Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009
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Errors of Refraction 141
Table 9-1: Standard notation table for presbyopia correction. The glasses may be unifocal, bifocal, progressive addition lens or multifocal
|
Age of the patient |
Near addition prescribed |
|
|
40 |
+1.00 DSPH |
|
|
42 |
+1.25 DSPH |
|
|
45 |
+1.50 DSPH |
|
|
47 |
+1.75 DSPH |
|
|
50 |
+2.00 DSPH |
|
|
52 |
+2.25 DSPH |
|
|
55 |
+2.50 DSPH |
|
|
60 |
+2.50 |
DSPH to +3.00 DSPH |
|
Aphakia |
+3.00 |
DSPH |
|
Pseudophakia |
+2.50 |
DSPH to +3.00 DSPH |
|
|
|
|
Fig. 9-11: Optical principle of conductive keratoplasty (CK)
Surgical (Conductive Keratoplasty)
Conductive keratoplasty (CK) is based on the principle of thermokeratoplasty, using radiofrequency energy. Conductive keratoplasty creates a purse-string effect that steepens the central cornea through collagen shrinkage that encompasses 80% of the corneal thickness (Fig. 9-11).
C H A P T E R |
Estimation and |
|
10 |
||
Correction of |
||
Refractive Errors |
INTRODUCTION
The methods employed in measuring and correction of refractive errors consist of:
I.Initial estimation by objective methods—Objective method refers to preliminary estimation of refractive error without any verbal response from the patient. However, some co-operation from the patient such as steady fixation is required. Objective methods are:
i.Retinoscopy (plane mirror or streak)
ii.Autorefractometry
iii.Photo refraction.
II.Subjection methods—Subjective method refers to refinement of refractive error estimated from objective method, to obtain best visual acuity. Subjective method requires verbal response from the patient. However, subjective method cannot be used for patients who cannot speak or are unable to respond. In those cases, the practitioner must rely on objective methods only.
RETINOSCOPY
It is the most common objective method used for estimation of refractive error, with accommodation at rest. It involves a study of the movements of the retinal image produced by a patch of light on the patient’s retina, through a peephole in the centre of the mirror. Retinoscopy can be done by either a plane mirror or a streak retinoscope.
Estimation and Correction of Refractive Errors 143
RETINOSCOPY WITH PLANE MIRROR–PROCEDURE
a.Cycloplegia–It is required in children and young subjects. In adults and older patients it is usually not required unless the pupil is very small. Cycloplegia in children (upto 6 years of age) is achieved by application of atropine sulphate (1%) eye ointment 2 times daily for 3 consecutive days prior to the day of examination. In older children and adults tropicamide (1%) eyedrop is applied 2 or 3 times at interval of 5 minutes and retinoscopy should be conducted between 20–30 minutes of the last application.
b.It is done in a dark room–The refractionist sits 1 metre away from the patient. The patient, with the trial frame on, is asked to look at the distance. The retinoscopy light is positioned behind and to the right of the head of the patient.
c.The refractionist looks through the central peephole of the retinoscopy mirror and reflects the light on the patient’s pupil.
d.The mirror is tilted very slowly vertically and horizontally and the movement of the shadow in the pupillary area is observed.
e.Depending on the refractive status of the patient’s eye the shadow will behave in relation to the tilting of the mirror as follows:
i.In myopia of > 1.00D–The shadow moves in the opposite direction. This is called “against the movement”.
ii.In myopia of –1.00D–There is absence of shadow. The pupil will be either totally dark or illuminated. This is called “point of reversal/point of neutralisation”.
iii.In hypermetropia, emmetropia and myopia of < 1.00D–The shadow moves in the same direction. This is called “with the movement”.
f.If the refractionist observes “against the movement”, point of reversal can be achieved by adding increasingly concave (minus) lenses from the trial set in the trial frame. If the refractionist observes “with the movement” point of reversal can be achieved by adding increasingly convex (plus) lenses from
144 Textbook of Visual Science and Clinical Optometry
the trial set in the trial frame. Speed and brightness of the retinoscopy reflex/shadow increases as point of reversal is approached.
g.The procedure of point of reversal is done in both vertical and horizontal meridians separately in each eye. In spherical refractive errors the point of reversal will be the same in both the meridians. In astigmatism, the point of reversal will be different in both the meridians. In astigmatism with oblique axes, the mirror is tilted aligning with the oblique axes.
h.In patient’s with high refractive errors the reflex/shadow is faint and the movement is often so slow it is quite difficult to discern. In this situation, try a high convex lens or a high concave lens to ascertain the nature of the movement. If the refractionist sits at 1 metre away from the patient and the light source, the point of reversal is –1.00D, i.e. the patient is –1.00D myopic with the addition of the lens required for neutralisation of movement. However, it is often convenient for the refractionist to hold the lens in his left hand instead of trial frame and perform the retinoscopy. Here, he sits at arm’s length distance from the patient, i.e. 2/3rd of a metre. The point of reversal will be then –1.50D.
