range of accommodation

Distance between the far point of accommodation and near point of accommodation is known as range of accommodation.

Amplitude of accommodation

The dioptric difference between the far point of accommodation and near point of accommodation is known as amplitude of accommodation.

Far point

The clear image at maximum distance from the eye is called the far point of accommodation. In emmetropes it is at infinity.

Near point

The clear image at closest distance from the eye is called near point of accommodation.

Positive portion of relative accommodation (PRA)

A measure of maximum ability to stimulate accommodation while maintaining clear single binocular vision.

Negative portion of relative accommodation (NRA)

A measure of maximum ability to relax accommodation while maintaining clear single binocular vision. PRA is tested using minus lens and NRA is tested using plus lens. PRA should be greater than NRA, otherwise the patient will have asthenopic symptoms while doing near work.

Changes in the eye during accommodation

Inner changes

- Ciliary muscles constrict
- Anterior surface of the lens increases its curvature, which increases the dioptric power of the crystalline lens

Other changes

- The pupil constricts. This increases the depth of focus and also eliminates peripheral rays which diminish spherical aberration
- Both eyes will converge

Unaccommodative state is maintained due to the following factors:
- Elastic force of the lens
- Elastic elements of the ciliary body
- Toughness of zonules and capsules

Accommodation is helped by

- Elasticity of lens
- Contraction of ciliary muscles
- Plasticity of the capsule and zonules which relax when the ciliary muscles contracts.

During the act of accommodation the following phenomenon occurs

- Lens decreases in equatorial diameter Centre of lens protrudes forward
- Relative flattening at the periphery of the lens
- Lens increases in thickness.

Changes in accommodation with age

As age advances, the amplitude of accommodation decreases. This is due to change in the consistency of the lens, which becomes harder with advancement of age. At about 40 years, a normal person cannot read when the reading material is about 33 cm away from the eyes. This is called presbyopia. (Normal reading distance - 33cm).

Amplitude of accommodation with age

Age	Accommodation in Diopter
5 yrs	16D
10 yrs	14D
15 yrs	12 D
20 yrs	10D
25 yrs	8.5D
30 yrs	7.0D
35 yrs	5.5D
40 yrs	4.5D
45 yrs	3.5D
50 yrs	2.5D
55 yrs	1.75D
60 yrs	1D

Accommodation in refraction

Hypermetropia

Hypermetropic patient will accommodate continuously to see a clear image. This excessive accommodation will result in pseudomyopia.

Pseudomyopia

The accommodative spasm causes blurry distance vision after prolonged near work. The individual may appear myopic.

Myopia

Myopic patients have clear near vision. There is no need for accommodation.

In hypermetropia

- Range of accommodation: normal
- Amplitude of accommodation: greater

In myopia

- Range of accommodation: less
- Amplitude of accommodation: shorter

Controlling Accommodation

If accommodation is not controlled, refractive error may vary while evaluation is performed.

There are two ways to control accommodation
- Cycloplegia
- Fogging method

Cycloplegic drops

These drops have a temporary paralyzing effect on the ciliary muscles.

Cycloplegics are drugs used to paralyse the ciliary muscles and also to dilate the pupil. Cycloplegic refraction is necessary in young children, especially for hypermetropia.

- Cyclopentolate is a good cycloplegic. Allow enough time for the full cycloplegic effect; 20-30 minutes
- Homotropine - action up to two days
- Atropine - Action up to 2 weeks, or sometimes even more (21 days). This is used therapeutically also Fogging

Fogging

This method is used to control accommodation during a manifest refraction. This test is not needed for all patients. The objective of fogging is to blur the vision in the eye being tested to about 6/36. This is done by increasing the convex (+) spherical lens or by reduction of concave (-) spherical lens power. The convex (+) power is reduced before the eye or concave (-) power is added in steps of 0.25 D until the visual acuity is improved to the maximum.

Anomalies of accommodation

They are four types:
- Excessive accommodation
- Insufficiency of accommodation
- Spasm of accommodation
- Ill sustained accommodation

Excessive accommodation

Excessive accommodation is associated with excessive convergence and is found most frequently in younger people.

Spasm of accommodation

Tone of ciliary muscle is increased and a constant accommodation effort is expanded by the para sympathetic nervous system. Pseudomyopia is produced.

Insufficiency of accommodation

In this condition, the patient’s accommodative amplitude is consistently lower than what is normal for his/her age. This condition is also called premature presbyopia.

The insufficiency may be of:
a. Lenticular origin
b. Ciliary origin

Ill sustained accommodation

Amplitudes are normal, but rapidly diminish with age. When the patient uses his eyes for near, for a long duration, his accommodation fails, near point recedes gradually and his near vision becomes blurred.

Paralysis of accommodation

i) Artificial – by drug :

1. Atropine
2. Homatropine

ii) Due to diseases :

1. Paralysis of oculomotor nerve
2. Paralysis of ciliary muscle due to any cause. Usually it is accompanied by paralytic dilatation of pupil.

Inertia of accommodation

Some difficulty is noted altering the range of accommodation, manifested as difficulty in changing focus from distance to near and near to distance. This is a rare condition.

Symptoms

Associated with close work;

1. Asthenopia
2. Transient blurred vision for near
3. Photophobia
4. Abnormal fatigue
5. Headache
6. Watering
7. Difficult sustaining near visual function
8. Dizziness
9. Abnormal working distance
10. Orbital pain

Treatment

Orthoptic Excercise

- Prism glasses (Bases out)
- Convex (+) power glasses
- Accommodative flipper

Clinical findings in accommodation problems

Condition	Symptom	Findings	Etiology	Ocular treatment
Accommodation fatigue	Blurred near Vision	Normal NPA which decreases with repeated testing	Refractive errors, medication, illness, non-specific	Spectacle correction/reading excercises
Accommodation failure (insuffiency ill-sustained)	Blurred near Vision	Decreased NPA heterophoria	refractive Errors, local eye trauma, medication illness, non-specific	Spectacle correction/reading excercise, fusion excercise
Accommodative paralysis	Blurred Vision, micropsia, diplopia	Decreased NPA, heterophoria,tropia	Trauma, medication, neurologic/medical	Spectacle correction/reading excercise,base-in prism
Accommodative Spasm	Blurred fluctuating vision,diplopia,asthenopia	Increased NPA, pseudomyopia, variable eso	Psychogenic, trauma neurologic/medical	Cycloplegics (or) miotics Spectacle correction/reading excercise

NPA - Near Point Accommodation

Students Excercise

True or false

1. Accommodation is the mechanism by which the eye changes its size (  )
2. There are three types of accommodation (  )
3. The amplitude of accommodation declines with age (  )
4. Cycloplegic drops have a temporary paralyzing effect on ciliary muscle (  )
5. Asthenopia is not a symptom of accommodation insufficiency (  )

Match the following

1. PRA - Exercise accommodation
2. NRA - Accommodative flipper
3. Fogging - Minus lens
4. Accommodation weakness - Lens
5. Pseudomyopia - Controlling accommodation

Answer the following

1. What is accommodation?
2. What are the symptoms of accommodation problems?
3. What are the types of accommodation?
4. What is the anomaly of accommodation?
5. What is the treatment for accommodation?