Optics, Errors of Refraction and Anomalies of Accommodation

Optics, Errors of Refraction and Anomalies of Accommodation

LIGHT AND GEOMETRICAL OPTICS

• Light is the visible portion (400-700 nm) of the electromagnetic spectrum
• Cornea absorbs rays shorter than 295 nm Lens absorbs rays shorter than 350 nm.
• Critical angle refers to the angle of incidence in the denser medium, corresponding to which angle of refraction in the rarer medium is 90°.
• Prism produces displacement of the objects seen through it towards the apex

OPTICS OF THE EYE

• Total dioptric power is about +60 D
• The principal point lies 1.5 mm behind the anterior surface of cornea
• Nodal point is situated 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
• Anterior focal length is 17.2 mm (15.7 + 1.5)
• Posterior focal length is 22.9 mm (24.4 – 1.5)

Axis of the eye

1. Optical axis. It is the line passing through centre of the cornea, centre of the lens and meets the retina on the nasal side of the fovea.

2. Visual axis. It is the line joining the fixation point, nodal point and the fovea.
3. Fixation axis. It is the line joining the fixation point and the centre of rotation.

Angles of the eye

1. Angle alpha. It is formed between the optical axis and visual axis at the nodal point.
2. Angle gamma. It is formed between the optical axis and fixation axis at the centre of rotation of the eyeball.
3. Angle Kappa. It is formed between the visual axis and central pupillary line. A positive angle kappa results in pseudoexotropia and a negative angle kappa in pseudoesotropia.
4. Visual angle is the angle subtended by an object at the nodal point.

Refractive power of the eye

• Total : +60D
• Cornea : +44D
• Lens: +16D

Refractive indices of the media of the eye

• Cornea : 1.37
• Aqueous humour : 1.33
• Crystalline lens : 1.39
• Vitreous humour : 1.33

Atractive indices of the other media

• Air : 1
• Water : 1.33
• Tear fluid : 1.338
• HEMA : 1.43
• PMMA : 1.496
• Spectacle, crown glass : 1.52
• Flint glass : 1.61
• Diamond : 2.52.

ERRORS OF REFRACTION

Emmetropia (optically normal eye)

• An emmetropic eye is slightly hypermetropic for red rays and myopic for green rays.
• At birth, eye is hypermetropic by +2 to +3 D and usually becomes emmetropic by the age of 5 to 7 years.

Hypermetropia (long sightedness)

Axial hypermetropia due to decreased axial length of the eye ball is the most common cause. One mm decrease in axial length causes three dioptres of hypermetropia.
Curvatural hypermetropia occurs due to flattening of the cornea. One mm increase in the radius of curvature of cornea causes six dioptres of hypermetropia.
Index hypermetropia occurs due to increase in the refractive index of the cortex of the crystalline lens as seen in early cortical cataract and in diabetics.
Positional hypermetropia results due to backward displacement of the lens.

Components of hypermetropia

• Latent hypermetropia. It is corrected by the ciliary tone. It is detected when refraction is carried out after abolishing the ciliary tone with atropine.
• Manifest hypermetropia. It consists of two components.
– Facultative hypermetropia. It can be overcome by an effort of accommodation.
– Absolute hypermetropia. It cannot be overcome by an effort of accommodation.

Optical change In aphakia

• Eye becomes highly hypermetropic.
• Total power of eye is reduced from +60 D to +44 D.
• Anterior focal point becomes 23.2 mm in front of the cornea.
• Posterior focal point is 31 mm behind the cornea.
• Loss of accommodation.

Disadvantages of aphakic glasses

• Produce image magnification by 25 to 30 percent
• Pincushion distortion due to spherical aberration is the most frequently noticed problem
• Limited field of vision
• Roving ring scotoma (Jack in the box phenomenon)
• Chromatic aberration

Intraocular lens In aphakia

• Exact power is calculated by biometry using keratometer and A-scan ultrasound.
• Standard power of +20 D of posterior chamber IOL is equivalent to +10 DS spectacles.
• All disadvantages of aphakic glasses are eliminated by intraocular lens use.
• Average weight of an IOL in air is 15 mg and in aqueous humour is 5 mg.
• Power of an IOL in air is more (+60D) than that in the aqueous humour (+20D)
• Equivalent power of an anterior chamber IOL is about +3D less than that of the posterior chamber IOL
• Further details of IOL.

Surgical treatment of hypermetropia:

Hyperopic LASIK is effective in correcting hypermetropia upto +4D.
Other procedures includes hyperopic PRK, Holmium laser thermoplasty and conductive keratoplasty (CK).

Myopia

• Axial myopia. One mm increase in axial length produces myopia of 3 D.
• Curvatural myopia. One mm decrease in radius of curvature of cornea produces myopia of 6 D.
• Congenital myopia. Present since birth, usually unilateral, usually the error is of about 8-10 D which mostly remains constant.
• Simple or developmental myopia. It is the commonest type of myopia. It does not progress after adolescence; and the error usually does not exceed -6 to -8 D.
• Pathological (degenerative) myopia. It results from a rapid axial growth of eyeball (posterior to equator) which is strongly linked with heredity; since it is familial and more common in certain races (Chinese, Jews, Japanese). Fundus examination may reveal; patches of chorioretinal degeneration, and Foster-Fuch’s spot (dark circular area due to intrachoroidal haemorrhage at the macula).