Calculation of Refractive Error Based on the Retinoscopy
It is calculated from the simple formula:
1
RE = ——— + D + A
– d
RE = Refractive error; d = Distance in metres of the retinoscope from the patient’s eye; D = Power of the lens in diopter in trial frame at the point of reversal; A = Accommodation in diopters (accommodation is zero in static retinoscopy).
i.If the point of reversal is + 3.00D in 1 metre static retinoscopy, the refractive error will be – 1.00D + 3.00D = + 2.00D.
ii.If the point of reversal is – 4.00D in 2/3rd metre static retinoscopy, the refractive error will be – 1.50D –4.00D =
– 5.50D.
iii.In case of astigmatism, i.e. where the point of reversal is different in two meridians, it is calculated as follows—
Estimation and Correction of Refractive Errors 145
(Conventionally, the retinoscopy finding is recorded as above) The point of reversal is marked in a cross as above. So, in the right eye the refractive error will be calculated from the optical cross drawn below;
The final refractive error is calculated from the formula given below:
The power of the sphere = lower value ignoring the sign
Axis of the cylinder (Astigmatism) = the meridian of the sphere, i.e. the lower value
The power of the cylinder = deduct the lower value from the higher value keeping the signs intact.
So, in right eye the refractive error will be + 1.00DSPH with + 2.00 – (+ 1.00) DCYL 180° = + 1.00 DSPH with + 1.00DCYL 180°.
So in left eye the refractive error will be –3.00DSPH with – 4.00– ( – 3.00) DCYL 90° = – 3.00DSPH with – 1.00DCYL 90°.
RETINOSCOPY WITH STREAK RETINOSCOPE
Modern retinoscopes are fitted with a long straight filamented bulb in order to produce a bright streak shaped object. Parts of a streak retinoscope are (Fig. 10-1):
146 Textbook of Visual Science and Clinical Optometry
Fig. 10-1: Streak retinoscope with parts
i.Battery handle (to accommodate the batteries).
ii.Light source–A bulb with long straight filament.
iii.Condensing lens–It forms an image of the filament and focusses onto the mirror.
iv.Mirror with a central peephole–It is located in the head of the streak retinoscope and is oriented at an angle of 45° to the axis of the handle. It reflects the streak of light from the condensing lens towards the patient’s pupil.
v.Focussing sleeve–It changes the distance between the bulb and the condensing lens to project either divergent rays (to produce plane mirror effect) or convergent rays (to produce concave mirror effect). Width of the streak varies as the sleeve is raised or lowered (Fig. 10-2). In some retinoscopes the bulb is moved (Copeland) while in others the condensing lens is moved (Welch Allyn, Keeler, etc.) by the focussing sleeve. The sleeve is also turned to rotate 360° the streak of rays in different meridians. The plane mirror effect is usually used in streak retinoscopy.
Estimation and Correction of Refractive Errors 147
Figs 10-2A and B: Streak retinoscope–use of focussing sleeve to obtain.
(A) Concave mirror effect, (B) Plane mirror effect
Retinoscopy with Streak Retinoscope–Procedure
a.Set the focussing sleeve in lowest position in Welch Allyn, Keeler streak retinoscopes and in upper most position in Copeland streak retinoscope to get plane mirror effect.
b.It is convenient for the refractionist to work at a distance of 66 cm (or 2/3rd of a metre or 26”), which corresponds to an arm’s length, in streak retinoscopy. This implies a working lens of + 1.50D. Some refractionist uses a working distance of 50 cm i.e. ½ metre with a working lens of + 2.00D.
c.The patient is asked to look at a fixation point at least 15’ from the eye.
d.The streak is aligned vertical by rotating the focussing sleeve.
e.Now observe the retinal reflex/shadow and it will appear as in Figure 10-3A. if oblique astigmatism is absent. If oblique astigmatism is present, the retinal reflex will appear as in Figure 10-3B, where the reflex within the pupil is not aligned vertically.
