ANOMALIES OF ACCOMMODATION

MODERATOR: Dr. S.B PATIL

PRESENTER: Dr. KRISHNAPRABHA SHASTRY

WHAT IS ACCOMMODATION??

Accommodation is a dynamic optical change in the dioptric power of the eye by altering the shape of the crystalline lens in order to focus objects at variable distances.

TERMINOLOGIES

Far point: farthest point at which small objects can be seen clearly

Near point: nearest point at which small objects can be seen clearly.

Range of accommodation : distance between the near point and the far point

Amplitude of accommodation : the difference between the dioptric power, needed to focus at near point and to focus at far point.

Depth of field:

When an object is accurately focused monocularly,often the objects somewhat near and somewhat farther away are also seen clearly, without any change in accommodation.

This range of distance from the eye in which an object appears clear without change of accommodation is termed depth of field.

Depth of focus:

The range at retina in which an optical image may move without impairment of clarity.

The change in accommodation when fixation is changed from a distance object to a near object is termed positive accommodation

While the change from a near object to a distant object is termed

negative accommodation

ANATOMY OF ACCOMMODATIVE APPARATUS

Ciliary body The ciliary muscle The choroid Anterior and posterior zonular fibres Lens capsule Crystalline lens

NATURAL HISTORY OF ACCOMMODATION

Accommodation is non functional at birth and matures by five to seven months of age.

Recent studies have shown that the accuracy of the accommodation system similar to that of adult levels, was present in most infants, aged two to three months and all infants by

six months.

A study by Braddick et al showed that neonates are able to accurately accommodate for distances less than 75cm, while infants at six months or older demonstrated accurate accommodation for targets at 150cm or closer.

These results support that from one to six months, the accommodative ability improves, but whether this improvement is due to an enhancement in the retinal imaging process is still uncertain.

THEORIES OF ACCOMMODATION

The exact mechanism of accommodation is not

known.

In the year 1801, Thomas Young,an English physician, reported that the crystalline lens is responsible for accommodation.

RELAXATION THEORY OF HELMOLTZ

Also known as capsular theory

Lens is elastic and in normal state is stretched and flattened by the tension of the

suspensory ligaments.

During accommodation, contraction of ciliary muscle causes the ciliary ring to shorten and move toward the equator of the lens.

As the result, the zonules are relaxed, the tension on the capsule is relieved and the lens attains a more spherical shape, increase in convexity of the lens increases the Dioptric power and thus allows the near object to be focused clearly on retina.

TSCHERNING’S THEORY OF INCREASED TENSION

This theory attributed the increased curvature of the capsule in increasing tension on the zonules.

It states that contraction of the ciliary muscle pulls on the zonules directly and increases tension of the capsule at equator of the lens which leads to bulging of the poles.

COTENARY THEORY Cotenary theory of accommodation

was proposed by Coleman

The Cotenary(hydraulic suspension) theory proposes that lens, zonules and anterior vitreous comprise a diaphragm between aqueous and vitreous

As ciliary muscle contracts, it forms a

pressure gradient, causing anterior

movement of lens, zonules diaphragm and increasing anterior curvature.

SCHACHAR’S THEORY

Accommodation occurs when ciliary muscle contraction tenses the zonules and not when it relaxes the zonules.

MECHANISM OF ACCOMMODATION

At rest, when eye is focused for distance, resting tension on the zonular fibers apply an outward directed tension on the lens equator through the lens capsule. This holds the lens in a relatively flattened and un-accommodated state.

When accommodation needs to occur, the ciliary muscle contracts and the apex of the ciliary body moves forward, this movement releases the tension on all the zonular fibres,thus the lens capsule molds the lens into a more spherical, accommodated form. the lens diameter decreases, the anterior and posterior lens surface curvatures increase and the lens thickness increases.

ACCOMMODATION REFLEX

The accommodation reflex is activated in the retina where there is a loss of high spatial frequency which results in a blurred retinal image.