Complications of pathological myopia

• Complicated cataract
• Choroidal haemorrhage
• Tears and haemorrhages in the retina
• Vitreous haemorrhage
• Retinal detachment.
• Diseases associated with myopia are microphthalmos, congenital glaucoma, microcornea, retrolental fibroplasia, Marfan’s syndrome, Turner’s syndrome, and Ehlers-Danlos syndrome.

Optics, Errors of Refraction and Anomalies of AccommodationSurgical treatment of myopia

• Radial keratotomy. Multiple radial incisions are given in the periphery of cornea (leaving central 4 mm optical zone) in order to flatten the curvature of cornea. (Not done now-a-days).

• Photorefractive keratectomy (PRK). In it reshaping of the cornea is done with excimer laser. (Not preferred presently).

• Laser assisted in situ keratomileusis (LASIK) is presently the preferred surgical technique for correcting myopia of up to -12D. In it, the midstromal tissue is ablated with excimer laser after raising a 130-160 micron thick flap of anterior corneal tissue.

• Removal of clear crystalline lens by extracapsular cataract extraction (preferably by phacoemulsification) with IOL implantation of appropriate power is being recommended for surgical treatment of myopia of >12D.

• Phakic intraocular lenses and intracomeal ring implantation are the other surgical techniques being tried for correction of myopia.

• Orthokeratology a non-surgical reversible method of molding the cornea with overnight wear unique rigid gas permeable contact lenses, is also being considered for correction of myopia upto -5D. It can be used even in the patients below 18 year of age.

Astigmatism

Depending upon the axis and the angle between the two principal merldla

• With-the-rule astigmatism, wherein the vertical meridian is more curved than the horizontal and the two principal meridia are at right angle to each other. Thus, correction of with-the-rule astigmatism will require a concave cylinder at 180 ± 20° or a convex cylinder at 90 ± 20°.

• Against-the-rule astigmatism. It is just reverse of with-the-rule astigmatism, i.e., herein the horizontal meridian is more curved than the vertical.

• Oblique astigmatism. Herein the two principal meridia are at right angle to each other but these are not the horizontal and vertical, e.g., these may be 45° and 135°.

• Bioblique astigmatism. In this condition the two principal meridia are not at right angle to each other e.g., these may be 30° and 100°.

Depending upon the type of refractive error

• Simple astigmatism. Herein the rays of light are focused on the retina in one meridian and either in front (simple myopic astigmatism) or behind (simple hypermetropic astigmatism) the retina in the other meridian.

• Compound astigmatism. In this type light rays are focused in both the principal meridia either in front (compound myopic astigmatism) or behind (compound hypermetropic astigmatism) the retina.

• Mixed astigmatism. Here the light rays are focused in front of the retina in one meridian and behind the retina in the other meridian. Thus, eye is myopic in one meridian and hypermetropic in the other. Such patients have comparatively less visual symptoms as circle of least diffusion is formed on the retina.

Irregular astigmatism

• It is seen in patients with irregular corneal scars and keratoconus. In it there are multiple meridia which admit no geometrical analysis.

• It can be best treated by rigid gas permeable contact lens which replaces the anterior surface of cornea for refraction.

• Penetrating keratoplasty is indicated in extensive corneal scarring.

Anisometropia

In this refractive condition, degree of error is unequal in two eyes.

• Simple anisometropia. One eye is emmetropic and.the other either myopic or hypermetropic.
• Compound anisometropia. Both eyes are either hypermetropic (compound hypermetropic anisometropia) or myopic (compound myopic anisometropia) but error in one eye is higher than the other.
• Mixed anisometropia. One eye is myopic and the other hypermetropic.
• Simple astigmatic anisometropia. One eye emmetropic and the other has either simple myopic or simple hypermetropic astigmatism.

Aniseikonia

In this condition the images projected on to the visual cortex from the two retinae are abnormally unequal in
size and/or shape.

• Optical aniseikonia occurs in high anisometropia.
• Retinal aniseikonia may develop due to stretching or oedema of the retina in macular area.
• Cortical aniseikonia refers to asymmetrical simultaneous perception inspite of equal size of the images formed on the two retinae.

ACCOMMODATION AND ITS ANOMALIES

Accommodation

• Near point or punctum proximum. It is the nearest point at which small objects can be seen clearly. It varies with age, being about 7 cms at the age of 10 years, 25 cms at 40 years and 33 cms at 45 years.

• Far point (punctum remotum). It is the farthest point from where objects can be seen clearly. In an emmetropic eye far point is infinity, in a hypermetropic eye it is virtual and lies behind the eye, and in myopia it is real and lies in front of the eye.

• Range of accommodation. It is the distance between the near point and the far point of an eye.

• Amplitude of accommodation. It is the difference between the dioptric power needed to focus at near point and far point.

