Ординатура / Офтальмология / Английские материалы / Comprehensive Ophthalmology_Khurana_2007

In low degrees of refractive errors the shadow (red reflex) seen in the pupillary area is faint and moves rapidly with the movement of the mirror; while in high degrees of ametropia it is very dark and moves slowly. In the presence of astigmatism, when the axis does not correspond with the movement of the mirror, the shadow appears to swirl around.

Use of cycloplegics in retinoscopy

Cycloplegics are the drugs which cause paralysis of accommodation and dilate the pupil. These are used for retinoscopy, when the examiner suspects that accommodation is abnormally active and will hinder the exact retinoscopy. Such a situation is encountered in young children and hypermetropes. When retinoscopy is performed after instilling cycloplegic drugs it is termed as wet retinoscopy in converse to dry retinoscopy (without cycloplegics). The commonly employed cyclopegics are as follows:

1.Atropine is indicated in children below the age of 5 years. It is used as 1 percent ointment thrice daily for 3 consecutive days before performing

retinoscopy. Its effect lasts for 10 to 20 days.

2.Homatropine is used as 2 percent drops. One drop is often instilled every 10 minutes for 6 times and the retinoscopy is performed after 1 to 2 hours. Its effect lasts for 48 to 72 hours. It is used for most of the hypermetropic individuals between 5 and 25 years of age.

3.Cyclopentolate is a short acting cycloplegic. Its effect lasts for 6 to 18 hours. It is used as 1 percent eyedrops in patients between 8 and 20 years of age. One drop of cyclopentolate is instilled after every 10-15 minutes for 3 times (Havener’s recommended dose) and the retinoscopy is performed 1 to 1/½ hours or 60 to 90 min. later, after estimating the residual accommodation which should not exceed one dioptre.

4.Only mydriatic (10% phenylephrine) may be needed in elderly patients when the pupil is

narrow or media is slightly hazy.

Salient features of the common cycloplegic drugs are summarized in Table 23.1.

Note: The mydriatics should be used with care in adults with shallow anterior chamber, owing to the danger of an attack of narrow-angle glaucoma. In older people, mydriasis should be counteracted by the use of miotic drug (2% pilocarpine).

Observations and inferences

Depending upon the movement of the red reflex (Fig. 23.13) when a plane mirror retinoscope is used at a distance of 1 metre) the results are interpreted as:

1.No movement of red reflex indicates myopia of 1D.

2.With movement of red reflex along the movement of the retinoscope, indicates either emmetropia or hypermetropia or myopia of less than 1 D.

3.Against movement of red reflex to the movement of the retinoscope implies myopia of more than 1 D.

Above assertions can be easily remembered from

the Fig. 23.14.

Neutralization

When the red reflex moves with or against the movement of retinoscopy we do not exactly know the amount of refractive error. However, when the red glow in the pupil does not move then we know for certain that patient has myopia of 1D. Therefore, to estimate the degree of refractive error, the movement of red reflex is neutralized by addition of increasingly convex (+) spherical lenses (when the red reflex was moving with the movement of plane mirror) or concave (–) spherical lenses (when the red reflex was moving against the movement of plane

mirror). When a simple spherical error alone is present, the movements of red reflex will be neutralized in both vertical as well as the horizontal meridia. However, in the presence of an astigmatic refractive error, one meridian is neutralized by adding appropriate cylindrical lens with its axis at right angle to the meridian to be neutralized. It is important to note that sometimes, especially, when pupil is dilated, two light reflexes—one central and other peripheral—may be seen. Under such circumstances one should neutralize the central glow because the central parts of cornea and lens are more important in forming the image on the retina.

The end point of retinoscopy

With simple plane mirror retinoscope the end point of retinoscopy is neutralization of red reflex in all the meridia, i.e., either no movement or just reversal of the movement.

With a streak retinoscope at the end point streak disappears and the pupil appears completely illuminated or completly dark (Fig. 23.13).

Problems in retinoscopy

Certain difficulties encountered during the procedure of retinoscopy are summarized below:

1. Red reflex may not be visible or may be poor.

This may happen with small pupil, hazy media and high degree of refractive error. In most cases, this difficulty is overcome by causing mydriasis and/or use of converging light with concave mirror retinoscope.

2.Changing retinoscopy findings are observed due to abnormallyactive accommodation and is corrected by use of cycloplegia.