These visual fibres which are relayed in the lateral geniculate nucleus travel to the calcarine cortex (area 17).

Thereafter these fibres go to area 19 and area 22.

From here the fibres are then relayed to the

pre-tectal nuclei and thereafter to the Edinger-Westphal nuclei.

Emerging from the rostral part of the Edinger-Westphal nucleus, these

efferent fibres travel along the Oculomotor nerve.

The fibres then reach the ciliary ganglion and enter the eye in the short ciliary nerves.

After ciliary muscle contraction and the subsequent zonular relaxation, accommodation results.

REFLEX

TYPES OF ACCOMMODATION

Accommodation may be divided into the following components parts as described by von Noorden

1. Tonic accommodation

2. Proximal accommodation

3. Reflex accommodation

4. Convergence-accommodation

1) Tonic accommodation also known as dark accommodation, is

the passive state of accommodation. It is due to the tonus of the ciliary muscle and is active in the absence of a stimulus.

The resting state of accommodation is not at infinity but rather at an intermediate distance. The value obtained varies between individuals and usually is between 0.75 to 1.50D.

2) Proximal accommodation is also known as psychic accommodation and is induced by the awareness of the nearness of a target. This is independent of the actual dioptric stimulus.

3) Reflex accommodation is an automatic adjustment response to blur which is made to maintain a clear and sharp retinal image.

4)Convergence-accommodation is the amount of accommodation stimulated or relaxed associated with convergence.

STIMULUS FOR ACCOMMODATION

1)image blur

2)apparent size and distance of object

3)chromatic aberrations

4)oscillation of accommodation

5)scanning movements of eye

OCULAR CHANGES IN ACCOMMODATION

1) Zonules are slackened

2) curvature of lens surface increases

3) anterior pole of the lens moves forward

4) anterior capsule becomes slack

5) the lens sinks down

6) pupillary constriction occurs

7) eyeballs converge

8) choroid is stretched forward

9) ora serrata also moves forward

THERE ARE 4 ASPECTS INVOLVED WHEN TESTING THE ACCOMMODATIVE STATUS OF A PATIENT:

The accommodative amplitude

The accommodative response

The accommodative flexibility

The relative ranges of accommodation

Hence accommodative dysfunction can be characterised by a reduction of the accommodative amplitude, an inability to sustain the accommodative response, decreased flexibility or a defect in the accuracy of the accommodative response.

ANOMALIES OF ACCOMMODATION

Amplitude of accommodation varies with age and has a fairly wide range which is normal.

Variations in either direction above or below this range are termed as,

‘anomalies of accommodation’

CLASSIFICATION1) Diminished accommodation a)Physiological (presbyopia) b)Pharmacological(cycloplegia) c)Pathological Insufficiency of accommodation Ill sustained accommodation Inertia of accommodation Paralysis of accommodation

2) Increased accommodationa)Excessive accommodationb)Spasm of accommodation

PRESBYOPIA Age related (physiological)

insufficiency of accommodation , leading to progressive fall in near vision.

PATHOPHYSIOLOGY OF PRESBYOPIA In an emmetropic eye , far point is infinity

and the near point varies with age (7 cm at 10 years,25 cm at 40 years and 33 cm at 45 years)

we read from 25 cm,after 40 years, the near point recedes beyond reading or working range. This condition of failing near vision

is called Presbyopia.

CAUSES OF PRESBYOPIA

1. Age-related changes in the lens which include :

decrease in the elasticity of lens capsule and

sclerosis of lens

2. Age related decline in ciliary muscle power

SYMPTOMS

1)Difficulty in near vision : difficulty in reading

small prints difficulty in threading a

needle (to start with in the evening and

later even in good light).

2. Asthenopic symptoms due to fatigue of the ciliary muscle

are also complained after reading or doing any near work.

TREATMENT

prescription ofappropriate convex glassesfor near work.

A rough guide for providing presbyopic glasses in an emmetrope can be made from theage of the patient.

