accommodation.pptx

ACCOMMODATI ON Learning Objectives Definition of accommodation Mechanism of accommodation Components of accommodation Terminologies of accommodation Assessment of accommodation Definitio n Accommodation is the mechanism by which the eye changes refractive power by altering the...

ACCOMMODATI ON Learning Objectives Definition of accommodation Mechanism of accommodation Components of accommodation Terminologies of accommodation Assessment of accommodation Definitio n 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.  Accommodation is caused by the increased curvature of anterior area of the eye lens. Accommodation is usually the same in both eyes. Mechanism of accommodation Step 1: Blurred retinal image Step 2: Parasympathetic nerve system causes ciliary muscle contraction. Step 3: Zonule fibers relax. Step 4: Lens capsule becomes more curved especially anterior surface. - Increase lens thickness - Reduced equatorial diameter - Pupil constriction - Convergence Components of Accommodation There are two components: Physical Accommodation Physiological Accommodation 1. Physical Accommodation  The actual physical deformation of the lens.  It is measured in diopter.  Converging power of the eye is increased by 1D. 2. Physiological Accommodation  The contractile power of the ciliary muscle required to arise the refractive power of lens by 1D.  It measured in myodiopter. Terminologies of accommodation Far point: Position of an object when its image clearly falls on retina with no accommodation. Emmetropic eye : at INFINITY Myopic eye : REAL and lies in FRONT OF the eye Hypermetropic eye : VIRTUAL and lies BEHIND the eye Near point : Nearest point at which small objects can be seen clearly. Also called punctum proximum. Itvaries with age ; about 7 cm at 10 years of age 25 cm at 40 years of age 33 cm at 45 years of age Range of accommodation: Distance between far point and near point. Range of accommodation= Far point – Near point (in cm) (in cm)  Amplitude of Accommodation : Difference between the dioptric power, needed to focus at near point and to focus at far point. Amplitude = Near point – Far point (in Dioptre) (in Dioptre) Example: Given an emmetropic eye with near point of accommodation located at 10cm in front of the eye calculate the range and amplitude accommodation. Far point(R) = infinity Near point(P) = 10cm Range of accommodation : (R-P)=infinity Amplitude of accommodation – (P- R)=10D Example: Given uncorrected myopic eye with far point of accommodation located 50cm in front of the eye and near point at 10cm in front of eye calculate range and amplitude of accommodation. Far point – 50cm Near point – 10cm Range of accommodation – (R- P)=40cm Amplitude of accommodation – (P-R)=8D Depth of (object) field and Depth of (image) focus The range of object distances for which the circles of confusion are so small that the image is sharp enough to be considered 'in focus' is called the depth of field. In photography depth of field is controlled by adjusting the camera aperture’s aperture ,lens focal length, and distance of subject The range of image distances over which the image of an improperly focused object is acceptably sharp is called the depth of focus. Depth of focus depends on age, aberrations, and the amount of light entering the eye Assessment of Accommodation A)Assessment of NPA and amplitude of accommodation 1. Pushup Test: RAF rule/ ruler with accommodative target 2.Measurement of Amplitude of accommodation using minus lenses B) Assessment of Accommodative Response Techniques of Dynamic Retinoscopy C) Assessment of Dynamic Accommodation: Accommodative flipper D) Range of accommodation: NRA-PRA E) Accommodative –convergence relationship: AC/A 1. RAF rule or Prince’s rule 1.A reduced snellen chart:subtend 5min arc at 35 cm 2.Time roman face:N12,N8,N10,N5 3.GENERAL post office telephone directory 4.Dot on a black line 1,2,3 are used for NPA & amplitude of accommodation 4 is used for measuring NPC  Using RAF rule we can determine the NPA Patient wear full correction A sliding target with best corrected near acuity is moved towards the eye until the closest is found at which it still can be seen clearly& stop where sustained blur vision found. Repeat the test for atleast thrice The NPA is determined first for each eye separately and then for both eyes together.  The side of bar marked in dioptres will indicate the amplitude of accommodation in dioptres. The third side of the bar shows the age corresponding to the accommodation. If while measuring the NPA, the patient’s amplitude of accommodation is found so low that his or her near point is beyond the length of the instrument, PLUS lenses are added to his or her correction until the near point is brought within limit. Push-down: The push down method considered as a variation of push-up method. In this procedure the test target is moved away until the patient reports when it first becomes clear. Then the distance is measured and converted into dioptric value 2. Measurement of Amplitude of Accommodation using minus lenses Performed monocularly and the patient has to wear his or her full refractive correction. The patient is asked to fixate the best corrected near vision target at 40 cm distance and minus lenses of progressively increasing power are added before the eye till the patient reports the first sustained blur. The power of this minus lens + 2.5 D (for 40 cm distance of testing) is equivalent  For example, At 40 cm distance of testing , if the patient reports blur with -3 D spherical lens, the amplitude of accommodation would be, 3 + 2.5 = 5.5 D 3. Pushup Test without RAF ruler A very simple test done which can be done monocularly as well as binocularly with patient wearing full refractive correction. The patient is asked to fixed it at the best corrected near vision target at a distance where the target is seen clearly. The near vision chart is moved closer till the patient reports first sustained blur. Techniques of Dynamic Retinoscopy Monocular Estimation Method NOTT Bell Have to know elaborately Interpretation of Accommodative Response Lead of Accommodation: The amount by which the accommodative response of the eye is greater than the accommodative demand. It indicates accommodative excess. Lag of Accommodation: The amount by which the accommodative response of the eye is less than the accommodative demand. Assessment of Dynamic Accommodation Dynamic of accommodation can be assessed by testing accommodative facility. An accommodative Flipper of +2.00 DS with -2.00 DS is used to test accommodative facility by rapidly flipping the lenses  Difficulty with plus lenses is seen in patients with accommodative excess. Difficulty with minus lenses is seen in presbyopes.