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Accommodation of eye

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Information about Accommodation of eye
Education

Published on March 2, 2014

Author: RohitRao2

Source: slideshare.net

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in this ppt you can find out about the theories and anomalies of accommodation
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Theories & Anomalies Of Accommodation Presented by Dr. Rohit Rao

References • Duke-Elder’s practice of refraction by David Abrams • Optics & Refraction By A K Khurana • Textbook of Ophthalmology by E Ahmed • Clinical Optics By A R. Elkington • Borish's Clinical Refracfion By W J. Benjamin • Werner L, Trindade F, Pereira F,Werner L Physiology of Accommodation and Presbyopia, ARQ. BRAS. OFTALMOL. 63(6), December 2000.

Definition Accommodation is the mechanism by which the eye changes refractive power by altering the shape of lens in order to focus objects at variable distances

• Far point: Position of an object when its image clearly falls on retina with no accommodation. • Near point: Nearest point clearly seen with maximum accommodation. • Range of accommodation: Distance between far point and near point.

• Amplitude of accommodation: Dioptric power difference between rest and fully accommodated eye. – A=P-R ( A: amplitude of accommodation; P:dioptric value of near point; and R: dioptric value of far point.) • Accommodative Convergence/Accommodation Ratio – To view near object: Accommodation for clear retinal images, & convergence for binocular single vision. – The number of prism dioptres of convergence which accompanies each dioptre of accommodation is (AC/A) ratio – The normal range for the AC/A ratio is 3:1 to 5:1.

Theories of accommodation • The exact mechanism of accommodation is not known. • In year 1801 YOUNG reported lens is responsible for accommodation. “Principal fact that ACCOMMODATION is a feature of increase in the curvature of the lens which affects anterior surface mainly”

Relaxation theory of HELMHOLTZ • Also known as the “Capsular Theory”. • He considered that lens was elastic and in normal state it is stretched and flattened by tension of the suspensory ligaments. • During accommodation, contraction of ciliary muscle shortens ciliary ring and moves towards the equator of the lens. • Relax the suspensory ligaments, relieving strain. • Lens assumes more spherical form, increasing thickness and decreasing diameter.

Helmholtz Accommodation

Points in favour of the relaxation theory • Imaging technique showed that ciliary muscle move anteriorly & the equatorial edge of lens move away from sclera during accommodation. • Gonio-videography show zonular fibers extending from ciliary processes to lens equator, are relaxed during accommodation

Unaccommodated Pharmacologically stimulated

Points against the theory • It is not clear how lens alters its shape when tension in suspensory ligaments is relaxed? • what is responsible for decline in power of accommodation with age?

GULLSTRAND mechanical model of accommodation • It is based on HELMHOLTZ hypothesis • GULLSTRAND devised a mechanical model to explain accommodation. • It shows in unaccommodated state elasticity of choroid is stronger than lens. When accommodation comes into play weight i.e ciliary muscles contract to overcome elasticity of choroid. • It helps lens to take accommodated shape.

SCHACHAR’S theory • Presbyopia is due to growth in equatorial diameter, leads to decrease in perilenticular space. • Contraction of ciliary muscle cannot tense zonules and expand lens coronally. • SCHACHAR introduced use of scleral expansion bands (SEB).

TSHERNING’S theory • This theory attributed increased curvature of capsule to increasing tension of the zonules. • It states that contraction of ciliary muscle pulls zonules directly and increases tension of capsule at equator of lens, which leads to bulging of poles.

COTENARY theory • COTENARY theory of accommodation was proposed by COLEMAN. • The COTENARY (hydraulic suspension) theory proposes that lens, zonules & 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 central curvature. • Presbyopia is due to increase in lens volume, results in reduced response to pressure gradient created by ciliary body contraction.

Types of Accommodation • Tonic accommodation – It is due to tonus of ciliary muscle and is active in absence of a stimulus. The resting state of accommodation is not at infinity but rather at an intermediate distance. • Proximal accommodation – Is induced by the awareness of the nearness of a target. This is independent of the actual dioptric stimulus.

• Reflex accommodation – Is an automatic adjustment response to blur which is made to maintain a clear and sharp retinal image. • Convergence-accommodation – Amount of accommodation stimulated or relaxed associated with convergence. – The link between accommodation and convergence is known as accommodative convergence and is expressed clinically as AC/A ratio.