148 Textbook of Visual Science and Clinical Optometry
Figs 10.3A to C: (A) Oblique astigmatism absent (horizontal axis), (B) Oblique astigmatism present, (C) Oblique astigmatism absent (vertical axis)
f.Tilt the vertical streak horizontally across the pupil and notice whether the reflex in the pupillary area moves in the same direction as the streak or in the opposite direction.
g.Rotate the focusing sleeve until the streak is horizontal (Fig. 10-3C) and tilt the streak vertically across the pupil and notice whether the reflex in the pupillary area moves in the same direction as the streak or in the opposite direction.
h.Depending on the refractive status of the patient the reflex will behave as follows:
i.In myopia of >1.50D–The reflex and the streak moves in the opposite direction (against the movement–Fig.10-4A)
ii.In myopia of –1.50D–It is the point of reversal, i.e. neutralisation. The streak disappears and the pupil is flooded with light. There is absence of against the movement or with the movement (Fig.10-4C)
iii.In hypermetropia, emmetropia and myopia of < 1.50D–the reflex and the streak moves in the same direction (with the movement (Fig.10-4B)
i.Point of reversal is confirmed by:
i.Lean forward a little towards the patient and definite “with the movement” appears and move a little away from the patient and definite “against the movement” appears.
ii.Put an additional + 0.25D lens in the trial frame and “against the movement” appears.
j.If “with the movement” is observed, add increasingly convex lenses until point of reversal is reached. Similarly, if against the movement is observed, add increasingly concave lenses until
Estimation and Correction of Refractive Errors 149
Figs 10.4A to C: (A) Against the movement, (B) With the movement
(C) Point of reversal
point of reversal is reached. The procedure is done in both vertical and horizontal meridian. The speed, brightness and width of the retinal reflex increases as the refractionist approaches the point of neutralisation.
k.In case of oblique astigmatism the streak is aligned with the retinal reflex and the procedure is continued as discussed above. The point of reversal is estimated in one meridian first and the point of reversal is estimated 90° away in the second meridian.
l.Determination of the axis (principal meridians) in astigmatism– Axis is determined by the following factors:
i.Break–When the streak and the retinal reflex are not parallel, i.e. the streak is not aligned with a principal meridian, break is observed (Fig. 10-5). When the reflex is aligned with the streak the break disappears.
ii.Width–The reflex is narrowest when the streak aligns with a principal meridian. The width of the reflex increases when the streak is rotated to either side of the actual axis.
iii.Brightness–The reflex is brightest when the streak is aligned with the correct axis. Hence, when the streak is properly aligned with the axis, the break disappears and the reflex becomes narrowest and brightest.
Fig. 10-5: Observation of break
150 Textbook of Visual Science and Clinical Optometry
Static retinoscopy is used for estimation of refractive error. In static retinoscopy, it is assumed that the accommodation is nil, when the patient is asked to look at a distant fixation point.
Dynamic retinoscopy is used to estimate accommodative response. Hence, in dynamic retinoscopy the patient is asked to fixate at a near fixation target. The near fixation target may be words, shapes etc. which is appropriate for the age of the patient. These can be fitted onto the streak retinoscope. Dynamic retinoscopy also provides refractive information, adequacy of cycloplegia and may help to decide amblyopic therapy.
AUTOREFRACTOMETRY
It is a very precise objective automated electronic method to measure refractive error. They can be operated either manually or in automatic mode. The autorefractometers work on the basic principal of retinoscopy and Badal optometer. The patient is asked to look at a visible coloured fixation target. The refractionist aligns the instrument panel to sharp focus and pushes a button which initiates the process of estimation of refractive error. An inbuilt microprocessor analyses the focal power of emitted rays from the patient’s eye and processes it into accurate refractive error correction in diopters in few seconds. Usually 3 readings in each eye are averaged to give a final assessment in a print out. Modern autorefractometers are very quick and accurate.
PHOTOREFRACTION
It is an objective method of measurement of refractive errors by employing photographic method. It is ideal in patients who cannot maintain fixation, e.g. in infants and mentally unstable. However, it is less accurate than retinoscopy and autorefractometry.
SUBJECTIVE REFRACTION
The assessment of objective methods are further refined by subjective methods. If cycloplegia is used in objective (retinoscopy) method the subjective method is tested after the effect of cycloplegia wears off. Hence, often it is termed as postcycloplegic test/postmydriatic test.