accommodation Covergence ACnA ratio.pptx

It's talks about Accommodation

1. A SEMINAR PRESENTATION ON ACCOMMODATION CONVERGENCE AND AC/A BY DR. EDODIONG EKANEM UMOH MARCH 8, 2024

2. TABLE OF CONTENT ACCOMMODATION STRUCTURES OF ACCOMMODATION THEORIES OF ACCOMMODATION ACCOMMODATION ANOMALIES AND TREATMENT CONVERGENCE TYPES OF CONVERGENCE ANOMALIES OF CONVERGENCE AND TREATMENT AC/A RATIO (ACCOMMODATIVE CONVERGENCE/ ACCOMMODATION RATIO) CLINICAL METHOD TO DETERMINE AC/A RATIO

3. ACCOMODATION Accommodation is the process in which the eyes see objects at different distances and maintain clear images of the objects by the convergence and divergence of light. Scheiner (1612) was the first to observe this phenomenon and he did this by making two tiny pinholes in a card, the distance between the pinholes were not wider than the diameter of the pupil; an object was viewed through the pinhole. Objects appeared single when viewed through the pinhole but when the object was brought closer to the eye, they appeared double but after some seconds they appeared single again. This phenomenon of the eyes adjusting from seeing the second target from double to single within seconds is due to the eyes’ change in optical power.

4. fig1: illustration of accommodation (accuvision 2020)

5. Structures of accommodation The ocular structures involved in accommodation include the ciliary muscle, lens, and pupil. Below is a short description of these structures. Ciliary Muscle   - The ciliary muscle is a smooth muscle that is shaped like a ring and it’s located in the middle of the eyes. It holds the lens with the suspensory ligaments and also adjusts the optical power or shape of the lens during accommodation. Lens - This is a transparent structure in the eye, it is biconvex in shape (both surfaces are shaped like the exterior of a circle) Pupil - The pupil is located in the middle of the eyes, it is black in color and constricts to prevent light rays that have diverged from touching the retina and causing blurred vision.

6. Theories of accommodation Different theories which are still in contention were proposed to describe the mechanism of accommodation. One of the theory is: Helmholtz theory of accommodation (1855) – This theory is also called the capsular theory of accommodation. Helmholtz theorized that when the eyes are viewing a distant object, the ciliary muscle relaxes and the zonular fibers between the ciliary body and the equator of the lens stay flattened but when the object of focus is close, the ciliary muscles contract and the zonular fibers loosen. In the Helmholtz theory, the lens equator slides away from the sclera during accommodation and closer to the sclera when accommodation ends. In this theory, the zonular fibers are relaxed during accommodation and the zonular fibers are under tension when there is no accommodation reflex. OTHER theories include: catenary and schacher theories

7. Anomalies of Accommodation 1. Presbyopia – This is the inability of the eyes to focus near objects, it happens progressively and is common with elderly. Symptoms are eye strain, inability to focus on fine prints and small objects. The ability to focus on near objects declines throughout life, from an ability to focus at 50 mm away in children, to an ability to focus at 100 mm at 25 years of age, to an ability to focus at only about 2 m away by 60 years of age. treatments include Reading glasses Bifocal lens, Trifocal lens, Multi focal/ progressive addition lens. 2. Accommodative infantile – This is when the eyes lack the speed and accuracy to return to normal function during accommodation.

8. anomalies cont’d 3. Accommodation spasm – It is also called pseudomyopia as the name implies “ Pseudo”, its false myopia or false nearsightedness. This is when the ciliary muscle contracts continuously when viewing distant object resulting in blurred images. Etiology – Spasm can further be categorized into a) Functional spasm – This is a response to over fatigue and “eye strain”, also response to bad visual hygiene like poor lighting and glare unaccustomed work and optical or ocular motor difficulty e.g. anisometropia, early Presbyopia. b) Organic spasm – Ciliary spasm, drug induced e.g. pilocarpine, morphine, inflammation. Treatment Relaxation of the ciliary muscle with cycloplegics Consider occupation, general health and mental health of the patient. Correct refractive error Manage the cause.

9. 4) Accommodation insufficiency – The accomodative power is significantly less than normal physiological limits for patients age. Differential diagnosis with Presbyopia is that it still has normal physiological accommodation for the patients age. Etiology of insufficiency Malnutrition Anemia POA Glaucoma Symptoms of insufficiency Asthenopia Blurred vision for near work Treatment Treat the systematic cause Near vision spectacle(weakest plus lens)

10. fig2: asthenopic symptoms of accommodation(S. Abdi 2007)

11. ill sustained accommodation (accommodation fatigue): Here, range of accommodation is normal but cannot be sustained for a long time. The ciliary muscle cannot maintain contraction while viewing a near target with a resulting shift in accommodation towards far point. Etiology Stage of convalescence Stage of general tiredness When patient is relaxed in the bed Symptoms Tiredness in near work Blurred near work Treatment Improve visual hygiene (illumination and posture)

