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Accommodation
The document defines accommodation as the eye's ability to focus on near objects by changing the lens curvature. It describes how the ciliary muscle controls the lens shape by relaxing the zonules attached to the lens capsule. When relaxed, the elastic capsule makes the lens more convex to focus on nearby objects. The range and amplitude of accommodation are defined. Presbyopia and other accommodation anomalies like insufficiency, paralysis, and spasm are explained in terms of causes, symptoms, and treatments.
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Accommodation
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- 2. DEFINITION • Our eyeshave been provided with a unique mechanism by which we can even focus the diverging rays coming from a near object on the retina in a bid to see clearly, this mechanism is called accommodation. • In this increase in the power of crystalline lens occurs due to increase in curvature of its surface.
- 3. • At restthe radius of curvature of anterior surface of the lens is 10mm and that of posterior surface is 6mm. • During accommodation curvature of the posterior surface remains almost the same but that of anterior changes. In strong accommodation radius or curvature of anterior surface also becomes 6mm.
- 4. MECHANISM OF ACCOMMODATION •WHEN THE EYE IS AT REST; The ciliary ring is large and keeps the zonules tense. because of zonules tension the lens is kept compressed(flat)by the capsule. • CONTRACTION OF THE CILIARY MUSCLE; Causes the ciliary ring to shorten and thus releases zonular tension on the lens capsule. This allows the elastic capsule to act unrestrained to deform the lens substance.
- 5. • The lensthen alters its shape to become more convex, the lens assumes conoidal shape due to configuration of the anterior lens capsule which is thinner at the centre and thicker at the periphery. • FOR POINT AND NEAR POINT; The nearest point at which small objects can be seen clearly is called near point or punctum proximum and the distant point called for point or punctum remotum.
- 6. RANGE AND AMPLITUDEOF ACCOMMODATION • RANGE OF ACCOMMODATION; The distance between the near point and the for point is called the range of accommodation. • AMPLITUDE OF ACCOMMODATION; The difference between the dioptric power needed to focus at near point(P) and for point (R) is called amplitude of accommodation (A). A=P-R.
- 7. ANOMALIES OF ACCOMMODATION •PRESBYOPIA. • INSUFFICIENCY OF ACCOMMODATION. • PARALYSIS OF ACCOMMODATION. • SPASM OF ACCOMMODATION.
- 8. PRESBYOPIA • Is notan error of refraction but a condition of physiological insufficiency of accommodation leading to a progressive fall in near vision. • PATHOPHYSIOLOGY; As we know, in an emmetropic eye for point is infinity (∞ and near point varies with age. • Being about 7cm at the age of 10years,25cm at the age of 40years, and 33cm at the age of 45 years.
- 9. • At theage of 10years, amplitude of accommodation(A)=100/7 (dioptric power needed to see clearly at near point)-1/∞ (dioptric power needed to see clearly at for point clearly). A (at age 10)= 14 Dioptres. Similarly at the age of 40; A=100/25 -1/∞ = 4 Dioptres.
- 10. • CAUSES; Decrease inthe accommodative power of crystalline lens with increasing age, leading to presbyopia occurs due to; 1.Age related changes in the lens; decrease in elasticity of lens capsule, progressive increase in size and hardness of the lens substance. 2.Age related decline in ciliary muscle power. uncorrected hyperopia.
- 11. • Premature sclerosisof the crystalline lens. • Chronic simple glaucoma. • SYMPTOMS; • Difficulty in near vision, • Asthenopic symptoms, • Intermittent diplopia, • TREATMENT; • Spectacle and surgical treatment.
- 12. INSUFFICIENCY OF ACCOMMODATION •The accommodative power is significantly less than the normal physiological limits for the patients age. • CAUSES; • Premature sclerosis of lens, • Weakness of ciliary muscle, • TREATMENT; • Treatment of underlying causes,
- 13. • Near visionspectacle, • Accommodation exercises,
- 14. PARALYSIS OF ACCOMMODATION •Is also known as cycloplegia refers to complete absence of accommodation. • CAUSES; • Drug induced cycloplegia • Paralytic internal opthalmoplegia • Complete third nerve paralysis
- 15. • CLINICAL FEATURES; •Blurring of near vision, • Photophobia, • TREATMENT; • Self recovery in drug induced cycloplegia, • Dark glasses to reduce glare, • Convex lenses if paralysis is permanent,
- 16. SPASM OF ACCOMMODATION Refersto exertion of abnormally excessive accommodation, CAUSES; Drug induced spasm of accommodation, In children who attempt to compensate for a refractive anomaly that impairs their vision, it usually occurs when the eyes are used for excessive near work.
- 17. • CLINICAL FEATURES; •Defective vision, • Asthenopic symptoms, • DIAGNOSIS; • It is made with refraction under atropine cycloplegia, • Relaxation of ciliary muscle, • Necessary psychotherapy, • Correction of associated causative factors,
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