Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

Errors of Refraction 131

It is not uncommon to detect significantly good distant vision in presence of high mixed astigmatism. This surprising fact occurs because the circle of least diffusion falls upon or near the light sensitive layer of the retina.

SYMPTOMS

i.Diminished distant visual acuity–This is least in mixed astigmatism

ii.Eye strain or asthenopia–It is often more common and worse in lower degree of astigmatism than the higher one. This is due to effort of the eye to accommodate so that the circle of least diffusion falls upon or near the retina

iii.Headache and eyeache

iv.Blurring of letters while reading.

DIAGNOSIS

Objective

i.Retinoscopy

ii.Keratometry

iii.Placido keratoscopic disc–This test reflects irregularities on the corneal surface. The examiner looks through a hole in the centre of the disc, with alternatingly painted black and white circles, at the corneal image reflected from a light behind the patient.

iv.Computerised corneal topography.

Subjective

i.Astigmatic fan (Fig. 9-7)–It is used to measure the strength of the cylindrical lens and it’s axis. The end point of cylindrical lens correction is achieved when the outline of the whole fan becomes equally clear and sharp. The axis of the cylinder is at right angles to the line which was initially most clearly defined.

ii.Stenopaeic slit test–In this test the slit is rotated till the patient sees the chart clearest. Then required spherical lenses are added. Then the slit is rotated 90° and again vision is adjusted

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a.Isoanisometropia–Here refractive status of both the eyes are either hypermetropic or myopic.

b.Antimetropia–Here refractive status of one eye is myopia and the other is hypermetropia.

II.Based on dioptric difference–Patients symptoms vary significantly with the degree of dioptric difference between the two eyes

a.Low = 0 to 2.50D

b.High = 2.50D to 6.00D

c.Very high = > 6.00D

In low anisometropia binocular vision is easily achieved and full optical correction is well-tolerated. A difference of 0.25D produces 0.5% difference in size of the retinal images of the two eyes. A difference of upto 5% of retinal image size is well-tolerated and two images can be fused without strain. So, a dioptric difference of more than 2.50D will lead to binocular vision problem and eye strain. Often the vision becomes uniocular and the worse eye becomes lazy, i.e. amblyopic and convergent if corrective measures are not undertaken in childhood.

OPTICAL PROBLEMS/DIFFICULTIES OF

ANISOMETROPIA

a.Binocular vision: > 2.50D of difference in dioptric strength between the two eyes leads to eye strain due to effort of fusion. Binocular vision is not possible with spectacle correction if the anisometropia is > 4.00D.

b.Amblyopia: Often a difference of > 2.00D in hypermetropic patient is sufficient to induce amblyopia in the more hypermetropic eye. However, in myopic patients with anisometropia amblyopia is less likely to develop unless the difference is very significant.

c.Squinting: Convergent squint in childhood and divergent squint in adults.

d.Diplopia: It develops due to difference in image size of > 8%.

e.A difference in stimulus to the accommodation between the two eyes.

Errors of Refraction 135

SYMPTOMS

a.Blurring of vision for both distance and near

b.History of cataract operation or injury

c.Patient may wear very thick convex glass.

SIGNS

If extracapsular (ECCE)/intracapsular (ICCE) cataract extraction is done:

•Vision is finger counting at few feet without glasses.

•Upper limbus—Presence of linear scar with or without sutures (10’0’ nylon—Usually interrupted/continuous) may be seen.

•Anterior chamber depth–Deep.

•Iridodonesis, i.e. tremulousness of iris due to lack of support.

•Peripheral button hole iridectomy (PBHI) may be seen.

•Pupil–Jetblack due to loss of IIIrd and IVth Purkinje image (in ICCE) and IIIrd image (in ECCE).

•Ophthalmoscopy–The optic disc is very small.

TREATMENT

Spectacles

Spectacles are usually advised after 6 weeks of surgery. The time is required for complete wound healing and stabilisation of refractive error particularly astigmatism. If the patient was previously emmetropic usual prescription for glasses will be roughly as follows:

Glasses advised Right Eye = +10.00DSPH with +2.00 DCYL 180° (astigmatism with-the-rule)

Add: +3.00DSPH for near vision

The +3.00DSPH near addition is due to loss of accommodation due to absence of the lens. Aspherical lenticular resin lens (CR– 39) is ideal for aphakic patients than crown glass lens.

Optical disadvantages of aphakic glasses:

i.Image magnification is 25–30%. So, in uniocular aphakia binocular vision is not possible due to aniseikonia. Hence, to avoid diplopia (where phakic eye vision is > 6/36),

136Textbook of Visual Science and Clinical Optometry

balanced (+10.00D) or frosted glass is dispensed for the phakic eye. The image magnification causes objects to appear closer to the eye then they are really.

ii.Spherical aberration–Pincushion distortion (see chapter - 7)

iii.Jack-in-the-box phenomenon–Due to prismatic aberration a ring scotoma is produced all around the edge of the lens. This causes an unseen object to suddenly pop up in front of the eyes or disappear into the ring scotoma, as the patient moves his eyes.

iv.Restricted visual field.

v.Lack of physical coordination which results from image magnification, restricted visual field, pincushion distortion and Jack-in-the-box phenomenon.

vi.Thick, heavy lenses are cosmetically deficient.

vii.Loss of lens often leads to coloured vision due to lack of natural filter offered by the lens. UV-A (315–400 nm) protection offered by the lens is lost.