12. Inertia of accommodation – Difficulty in changing focus from distance to near and vise versa Etiology Prolonged close work Ocular motor imbalance Treatment Correct ocular motor anomaly Discuss visual hygiene

13. Paralysis of accommodation – This may be partial or total, unilateral or bilateral. Etiology Congenital effects e.g. no ciliary muscle Cycloplegics and systemic drugs Degenerative conditions like Parkinson's disease Exogenous poison e.g. snake bites, bee stings Ocular disease like anterior uveitis, Glaucoma Symptoms Blurred near vision Photophobia Micropsia Management Self recovery in drug induced Dark sun shade

14. Excessive accommodation – Here patient exerts more than normal accommodation for performing certain near work. It’s an intermittent process(not continuous). Etiology Young myope/ hyperope / astigmatism Beginning of presbyopia Symptoms Asthenopia Varying visual acuity Treatment Spectacle correction after cycloplegic refraction 20-20-20 rule should be observed when doing near work

15. CONVERGENCE This is the disjugate movement of the eye in which the eye moves inwards so that lines of sight will intersect. It helps maintain bifoveal single image at any fixation distances. Unlike accommodation, amplitude of convergence doesn’t detoriate with age, although some convergence might reduce under abnormal circumstances. Power or reserve convergence can be improved with orthoptic exercises.

16. Near Point of Convergence(NPC) This is a point of intersection of lines of sight of the eyes when maximum convergence is utilized. In simple words, this is the closest point in which an object can be seen as single during bifoveal vision. This is the nearest point on which the eyes can converge. It doesn’t change with age except in the presence of accommodative convergence in adults. It is much close to eyes compared to near point of accommodation In clinical practice, near point of convergence of 10cm is considered adequate.

17. Measurement of NPC Near point of convergence can be measured by: Tip of sharp pencil (grossly) RAF rule Livingston binocular gauge Prince rule

18. Types of Convergence Voluntary convergence – This is convergence of eye at our own will. Here convergence is attained by accommodating eye more without accommodating stimulus. E.g. converging eyes to reduce nystagmus or comedians applying voluntary convergence to obtain crossed eyes. Reflex convergence – They’re four types of reflex convergence and they’re  Tonic convergence – This occurs due to normal muscle tone of Extra Ocular Movements, it helps to bring the eye from anatomical diverged position to a physiological position. Tonic convergence decreases with age and can be eliminated by patching(30 min) or deep anesthesia.

19.  Fusional convergence – This ensures similar images are imaged on corresponding retinal points. It is mainly induced by bi-temporal image disparity, magnitude of fusional convergence is 35 PD at near and 18 PD at distance. Fusional convergence can be improved by orthoptic exercises.  Proximal convergence – It is induced by proximity of object of regard, it is initiated by psychological factor. It may be induced when a person feels he is looking at a near object even when he is not doing so.  Accomodative convergence – This is convergence induced when a person accommodates, it is induced or stimulated by blurred retinal image. It is independent of binocular vision i.e. it can occur in one eye.

20. Anomalies of convergence  Convergence insufficiency (CI) – This is the inability to maintain or obtain adequate convergence over certain period of time without undue effort, it is mostly caused by asthenopia. Etiology Idiopathic (developmental delay, wide IPD) Refractive errors (high hyperopia, myopia) Presbyopia

21. Clinical features of CI – Commonly seen in school children, near workers, precise workers like painters and patients with asthenopic symptoms. Symptoms of muscle fatigue which includes eye ache, headache, eyestrain and difficulty to change focus. Symptoms of difficulty to maintain binocular vision, this includes intermittent diplopia and blurred near vision. CI can be confirmed by Remote NPC i.e. more than 10cm Reduced fusional convergence for near Sometimes associated with exophoria at near and orthophoria at far.

22. Convergence Paralysis – This is the total lack of ability to overcome any amount of base out prism. It occurs secondary to some organic diseases of the brain or the nucleus of 3rd cranial nerve. Here, there’s complete absence of convergence, adduction is still normal, accommodation is usually normal, although it might be reduced or absent in some cases. Differential Diagnosis: With prism base out, patient with convergence paralysis will have diplopia immediately, while patient with CI can cope certain value of prism power. Treatment: Base in prism is used to eliminate diplopia at near Plus lens and base in prism to patients having accommodation weakness Occlusion of one eye at near work to eliminate diplopia. Eye surgery is contraindicated.

23. Convergence Spasm This is characterized by intermittent episode of maximum convergence usually associated with accommodative spasm. Etiology Functional causes associated with hysteria and neurosis Organic causes like lesions and head traumas. Clinical Features Extreme convergence Homonymous diplopia Blurred vision due to accommodative spasm Miosis High induced Myopia (>5D) Management Neurological evaluation needed Psychiatric evaluation might reveal hysteria and neurosis

24. Therapies to improve amplitude of fusional convergence are: Convergence exercise with prism Convergence exercise with diploscope Convergence card.