+1 DS for age of 40-45 years,

+1.5 DS at 45-50 years,

+ 2 DS at 50-55 years, and

+2.5 DS at 55-60 years

BASIC PRINCIPLES FOR PRESBYOPIC CORRECTION ARE:

1. Always find out refractive error for distance and first correct it.

2. Find out the presbyopic correction needed in each eye separately and add it to the distant correction.

3. Near point should be fixed by taking due

consideration for profession of the patient.

4. The weakest convex lens with which an individual can see clearly at the near point should be prescribed, since overcorrection will also result in asthenopic symptoms.

SURGICAL TREATMENT

CORNEAL PROCEDURES

A)Non ablative corneal procedure1)Monovision conductive keratoplasty

B)Laser based corneal procedure1)Monovision LASIK2)Laser thermal keratoplasty3)Presbyopic bifocal lasik4)Presbyopic multifocal lasik

Intraocular refractive procedure1)Refractive lens exchange

2)Phakic refractive lens

3)Monovision with iols

Scleral based procedures

1)Anterior sclerotomy with tissue Barriers

2)Scleral spacing Procedure

3)Scleral ablation with erbium yag laser

PREMATURE PRESBYOPIACauses are :

1. Uncorrected hypermetropia.2. Premature sclerosis of the

crystalline lens.3. General debility causing pre-

senile weakness of ciliary muscle.4. Chronic simple glaucoma.

INSUFFICIENCY OF ACCOMMODATION

The term insufficiency of accommodation is used when the accommodative power is significantly less than the normal physiological limits for the patient's age.

It should not be confused withpresbyopia

CAUSES

1) Premature sclerosis of lens.

2) Weakness of ciliary muscle due to systemic

causes of muscle fatigue such as debilitatingillness, anaemia, toxaemia, malnutrition,

diabetesmellitus, pregnancy and stress

3) Weakness of ciliary muscle associated with

primary open-angle glaucoma.

SYMPTOMS

These patients are most likely to report symptoms associated with the performance of near tasks. The onset of the symptoms can either be gradual or sudden and the intensity varies between individuals.

Blurred vision at near Headaches Eyestrain Fatigue, sleepiness and a loss of comprehension with continued reading

Movement of reading print

A dull 'pulling' sensation around the eye

Diplopia

Inability to read or perform any near work

Loss on concentration

Irritability of the eye

TREATMENT

1. The treatment is essentially that of the systemic

cause.

2. Near vision spectacles in the form of weakestconvex lens which allows adequate vision shouldbe given till the power of accommodationimproves.

3. Accommodation exercises help in recovery, if the underlying debility has passed.

ACCOMMODATIVE FATIGUE

This condition is also known as ill-sustained accommodation. It is similar to accommodative insufficiency except these patients are likely to report difficulties after a period of near tasks.

They will characteristically report that they are able to perform these near tasks initially but experience difficulties after a period of time has elapsed. This is due to failure of the accommodative system to maintain the accommodative effort.

As the individual continues to perform the near task over a prolonged period of time, the accommodative power weakens and the near point of the patient starts to recede resulting in blurry vision at near.

These patients will demonstrate amplitudes of accommodation which are within the normal range expected for their age, but will deteriorate over a prolonged time.

Hence if accommodative fatigue is suspected, it is diagnostic to repeat the amplitude of accommodation measurement several times or possibly at the end of the eye examination.

The likely causes of accommodative fatigue are similar to accommodative insufficiency, however, it is most commonly linked to conditions of general muscle fatigue.

Accommodative fatigue can occur as a result of any uncorrected refractive errors of hyperopia and astigmatism. Small amounts of anisometropia may also lead to accommodative fatigue.

CAUSES

1)Usually ill sustained accommodation is the initial stage of true insufficiency.

2)Stage of convalescence from debilitating illness

3)general tiredness

4)When the patient is relaxed on bed.