Contact Lens

Advantages

All the disadvantages of glasses are neutralised:

i.Image magnification is 6–7%. Hence, binocular vision is possible in uniocular aphakia.

ii.Aberrations are lessened, i.e. pincushion distortion, etc.

iii.Increased visual field.

iv.Better physical coordination.

v.Cosmetically attractive.

Disadvantages

i.Inability of elderly patients to insert and remove contact lens efficiently.

ii.Foreign body sensation.

iii.Additional glasses required for reading correction. However, bifocal contact lenses are available and becoming increasingly popular.

Errors of Refraction 137

Secondary IOL Implantation

Advantages

i.Image minification is 0–2%. Hence, quick return to binocularity is achieved due to minimum aniseikonia.

ii.Absence of aberrations.

iii.Restoration of normal peripheral field of vision.

iv.Excellent physical coordination.

Disadvantages

It is significantly reduced to usual complications following primary IOL implantation surgery, e.g. corneal decompensation, infection, astigmatism, etc.

Secondary IOL implantation in aphakia may be;

a.AC IOL implantation in aphakia following intracapsular cataract extraction (ICCE )–If the aphakic eye was emmetropic earlier, AC IOL of +18.00D strength is required to focus the image on the retina.

b.PC IOL Implantation in aphakia following extracapsular cataract extraction (ECCE) - +20.00D strength of PC IOL is required to focus the image on the retina.

c.Hyperopic LASIK.

PSEUDOPHAKIA

Pseudophakia means replacement of the natural crystalline lens by a synthetic intraocular lens (IOL).

MATERIALS OF IOL

•Polymethyl Methacrylate (PMMA)

•Silicon

•Acrylic.

CALCULATION OF IOL POWER

It is done by:

•Axial length measurement by A-scan

•Keratometry

•Standard calculation formulas.

Errors of Refraction 139

Residual Refractive Error

Residual refractive error in pseudophakia consists of;

•Spherical error–Accurate biometry overcomes this error.

•Astigmatism–Phacoemulsification results in astigmatism against-the- rule, whereas PC IOL implantation with sutures results in astigmatism with-the-rule.

•Loss of accommodation–However, nowadays multifocal IOL’s,

accommodative IOL’s are increasingly available to correct this error.

After phacoemulsification glasses may be advised after only one week and after small incision cataract surgery (SICS) glasses may be advised after 3 weeks. However, after ECCE with PC IOL implantation glasses are advised only after 6 weeks.

PRESBYOPIA

Presbyopia is defined as the slow, gradual, age-related and irreversible decline in the physiological process of amplitude of accommodation, i.e. recession of near point beyond comfortable near work and reading distance. The condition becomes first noticeable, clinically, usually between the ages of 38 and 42.

AETIOLOGY

Lenticular Theories

Fincham theory and Hess-Gullstrand theory

•Increase in hardness and consequent decline in plasticity of the lens nucleus as part of an ageing process.

•Decline in elasticity of the lens capsule.

These two factors contribute to requirement of more energy to

deform the lens material with increasing age. The lens becomes more resistance to change in the shape age as advances.

Geometric theory

•Increase in size and curvature of the lens

•Change in orientation of the zonules due to shifting of zonule insertions. Following ciliary muscle contraction the zonular relaxation leads to less radial force of tension on the surface of the lens capsule.

140 Textbook of Visual Science and Clinical Optometry

•So, the zonular relaxation has lesser effects on the change in shape of the lens.

Extralenticular Theory

•Weakening of the ciliary muscle (Duane theory)

•Progressive deterioration in the elastic components of zonules and ciliary body.

The aetiology of presbyopia possibly is multifactorial. In recent years restoration of accommodation by implantation of an accommodative IOL in the capsular bag is demonstrated clinically. So, lenticular origin of presbyopia (Fincham theory) along with geometric theory seems most acceptable and scientific.

SYMPTOMS

•Blurring of vision particularly small prints, in the evening or in dim light, at the normal reading distance.

•Eyestrain and headache after close work.

•Reading materials are held at a distance further away from the normal position.

TREATMENT

Spectacles

Prescription of appropriate corrective convex spherical lens to bring the near point within normal reading distance. The following steps are followed;

i.Correction of distance power of each eye separately (static refraction).

ii.Addition of appropriate convex lens to both eyes for near work. Minimum near correction glass is +0.75DSPH. Usually following standard notation table is followed while correcting presbyopia (Table 9-1).

Contact Lens

Various types of bifocal contact lenses are available for presbyopic correction.