25. Accommodative convergence/ Accommodation ratio: also known as AC/A ratio, is a measurement of changes in accommodative convergence in prism diopters induced when the patient exert or relax 1 diopter of accommodation. Changes in the accommodation are either evoked by placing plus lens which relaxes accommodation or by minus which activate accommodation or when a person fixates near the target. The assessment of the AC/A ratio is significantly important . AC/A ratio plays a crucial role in reaching a final diagnosis and is an important factor to consider in determining the appropriate treatment approach. E.g., in a High AC/A ratio patients respond well to lens treatment whereas, in the case of a normal or low AC/A ratio, the appropriate treatment approach should be prism and/or vision therapy.

26. There are two methods used clinically to determine the AC/A ratio Calculated heterophoria method Gradient method Calculated Heterophoria method Calculated heterophoria method is based on the difference in deviation between distance phoria & near phoria and this method takes account of patient Interpupillary distance (IPD), which is an important factor if accommodative changes are induced by placing the target at near. As convergence require for a patient with a larger IPD is more compared to a patient with a small IPD, when the target is placed 40cm far away from the eyes. AC/A ratio is determined by using the formula: AC/A Ratio= IPD+NFD (Hn - Hd) Where; IPD is intra pupillary distance in centimeters. NFD is Near fixation distance in meters. Hn is Heterophoria at Near in prism diopters. Hd is Heterophoria at distance in prism diopters. Note: Esophoria is plus, Exophoria is minus.

27. For example: Calculate AC/A ratio, Patient IPD is 70 mm, Distance deviation is 2 PD esophoria and Near deviation is 6 PD esophoria at 40 cm. IPD + NFD (Hn – Hd) 7 + 0.4 [6 – 2] 7 + 0.4 (4) AC/A ratio = 8.6: 1

28. Procedure of Calculated heterophoria method Make sure the patient is wearing their appropriate correction. Measure the patient binocular Interpupillary distance Ask the patient to look at a 6m faraway target and measure the patient’s distance phoria. Ask the patient to look at the near target (usually 40 cm) and measure the patient near phoria. Use the formula to calculate the AC/A ratio

29. Gradient method The gradient AC/A measures the amount of convergence generated by a dioptre of accommodative effort. Changes in the accommodation are induced by placing plus lens which relaxes accommodation or by a minus which activates accommodation. AC/A ratio is determined by placing +1.00 DS or +2.00 DS in front of each eyes. AC/A ratio will be given by: AC/A ratio = Phoria with additional lenses - Baseline phoria/Power of additional minus lenses Note: Esophoria is plus, Exophoria is minus. For example, Baseline phoria is 2 exophoria, with additional 2D plus lenses 12 exophoria. AC/A ratio = [12 – (2)] / 2 = 5 AC/A ratio = 5 : 1

30. Procedure of Gradient method Make sure the patient is wearing their appropriate correction. Measure horizontal baseline near phoria without any additional lenses. Add +1.00 DS or +2.00 DS lenses in front of both eyes and measure again horizontal near phoria with additional minus lenses. Calculate the AC/A ratio Calculate AC/A using formula mentioned above

31. REFERENCES Gwiazda, J., Thorn, F., & Held, R. (2005). Accommodation, accommodative convergence, and response AC/A ratios before and at the onset of myopia in children.  Optometry and Vision Science, 82(4), 273-278. Bruce, A. S., Atchison, D. A., & Bhoola, H. (1995). Accommodation-convergence relationships and age.  Investigative ophthalmology & visual science, 36(2), 406-413. Schor, C. (1999). The influence of interactions between accommodation and convergence on the lag of accommodation . Ophthalmic and Physiological Optics, 19(2), 134-150. Ripps, H., Chin, N. B., Siegel, I. M., & Breinin, G. M. (1962). The effect of pupil size on accommodation, convergence, and the AC/A ratio.  Investigative ophthalmology & visual science, 1(1), 127-135. Paik, H. J., & Lim, H. T. (2012). Accommodation and convergence, anomalies of convergence . Journal of the Korean Ophthalmological Society, 53(12), 1719-1726. Lederer, J. (1946). The Anomalies of Accommodation and Convergence . Clinical and Experimental Optometry, 29(11), 500-513. Mutti, D. O., Jones, L. A., Moeschberger, M. L., & Zadnik, K. (2000) . AC/A ratio, age, and refractive error in children.  Investigative ophthalmology & visual science, 41(9), 2469-2478. Jackson, J. H., & Arnoldi, K. (2004). The gradient AC/A ratio: What's really normal?.  American Orthoptic Journal, 54(1), 125-132.