SYMPTOMS

Patients complain that while doing near work,they start feeling tired soon,their near point gradually recedes andnear vision becomes blurred

TREATMENT

Curtailing the near work during the situations described in etiology

Improved visual hygiene with particular reference to conditions of illumination and posture during studying

ACCOMMODATION INERTIA

It is a condition in which there is difficulty in adjusting accommodation according

to the distance of the object so as to gain clear vision

SYMPTOMS

The patient typically complains that it takes some time and involves some definite effort for him to focus a near object after looking at a distance.

TREATMENT

1)Correcting any associated refractive error

2)Practice of accommodation exercises

PARALYSIS OF ACCOMMODATION

This may occur from local or general causes; most important local cause is the exposure of the eye to parasympatholytic medication.

Occasionally trauma to the eye is also responsible.

Causes act either on midbrain or on

parasympathetic nerve supply in the Oculomotor

nerve or ciliary muscle itself.

Neurological causes include vascular disorders, cerebral

syphilis, and encephalitis lethargica

Also seen in mumps, infectious mononucleosis, tonsillitis, herpes zoster

In paralysis of accommodation, near point recedes and approximates the far point

Micropsia is seen in paralysis whereas macropsia is in spasm of accommodation

A progressive paresis should suggest incipient glaucoma

A sudden paralysis should suggest diabetes or encephalitis with a history of fever

Antihypertensives, antidepressants may cause slight paralysis

 

CYCLOPLEGIA

State of paralysis of ciliary muscle

Atropine , homatropine ,scopolamine cause cycloplegia,these also paralyze the sphincter papillae of the iris causing dialatation of the pupil.

CLINICAL FEATURES

1.Blurring of near vision.

2. Photophobia due to accompanying dilatation of

pupil is usually associated with blurring of near vision.

3. Examination reveals abnormal receding of Near point and markedly decreased range of accommodation.

TREATMENT

1.Self-recovery occurs in drug-induced Paralysis and in diphtheric cases

2. Dark-glasses are effective in reducing the glare.

3. Convex lenses for near vision may be prescribed

if the paralysis is permanent.

EXCESSIVE ACCOMMODATION

A situation in which an individual exerts morethan normal accommodation for performing

certainnear work. It is within voluntary control of theindividual and is an intermittent process.

CAUSES

1)Young hypermetropes use excessive accommodation as a physiological adaptation to see clear images

2)Young myopes performing excessive near work

3)Astigmatic error in young

4)Presbyopes in the beginning

5)Use of improper or ill fitting spectacles.

PRECIPITATING FACTORS

1) A large amount of near work is an important precipitating factor for this condition , especially when the work is habitually undertaken in deficient or excessive illumination

2) General debility

3) Physical or mental ill health

SYMPTOMS

1)Blurred vision

2)Accommodative asthenopic symptoms like headache, feeling of fatigue and discomfort in the eyes

3)Both far point and near point are brought nearer to the eye

TREATMENT

1)Refractive error should be corrected after carefully performing refraction

2)Near work should be forbidden for a period

3)General condition of the patient’s health should receive special attention.

4)A holiday with the change of air usually has a greater effect than anything else.

SPASM OF ACCOMMODATION

Spasm of accommodation refers to exertion ofabnormally excessive accommodation.

CAUSES

1. Drug induced spasm of accommodation is known

to occur after use of strong miotics such asEchothiophate.

2. Spontaneous spasm of accommodation isoccasionally found in children who attempt to compensate for a refractive anomaly that impairstheir vision.

It usually occurs when the eyes areused for excessive near work in unfavourablecircumstances such as bad illumination, badreading position, state of neurosis,mental stress or anxiety.

CLINICAL FEATURES

1)Defective vision due to induced myopia.

2. Asthenopic symptoms are more marked than the

visual symptoms.

DIAGNOSIS

Diagnosis is made with refraction under atropine.

TREATMENT

1. Relaxation of ciliary muscle by atropine for a few

weeks and prohibition of near work allow prompt

recovery from spasm of accommodation.