Accommodation Reflex

• The afferent – Retina (with the retinal ganglion axons in the optic nerve, chiasm and tract), – Lateral geniculate body (with axons in the optic radiations) – Visual cortex. • Ocular motor control neurons are interposed between the afferent and efferent limbs of this circuit and include the visual association cortex

• It determines the image is "out-of-focus” & sends corrective signals | internal capsule and crus cerebri | supraoculomotor nuclei (generates motor control signals) | oculomotor complex.

• The efferent – Edinger-Westphal nucleus - oculomotor nerve - ciliary ganglion short ciliary nerve - iris sphincter and the ciliary muscle/zonules/lens of the eye – oculomotor neurons oculomotor nerve medial rectus, converge the two eyes.

Anomalies of Accommodation • Classification (by Duane with some modification): – Accommodative insufficiency – Ill-sustained accommodation– Paralysis (or paresis) of accommodation – Unequal accommodation – Accommodative excess. – Inertia of accommodation

– Diminished or deficient accommodation – Physiological : Presbyopia – Pharmacological : Cycloplegia – Pathological – Insufficiency of accommodation – Ill sustained accommodation – Inertia of accommodation – Paralysis of accommodation – Increased accommodation

Presbyopia Presbyopia is a condition of physiological insufficiency of accommodation leading to a progressive fall in near vision.

Pathophysiology • In emmetropic eye far point is infinity and near point varies with age (being about 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 normal reading or working range. • Failing near vision due to age-related decrease in amplitude of accommodation is called presbyopia.

Causes • Decrease in accommodative power of lens with increasing age, leads to presbyopia, occurs due to: – Age-related changes in lens: o Decrease in elasticity of lens capsule, and o Progressive, increase in size and hardness (sclerosis) of lens substance which is not easily moulded. – Age related decline in ciliary muscle power.

Premature presbyopia: • Uncorrected hypermetropia. • Premature sclerosis of the crystalline lens. • General debility causing pre-senile weakness of ciliary muscle. • Chronic simple glaucoma.

Symptoms • Difficulty in near vision. • Patients complaint of difficulty in reading small prints • Asthenopic symptoms due to fatigue of the ciliary muscle are also complained after reading or doing any near work.

Presbyopia Rx

Optical treatment • Prescription of appropriate convex glasses for near work. • A rough guide for providing presbyopic glasses in an emmetrope can be made from patient’s age. – About +1 DS is required at the age of 40-45 years, – +1.5 DS at 45-50 years, – + 2 DS at 50-55 years, – +2.5 DS at 55-60 years.

Basic principles of presbyopic correction • Refractive error for distance is corrected first. • Correction needed in each eye should be tested separately and add it to distant correction. • Near point should be fixed according to the profession of patient. • Weakest convex lens with which one can see clearly at near point should be prescribed, overcorrection will also result in asthenopic symptoms. • Presbyopic spectacles may be unifocal, bifocal or varifocal.

Surgical Treatment • Corneal procedures – Non ablative corneal procedure – Monovision CK Near Vision – Laser based corneal procedure – Laser thermal keratoplasty (LTK) – Monovision LASIK. – Presbyopic bifocal LASIK – Presbyopic multifocal LASIK C Distant Vision

• Intraocular refractive procedure – Refractive lens exchange – Phakic refractive lens – Monovision with IOLs • Scleral based procedures – Anterior sclerotomy with tissue barriers – Scleral spacing procedure – Scleral ablation with erbium : yag laser

Insufficiency of accommodation • Condition in which accommodative power is constantly less than lower limit of normal range according to patient’s age.

Etiology • Premature sclerosis of lens • Weakness of ciliary muscle due to systemic causes: Debilitating illness, anemia, toxemia, malnutrition, dia betes mellitus, pregnancy, stress etc. • Weakness of ciliary muscle due to local causes: PAOG, mild cyclitis as during onset of sympathetic ophthalmia.

Clinical features • Features of eye strain and asthenopia. • Head ach, fatigue & irritability of the eyes, while attempting near work. • Near work is blurred & becomes difficult or impossible. • Disturbance of convergence : intermittent diplopia. • It is stable condition, if due to sclerosis of lens. • But is not stable in association with ciliary muscle weakness.

Treatment • Identification & treatment of any systemic cause. • Any refractive error should be corrected & if vision for near work is seriously blurred then additional near correction has to be prescribed same as presbyopia. • If associated with convergence excess then full spherical correction.

