Accommodation

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Accommodation

Transcript of Accommodation

  • Theories of Accommodation and its Anomalies Presenter: Dr. Rujuta Gore Moderator: Dr. Atul Seth
  • A dioptric change in the power of the eye to see clearly
  • Relaxation Theory of Helmholtz Proposed by Thomas Young Elaborated by Hermann von Helmholtz Most widely accepted
  • Relaxation Theory of Helmholtz eye is at rest and focused for distance ciliary muscle is relaxed eye makes an effort to focus on a near object ciliary muscle contracts bulk of the anterior ciliary body moves forward release in tension on the zonular fibres elastic capsule moulds the lens into a spherical form
  • Relaxation Theory of Helmholtz Increase in surface curvatures causes an increase in optical power of the lens and therefore an increase in power of the eye
  • Helmholtzs Theory: Disaccommodation ciliary muscle contraction ceases posterior zonular fibres pull the ciliary muscle backward increases tension on the zonular fibres increase in lens diameter, decrease in lens thickness and a flattening of the anterior and posterior lens surface curvatures decrease in optical power
  • Shortcomings of Helmholtzs Theory Since the equatorial diameter increases with age, zonules should relax, and power of the crystalline lens should increase. Lens should become unstable
  • Schachars Theory Proposed by Ronald Schachar Alternative theory Contradicts the classical Helmholtzs mechanism
  • Schachars Theory ciliary muscle contracts equatorial zonular tension is increased anterior and posterior zonules are simultaneously relaxed central surfaces of the lens steepen peripheral surfaces of the lens flatten
  • Helmholtzs and Schachars Theory
  • Shortcomings of Schachars Theory Based on his theory, Schachar introduced a new surgery in 1992 i.e. the use of scleral expansion bands to increase the distance between the lens equator and ciliary muscle. Poor results of this surgery challenged the validity of his theory
  • Catenary (hydraulic suspension) Theory Proposed by Coleman DJ in 1970 Demonstrated in 2001 Explains the precise anatomical reproducible shape of the lens in accommodated state Assumption : the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye
  • Catenary (hydraulic suspension) Theory What is catenary?
  • Catenary (hydraulic suspension) Theory ciliary muscle contracts initiates a pressure gradient between the vitreous and aqueous compartments anterior capsule and the zonule form a trampoline shape or hammock shaped surface steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens
  • Clinical Assessment Reading progressively smaller letters at near NPA using RAF rule Relative positive accommodation using minus lenses Accommodative flipper test using paired +/ lenses Dynamic Retinoscopy Dynamic Distant Direct Ophthalmoscopy
  • TERMS TO REMEMBER: Range of Accommodation Amplitude of Accommodation Relative Amplitude of Accommodation Lead Lag Facility of Accommodation
  • Range of Accommodation: The distance between the far point and near point ie the distance over which accommodation is effective Amplitude of Accommodation: The difference between dioptric power needed to focus at far point (at rest) and at near point (fully accommodated)
  • Relative amplitude of accommodation: The total amount of accommodation which the eye can exert while the convergence of the eyes is fixed It can be positive (using concave lenses until the image blurs). This is called positive relative accommodation (PRA). It can be negative (using convex lenses until the image blurs). This is negative relative accommodation (NRA)
  • Lead of Accommodation: The amount by which the accommodative response of the eye is greater than the dioptric stimulus to accommodation Lag of Accommodation: The amount by which the accommodative response of the eye is less than the dioptric stimulus to accommodation
  • Clinical Assessment Measurement of NPA: It is the closest point at which an object can be seen clearly Also called near point or punctum proximum Measured with the RAF rule
  • DDDO An emmetropic eye has with movement on retinoscopy and superior crescent on DDDO while 1D myopia (due to accommodation) shows no movement on retinoscopy and disappearance of superior crescent on DDDO DDDO is an easier test than DR Location of bright crescent moving from top to the bottom of the pupil is probably easier to recognize[Fig. 5] than change in the movement of the retinoscopy reflex (with movement to the against movement), more so when the pupils are dilated
  • Anomalies of Accommodation General symptoms: Problems are longstanding Intermittently blurred vision Eyestrain and/or headache with visual tasks Fatigue/sleepiness with visual tasks Inattentiveness over time
  • Anomalies of Accommodation Classification Decreased Accommodation Insufficiency Ill-Sustained Accommodation Inertia Paralysis Increased Accommodation Excess Spasm
  • Accommodation Insufficiency The accommodative amplitude is distinctly below the lower limit of the expected amplitude in relation to the age of the individual Similar to presbyopia Can result from systemic conditions such as diabetes mellitus, multiple sclerosis, anemia, general physical fatigue, myasthenia gravis, trauma, malnutrition, convalescence from debilitating illnesses and chronic alcoholism
  • Accommodation Insufficiency Specific symptoms: Blurred vision/eyestrain with NEAR visual tasks Intermittent diplopia due to associated disturbances of convergence Examination findings Reduced amplitude of accommodation Higher than normal lag of accommodation Difficulty clearing -2.00 D lenses on monocular and binocular accommodative facility testing PRA (positive relative accommodation) lower than -1.50
  • Causes of Unilateral Accommodation Failure: Congenital unilateral third nerve palsy Transient, post traumatic, accommodation failure associated with traumatic mydriasis Causes of Bilateral Accommodation Failure: Cortical vision impairment Foveal hypoplasia (albinism, aniridia) Down syndrome Iso-ametropic amblyopia Ectopia lentis Macular degeneration Nanophthalmos Near vision palsy Rule out
  • Treatment: Accommodation Insufficiency Spectacle correction For near- weakest convex lenses should be prescribed If there is associated convergence insufficiency base out prism may be added to patient comfort In cases with convergence excess full spherical correction should be prescribed ACCOMMODATION TEST-CARD EXERCISE
  • Vision Therapy: To stimulate accommodation mono- ocularly Small print targets that are slowly moved CLOSER to the eye Reading print through MINUS lenses (gradually increasing the power) using Monocular minus lens rock Monocular lens flippers Monocular minus lens clear/blur/clear (for fine voluntary control) Binocular lens flippers Treatment: Accommodation Insufficiency
  • Ill-sustained Accommodation Initial stage of true insufficiency Range is normal During prolonged near work, accommodative power weakens, the near point gradually recedes and vision becomes blurred
  • Inertia of Accommodation Rare condition Difficulty in altering the range of accommodation Requires time and effort to focus a near object after looking into distance Treatment: Correction of refractive error Accommodative Exercises
  • Paralysis of Accommodation Causes: Drug induced cycloplegia atropine ,homatropine Internal opthalmoplegia [paralysis of cilliary muscle & sphincter pupillae] Neuritis associated with chronic alcoholism, diabetes CNS infections Head Injury Specific Symptoms: Blurring of near vision Photophobia [glare]
  • Treatment: Paralysis of Accommodation Self recovery occurs in drug induced paralysis Dark glasses are effective in reducing the glare Convex lenses for near vision may be prescribed