Presbyopia

It is not an error of accommodation but a condition of age related physiological insufficiency of accommodation, leading to failing vision for near (usually after the age of 40 years).

Causes of presbyopia

• Decrease in the elasticity and plasticity (hardening) of lens with age.
• Age-related decrease in the power of ciliary muscle.

Causes of premature presbyopia

• Hypermetropia
• Primary open-angle glaucoma
• General debility, causing presenile weakness of ciliary muscle
• Premature sclerosis of the crystalline lens
• Excessive close work

Surgical treatment of presbyopia

The techniques still under trial are:

• Monovision LASIK
• LASiK-PRAM
• Anterior ciliary sclerotomy (ACL) with tissue barrier
• Bifocal or multifocal or accommoding IOLs
• Conductive keratoplasty (CK)

DETERMINATION AND CORRECTION OF REFRACTIVE ERRORS

Retlnoscopy (Skiascopy or shadow test)

When a plane miiror retinoscope is used at a distance of 1 m, depending upon the movement of the red reflex (shadow) the results are interpreted as below.

• No movement : Myopia of ID
• With movement : either emmetropia or hypermetropia or myopia less than 1 D
• Against movement : Myopia more than 1 D

Auto-refractometry

• Auto-refractometry is an objective method of finding out the error of refraction by using computerized autorefractometers.

• These are based on the principle of indirect ophthalmoscopy.

• Autorefractometer quickly gives information about the refractive error in terms of sphere, cylinder with axis and inter-pupillary distance.

• It is a good alternative to retinoscopy for busy practice, mass screening and epidemiological studies.

• Subjective refraction is must even after autorefractometry.

Tests for confirmation of subjective refraction

• Duochrome -test. It is based on the principle of chromatic aberration. When red letters are more clear than the green it indicates that patient is slightly myopic.

• Jackson’s cross-cylinder lest. It is used to verify the strength and axis of the cylinder prescribed. The cross cylinder is a combination of two cylinders of equal strength but with opposite sign, placed with their axis at right angles to each other. It is formed by a spherical lens (e.g., +0.5 DS) with a cylinder of the opposite sign and double power (e.g., -1DC).

• Astigmatic fan test. It is used to confirm the cylindrical correction. In the presence of astigmatism, some lines will be seen more sharply defined.

• Pin-hole test. An improvement in the visual acuity while looking through a pin hole indicates that optical correction in the trial frame is incorrect.

Spectacles

• Crown glass of refractive index 1.5223 is most commonly used for making spectacles.
• Resin lenses are made of allyl diglycol carbonate. These are light weight, unbreakable and scratch resistant.
• Meniscus lenses are used to make spectacles of small to moderate power. Periscopic lenses have a concave posterior surface of -1.2 D. Deep meniscus type lenses have a concave posterior surface of – 6D. Spherical correction added to the anterior surface of the meniscus lenses.
• Lenticular form lenses are used for high plus and high minus lenses.
• Aspheric lenses are also used to make high power (e.g., aphakic) lenses.

Contact lenses

• Hard contact lenses are made up of PMMA (Polymethyl methacrylate) which is a light weight and non-toxic but hydrophobic material, durable, cheap and of high optical quality.
Disadvantages: can cause corneal hypoxia and corneal abrasions (not used ).

• Soft contact lenses are made up of presently HEMA (hydroxy ethyl methacrylate) which is hydrophilic.

Advantages: Being soft and oxygen permeable, they are most comfortable and so well tolerated.
Disadvantages: These include problems of proteinaceous deposits, getting cracked, limited life, inferior optical quality, more chances of comeal infections and can not correct astigmatism of more than 1 D.

• Rigid gas permeable (RGP) contact lenses are classically made up of copolymer of PMMA and silicone containing vinyl monomer and fluoropolymers (which is permeable to O2).

Advantages are O2, permeability and their ability to correct astigmatism.
Disadvantages include difficult fitting technique and tolerance problems in early stages.

SOME SALIENT POINTS

• Astigmatism is the most common refractive error.
• Refractive error are the most common cause of defective visual acuity.
• The commonest problem associated with aphakic glasses is pin-cushion distortion.
• The commonest type of astigmatism encountered in general population is against-the-rule astigmatism.
• Curvatural myopia and hypermetropia occur commonly as a factor of astigmatism.
• The retinal image of the aphakic eye is about a quarter larger than the emmetropic retinal image.
• Pathological curvatural myopia is seen typically in conical cornea.
• Visual angle is the angle subtended by the object at the nodal point.
• All accommodation is lost in aphakia.
• S. G, H, and B are the easiest letters to recognize on the Snellen chart, where as L, T, U, V and C are the five most difficult ones.
• The pigment epithelium on the back of the iris and the retinal pigmentary epithelium at the back of the eye absorb radiation of all wavelengths.
• The eye is normally myopic for blue and green rays and hypermetropic for red rays.
• Distant vision is often found to be surprisingly good with mixed astigmatism due to circle of least diffusions.
• Asthenopia is worse in lower degree of astigmatism than the higher degrees of astigmatism due to circle of least diffusion.
• Image jump occurs with the use of bifocal lenses.