2. Correction of associated causative factors prevent recurrence.

3. Assurance and if necessary psychotherapy.

MEASURING ACCOMMODATION:

Tests of accommodation are performed monocularly.

When measuring the accommodative amplitude, it is assumed that you are testing an emmetropia, or someone who is corrected with spectacles, so that their far point is at infinity.

Target size, target illumination, and speed of target approach will affect the measurement of the amplitude of accommodation. The push up method works well for emmetropes, or fully corrected ametropes.

1. Near point of accommodation “Push Up Test”: For this test, use relatively small letters (0.4M or 0.5M) to help better control accommodation. Slowly move these letters closer to the eye until they become blurry. Measure the distance when the letters became blurry. This is the near point of accommodation.

2. Prince Rule:  It consists of a ruler scaled in diopters on one side and in millimeters on the other. One end of the ruler is held against the face and a test card is moved along the ruler towards the eye until a blur is noticed.

This is to locate both the near and far points as in the push up method.

3. Spherical Lens Test:  Spherical lenses are used in this test. The individual focuses on a stationery target while plus or minus lenses are used to measure the accommodative amplitude.

A reading card is put at a convenient distance, say 40 cm, and the individual fixates on threshold size type. Plus lenses are added until the print is blurred and then minus spheres are gradually added until the print blurs again. The difference is the accommodative amplitude.

Always test for accommodate relaxation with plus lenses before performing accommodative stimulation with minus lenses. This is because some individuals cannot adequately relax accommodation after exerting a maximum accommodative effort.

SEQUENTIAL MANAGEMENT OF ACCOMMODATIVE DYSFUNCTION:

Correction of refractive error

Added lenses

Vision therapy

Correction of the patients refractive error is usually the first step recommended when managing most, if not, all binocular vision disorders.

A clear retinal image and the reduction in stress on the ocular system as a whole will serve to ensure that the maximum possible benefit from the management plan will be obtained.

Added lenses, almost always convex, are very

important in the treatment of two of theaccommodative dysfunctions, namelyaccommodative insufficiency and

accommodativefatigue.

These conditions best respond to added convex lenses.

The underlying principle involved here, is these patients experience difficulties stimulating and maintaining accommodation, hence will benefit from added convex lenses.

Similarly those conditions in which

patients experience difficulties in relaxing accommodation require vision therapy rather than added lenses.

Vision therapy is inclusive of both office and home therapy with the objective of increasing the amplitudes and speed and decreasing the latency of the accommodative response.

The unit for accommodation is diopters (D), which is the reciprocal of the working distance. Thus for a fixation distance of 1m, the accommodation is said to be 1D.

Similarly if the fixation distance is 40cm, the accommodation is said to be 2.5D and so forth.

A corrected ametropic eye or an emmetrope is assumed to exert no accommodative effect to view an object at infinity

When viewing a detailed target at near, the accommodative response is usually less than the accommodative stimulus.

The optical difference between the stimulus and the response is denoted as the lag of accommodation and determined clinically using the Monocular Estimate Method (MEM) and Nott retinoscopy.

At near, the eye under-accommodates by 0.50 to 0.75 D.

It was concluded that both Nott and MEM dynamic retinoscopy techniques could be used interchangeably to asses the lag of accommodation at 40cm in patients.

An unusually high or low lag of accommodation in an emmetrope or corrected ametrope, is usually indicative of accommodative dysfunction, vergence imbalances and latent hyperopia.

Hence it is of extreme importance that the accommodative response be routinely assessed in visual examinations to facilitate the diagnosis of accommodative dysfunction

ACCOMMODATION EXERCISES

During the exercise the patient should always

wear the distance correction

Accommodation test card exercise is the most simple and most common accommodation exercise.

The accommodation test card consists of a black vertical line drawn on a white card.

The patient is instructed to hold the card at a considerable distance from eyes and bring it closer until the line appears blurred and indistinct.

By repeating this he should be encouraged to bring his near point as close as possible.

Patient should be advised to practice the test at short periods through out the day.

Thank you!