3.Scissors shadows may sometimes be seen in patients with regular astigmatism with dilated pupils. Mostly this difficulty is diminished with the undilated pupil.

4.Conflicting shadows moving in various directions in different parts of the pupillary area are seen in patients with irregular astigmatism.

5.Triangular shadow may be observed in patients with conical cornea (keratoconus), with its apex at the apex of cone. On moving the mirror the triangular reflex appears to swirl around its apex (yawning reflex).

Static versus dynamic retinoscopy

Static retinoscopy refers to the procedure performed without active use of accommodation (as described above). Dynamic retinoscopy implies when the procedure is performed for near vision with active use of accommodation by the patient. However, usefulness of performing dynamic retinoscopy has not yet been established in refraction.

Rough estimate of refractive error after retinoscopy

Objectively a rough estimate of error of refraction is made by taking into account the retinoscopic findings,

deductions for distance (e.g., 1D for 1 m and 1.5 D when retinoscopy is performed at 2/3rd m distance) and deduction for the cycloplegic when used (e.g., 1 D for atropine, 0.5 D for homatropine and 0.75 D for cyclopentolate).

Thus briefly

Amount of refractive error = Retinoscopic findings – deduction for distance – tonus allowance for cycloplegic drug used.

It is customary to do retinoscopy both vertically and horizontally and note the values separately (Fig. 23.15). In Fig 23.15 A, X denotes retinoscopy value along horizontal meridian and Y denotes the value along the vertical meridian.

When retinoscopy values along horizontal and vertical meridia are equal then there is no astigmatism and a spherical lens is required to correct the refractive error. For example: When retinoscopic finding is 7 D with the procedure preformed at 1m distance using atroprine as cycloplegic than appropriate refractive error will be: 7D-1D (for distance) – 1D (tonus allowance for atroprine) = 5D (Fig. 23.15B)

When retinoscopy values along horizontal and vertical meridia are unequal, then it denotes presence of astigmatism which is corrected by a cylindrical lens alone or in combination with a spherical lens (Figs. 23.15C and D).

The subjective verification of refraction is a must even after autorefractometry.

KERATOMETRY

The ‘keratometry’ or ‘ophthalmometry’ is an objective method of estimating the corneal astigmatism by measuring the curvature of central cornea. The keratometry readings are not of much value in routine refraction for prescribing glasses; but are of utmost value for prescribing contact lenses and for calculating the power of intraocular lens to be implanted.

Principle. Keratometer is based on the fact that the anterior surface of the cornea acts as a convex mirror; so the size of the image produced varies with its curvature. Therefore, from the size of the image formed by the anterior surface of cornea (first Purkinje image), the radius of curvature of cornea can be calculated. The accurate measurement of the image size is obtained by using the principle of visible doubling.

Types. Two types of keratometers used in practice are Javal-Schiotz model and Bausch & Lomb model.

The Javal-Schiotz model keratometer consists of two illuminated ‘mires’ (A and B) fixed on a rotatable circular arc (C) and a viewing telescope T (Fig. 23.17). The double images (aa1 and bb1) of the mires (A and B) are formed on the cornea. Keratometry readings are obtained by coinciding the images a1 and b as shown in Figure 23.18. The readings are noted first in the horizontal meridian and then the arc is rotated by 90o and the readings are noted in the vertical meridian.

Bausch & Lomb keratometer (Fig. 23.19). In it, the ‘mires’are in the form of circles (Fig. 23.20). With this keratometer the radius of curvature of cornea in horizontal and vertical meridia can be measured simultaneously without rotating the mires.

SUBJECTIVE REFRACTION

Subjective refraction is meant for finding out the most suitable lenses to be prescribed. It should always be carried out after getting a rough estimate of the refractive error by the objective refraction as described above. When a cycloplegic has been used the subjective refraction (postmydriatic test) should be carried out preferably after 3 to 4 days (when homatropine or cyclopentolate is used) and 3 weeks (when atropine is used).

Fig. 23.17. Basic structure of Javal and Schiotz keratometer.

Fig. 23.18. Mires during keratometry with Javal-Schiotz Keratometer.

Fig. 23.19. Bausch and Lomb Keratometer.

Fig. 23.20. Mires during keratometry with Bausch & Lomb keratometer.

The technique of subjective refraction requires the patient’s cooperation in arriving at the proper estimation of the refractive error. The proper subjective refraction includes three steps:

I.Subjective verification of refraction.