• Convergence insufficiency is there, then base in prisms can be added. • Prismatic correction added should bring near point of convergence to same distance as near point of accommodation. • Weakest convex lenses should be prescribed, so as to exercise and stimulate accommodation. • After recovery additional correction should be made weaker and weaker from time to time.

• Accommodative exercises. – While do exercises patient should wear correction for distance. – Should be done simultaneously in both eyes, even if associated with convergence insufficiency. – But with convergence excess then the exercise should done with one eye alternately. – Accommodation test card exercise. – Useless in generalized debility and sclerosis of lens.

Ill-Sustained accommodation • Accommodation fatigue. • It is a situation in which though range of accommodation is in normal range but it cannot sustain it for a sufficient period of time. • Initial stage of insufficiency of accommodation. • It occurs due to – Stage of convalescence from debilitating illness – Stage of generalized tiredness – When the patient is relaxed in the bed

Clinical features • These symptoms are most commonly reported at the end of the day • Blurred vision after prolonged near work. • Headaches • Eyestrain • Fatigue, sleepiness and a loss of comprehension with continued reading • A dull 'pulling' sensation around the eye.

Treatment • Near work should be curtailed during debilitating illness. • General tonic measures should be taken. • The condition of illumination and posture while doing near work, should be improved.

Inertia of accommodation • It is a condition in which patient faces difficulty in altering the range of accommodation. • Amplitude of accommodation is normal. • Ability to make use of this amplitude quickly and for long periods of time is inadequate.

Clinical features • Difficulty changing focus from one distance to another • Headaches • Eyestrain • Fatigue • Difficulty sustaining near tasks • Blurred vision Treatment: correcting any refractive error and accommodative exercises.

Paralysis of accommodation • Cycloplegia, refers to complete absence of accommodation. • Causes – Atropine, homatropine or other parasympatholytic drugs. – Internal ophthalmoplegia (paralysis of ciliary muscle and sphincter pupillae)due to neuritis associated with diphtheria, syphilis, diabetes, alcoholism, c erebral or meningeal diseases.

– Complete third nerve paralysis due to intracranial or orbital causes. – Systemic medications such as antihypertensive, antidepressants.

Clinical features • Blurred vision at near • Photophobia or a 'dazzling' effect • Diplopia • Micropsia: objects may appear smaller than they are due to a false sense of distance • Enlarged pupil.

Treatment • An effort should be made to find out the cause and try to eliminate it. • Self-recovery occurs in drug-induced paralysis and in diphtheric cases (once systemic disease is treated). • Dark-glasses effective in reducing glare. • Convex lenses for near vision, if the paralysis is permanent.

Excessive accommodation • Accommodative response is greater than the accommodative stimulus. • There is functional increase in tonus of ciliary muscle, results in a constant accommodative effect.

Causes • Young hypermetropes frequently uses excessive accommodation as a physiological adaptation • Young myopes performing excessive near work, associated with excessive convergence. • Astigmatic error in young patients • Presbyopes in the beginning • Use of improper and ill fitting spectacles

Precipitating factors • Excessive near work done, especially in dim or excessive illumination. • General debility, physical or mental ill health

Symptoms Blurred vision at near is uncommon Blurred vision at distance Headaches Eyestrain Photophobia Difficulty changing focus from distance to near • Diplopia • • • • • •

Treatment • It has a good prognosis. • Refractive error should be corrected after carefully performed cycloplegic refraction. • Near work should be stopped for some time, after that it should be done with proper illumination conditions.

Spasm of accommodation • Spasm of accommodation refers to exertion of abnormally excessive accommodation.

Causes • Drug induced spasm of accommodation is known to occur after use of strong miotics. • Spontaneous spasm of accommodation: attempt to compensate for a refractive anomaly. • Occurs when excessive near work is done with bad illumination, bad reading position, state of neurosis, mental stress or anxiety.

Clinical features • Defective vision: due to induced myopia. • Asthenopic symptoms • Precipitating factors like marked degree of muscular imbalance, trigeminal neuralgia, a dental lesion, general intoxication.

Treatment • Relaxation of ciliary muscle by atropine for 4 weeks or more and • Prohibition of near work allow prompt recovery from spasm of accommodation. • Elimination of the associated causative factors to prevent the recurrence.

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