II. Subjective refinement of refraction, and III. Subjective binocular balancing.

I. The subjective verification of refraction

The subjective verification of refraction can be performed by: the ‘trial-and-error’ method. For this, the patient is seated at a distance of 6 metres from the Snellen’s vision chart. A trial frame is put on the face of the patient and the visual acuity is noted for both the eyes, separately. Then an occluder is put in front of one eye and the appropriate lens combination (as indicated by retinoscopy or automated refractometry) is placed in front of the other eye. By increasing or decreasing the power of lens the most suitable spherical lens is chosen (the strongest convex lens and the weakest concave lens providing best vision should be chosen in patients with hypermetropia and myopia, respectively). Then the axis of the cylinder and finally its strength should by finalized using the same ‘trial-and-error’ method. The similar procedure is repeated for the second eye.

II. Subjective refinement of refraction

The most suitable combination of lenses chosen after the subjective verification of refraction is refined

before the final prescription is made. It is always better to first refine the cylinder and then sphere.

1. Refining the cylinder. Cylinder can be refined by either use of Jackson’s crosscylinder (more commonly) or by astigmatic fan test.

i. Jackson’s crosscylinder test. It is used to verify the strength and axis of the cylinder prescribed. The crosscylinder is a combination of two cylinders of equal strength but with opposite sign placed with their axes at right angles to each other and mounted in a handle (Fig. 23.21). The commonly used crosscylinders are of ±0.25 D and ±0.5 D.

Verification of strength of the cylinder. To check the power of the cylinder, the crosscylinder of

± 0.25 D is placed with its axis parallel to the axis of the cylinder in the trial frame first with the same sign and then with opposite sign. In the first position, the cylindrical correction is enhanced by 0.25 D and in the second it is diminished by the same amount. When the visual

acuity does not improve, in either of the positions the power of cylinder in trial frame is correct. However, if the visual acuity improves in any of the positions a corresponding correction should be made and reverified till final correction is attained.

Verification of axis of the cylinder. Crosscylinder (±0.5 D) is placed before the eye with its axis at 45o to the axis of cylinder in trial frame (first with –0.5 D cylinder and then +0.5 D cylinder or viceversa) and the patient is asked to tell about any change in the visual acuity. If the patient notices no difference between the two positions, the axis of the correcting cylinder in the trial frame is correct. However, if the visual improvement is attained in one of the positions, a ‘plus’ correcting cylinder should be rotated in the direction of the plus cylindrical components of the crosscylinder

(and vice-versa). The test is then repeated several times until the neutral point is reached.

ii. The astigmatic fan test. It is used to confirm the cylindrical correction. The astigmatic fan consists of a dial of lines radiating at 10° interval to one another (Fig. 23.22). In this test the patient is asked to see the fan after fogging by +0.5 D added over and above the best suitable combination of lenses chosen. The stigmatic patient will see all the lines equally clear. In the presence of astigmatism, some lines will be seen more sharply defined. The concave cylinder is then added with its axis at right angles to the clearest line until all the lines are equally sharp.

iii. A stenopaeic slit-test. Though not practically used now, this test also helps in checking the

correction of astigmatism. The 1-mm wide stenopaeic slit (Fig. 23.23) when placed in front of the eye allows clearest vision when it is rotated into the axis of astigmatism and the refraction will then be indicated by the strongest convex lens which allows full vision in this axis and again in the axis perpendicular to it.

2. Refining the sphere. The spherical correction is refined after refining the cylinder power and axis. Refining of the sphere is done by using following tests:

i.The fogging technique. After the cylinder power and axis have been refined, the eye to be tested is fogged by insertion of about +2D spherical lens in myopic patients and about +4D in hypermetropic patients over the previously verified sphere. The patient is instructed to see the distant test types through this, and gradually the additional convex lens is reduced (by about 1/2 D at a time) until full vision is restored. This method is more useful in hypermetropia.

ii.Duochrome test. It is based on the principle of chromatic aberration. In this, the patient is asked to read the red and green letters. In an emmetropic eye the green rays are focused slightly anterior and red rays slightly posterior to the retina. Therefore, to an emmetropic patient letters of both colours look equally sharp. When the patient tells that he or she sees red letters more clearly than the green, it indicates that he or she is slightly myopic. His or her spherical lenses should be adjusted such that he or she sees letters of both colours with equal clarity.

iii.Pin-hole test. It helps in confirming whether the optical correction in the trial frame is correct or not.

Fig. 23.23. Stenopaeic slit.

Fig. 23.24. Pin-hole.

An improvement in visual acuity while looking through a pin hole (Fig. 23.24) indicates that optical correction in the trial frame is incorrect.

III. Subjective binocular balancing

The final step in the subjective refraction is binocular balancing—a process sometimes known as ‘equalizing the accommodative effort’ or ‘equalization of vision’. This allows both eyes to have the retinal image simultaneously in focus. The details of the techniques of binocular balancing are beyond the scope of this book.

Correction for near vision

Correction for near vision is indicated usually after the age of 40 years. When the distance vision has

been satisfactorily corrected, the visual acuity at working distance of the patient should be estimated using any of the near vision charts (Jaeger’s chart or Snellen’s reading test types or number points types standardized by the faculty of ophthalmologists, N5 to N48). In case near vision is defective, a suitable convex lens addition (tested separately for each eye) should be made over the distant correction. The near correction added should be such that about one-third of the amplitude of accommodation should remain as reserve. In general, it is better to undercorrect than to overcorrect the presbyopia (also see page 42).

RELATED QUESTIONS

LIGHT AND GEOMETRICAL OPTICS

What is the wavelength of visible spectrum of the

light?

Between 390 and 700 nm.

Which light rays are absorbed by the cornea and

crystalline lens of the eye?

Cornea absorbs rays having wavelength shorter than 295 nm and the crystalline lens of the eye absorbs rays having wavelength shorter than 350 nm.

White light consists of how many colours?

Seven, viz. violet, indigo, blue, green, yellow, orange and red (VIBGYOR).

What do you mean by reflection of light?

Reflection of light is a phenomenon of change in the path of light rays without any change in the medium.

What are the features of an image formed by a

plane mirror?

It is: (i) virtual, (ii) erect and laterally inverted, (iii) of the same size as object, and (iv) at the same distance behind the mirror as the object is in front.

What do you mean by refraction of light?

Refraction of light is a phenomenon of change in the path of light when it goes from one medium to another.

Describe the features of the images formed by a concave mirror for different positions of the

object.

See Table 3.1

What is total internal reflection?

When a ray of light travelling from an optically denser medium to an optically rarer medium is incident at an angle greater than the critical angle of the pair of media in contact, the ray is totally reflected back into the

denser medium. This phenomenon is called total internal reflection.

What is the critical angle ?

Critical angle refers to the angle of incidence in the denser medium corresponding to which angle of refraction in the rarer medium is 90°.

What do you mean by Sturm conoid focal interval

of Sturm and circle of least diffusion?

Sturm conoid refers to the configuration of the light rays refracted through an astigmatic (toric) surface. The parallel rays of light when refracted through a toric surface are not focused at one point but form two focal lines. Distance between the two lines is called focal interval of Sturm. Circle of least diffusion is formed between these two lines.

Why a patient with mixed astigmatism has

comparatively better vision?

Because in such patients the circle of least diffusion is formed on the retina.

OPTICS OF THE EYE

What is a ‘reduced eye’ ?

The focusing system of the eye is composed of cornea, aqueous humour, crystalline lens and vitreous humour, the optics of which, otherwise is very complex. However, Listing has chosen a simple data to understand the optics of eye. This is called Listing’s reduced eye. Its cardinal points are:

Single nodal point situated (in the posterior part of crystalline lens) is 7.2 mm behind the anterior surface of cornea.

Anterior focal point is 15.7 mm in front of the anterior surface of cornea.

Posterior focal point (on the retina) is 24.4 mm behind the anterior surface of cornea.

Total dioptric power is about +60D.

What is nodal point of the eyeball?

It is the optical centre of the entire focusing system of the eye consisting of cornea, aqueous and lens when considered as one lens.

What is optical axis of the eyeball ?

It is a line passing through the centre of cornea and centre of the lens which meets the retina on the nasal side of fovea.

What is visual axis ?

It is a line joining the fixation object, nodal point and the fovea.

What is fixation axis ?

It is the line joining the fixation point and the centre of rotation of the eye.

What is visual angle ?

It is the angle subtended by an object on the nodal point of the reduced eye.

What are angles alpha, gamma and kappa of the eyeball ?

1.Angle alpha is the angle formed between the optical axis and visual axis at the nodal point of the eyeball.

2.Angle gamma is the angle formed between the optical axis and fixation axis at the centre of rotation of the eyeball.

3.Angle kappa is formed between the visual axis and control pupillary line. A positive angle kappa results in pseudoexotropia and a negative angle kappa is seen in esotropia.

What is the refractive power of the eyeball (total),

of the cornea and the crystalline lens.

Total refractive power of the eyeball is about +60D; out of this +44 D is contributed by the cornea and about +10D by the crystalline lens.

What are the refractive indices of the media of

the eye?

Refractive indices of the media of the eye are as

REFRACTIVE ERRORS

What is emmetropia ?

Emmetropia (optically normal eye) is a state of refraction when the parallel rays of light coming from infinity are focused at the sensitive layer of retina with accommodation at rest.

Define ametropia.

Ametropia (a condition of refractive error) is defined as a state of refraction when the parallel rays of light coming from infinity are focused either in front or behind the retina. It includes myopia, hypermetropia and astigmatism.

Define hypermetropia (long-sightedness).

Hypermetropia is the refractive state of the eye wherein parallel rays of light coming from infinity are focused posterior to the retina, with accommodation at rest.

What is the refractive status of the eye at birth?

At birth the eyeball is relatively short and thus most infants are born with +2 to +3 D hypermetropia. This is gradually reduced and by the age of 5 to 7 years usually the eye becomes emmetropic.

What are aetiological types of ametropic refractive errors?

1.Axial ametropia: There is abnormal axial length of the eyeball, too long in myopia and too short in hypermetropia.

2.Curvatural ametropia: There is abnormal curvature of the cornea or lens or both; too strong in myopia and too weak in hypermetropia.

3.Index ametropia: There is abnormal refractive index of the media; too high in myopia and too low in hypermetropia.

4.Positional ametropia: Forward displacement of the lens causes myopia and backward displacement results in hypermetropia.

What are the components of the hypermetropia?

Total hypermetropia = latent + manifest (facultative + absolute)

1.Total hypermetropia: It is the total amount of refractive error estimated after complete cycloplegia with atropine.

2.Latent hypermetropia is that which is corrected by inherent tone of the ciliary muscle.

3.Manifest hypermetropia: It is the remaining portion of total hypermetropia, which is not corrected by the ciliary tone.

Name the most common factor responsible for

myopia and hypermetropia.

Too long axial length and too short axial length are responsible for myopia and hypermetropia, respectively.

Name the complications which may occur in nontreated cases of hypermetropia.

1.Recurrent styes and blepharitis,

2.Accommodative convergent squint,

3.Amblyopia.

Define aphakia.

Aphakia literally means absence of the crystalline lens from the eye. However, from the optical point of view, it may be considered as a condition in which the lens is absent from the pupillary area and does not take part in refraction.

Enumerate the refractive changes which occur in an aphakic eye.

1.Eye becomes highly hypermetropic.

2.Total power of the eye is reduced to +44Ds from +60 Ds

3.Anterior focal distance becomes 23 mm (from 15

mmin normal phakic eye)

4.Posterior focal distance becomes 31 mm (from 24

mmin normal phakic eye).

Name the various modalities for correction of aphakia and enumerate advantages and

disadvantages of each.

1. Spectacles

Advantages: It is cheap, easy and safe method of correcting aphakia.

Disadvantages: (i) Image is magnified by 30 percent, so not useful in unilateral aphakia (produce diplopia), (ii) problems of spherical and chromatic aberrations may be troublesome, (iii) field of vision is limited, (iv) prismatic effect of thick glasses causes, ‘roving ring scotoma’ (v) cosmetic blemish, especially in young aphakics.

2. Contact lenses

Advantages: (i) Less magnification (5%) of the image, (ii) elimination of aberrations and prismatic effect of thick glasses, (iii) wider and better field of vision, (iv) cosmetically better accepted by young persons.

Disadvantages: (i) More cost, (ii) cumbersome to wear, especially in old age and in childhood, (iii) corneal complications may occur.

3. Intraocular lens implantation

It is the best available method of treatment. Advantage: It offers all the advantages which the contact lenses offer over the spectacles. In addition, the disadvantages of contact lenses are also taken care of.

Disadvantages: It requires more skilled surgeons and costly equipments.

4. Refractive corneal surgery

It is still under trial and includes keratophakia and epikeratophakia.

What are fundus findings in a patient with high

hypermetropia?

Fundus examination in a patient with high hypermetropia may show:

–Pseudopapillitis

–shot silk appearance of the retina.