Ординатура / Офтальмология / Английские материалы / Comprehensive Ophthalmology_Khurana_2007

Tarsal glands are formed by ingrowth of a regular row of solid columns of ectodermal cells from the lid margins.

Cilia develop as epithelial buds from lid margins.

Conjunctiva

Conjunctiva develops from the ectoderm lining the lids and covering the globe (Fig.1.12).

Conjunctival glands develop as growth of the basal cells of upper conjunctival fornix. Fewer glands develop from the lower fornix.

The lacrimal apparatus

Lacrimal gland is formed from about 8 cuneiform epithelial buds which grow by the end of 2nd month of fetal life from the superolateral side of the conjunctival sac (Fig. 1.12).

Lacrimal sac, nasolacrimal duct and canaliculi.

These structures develop from the ectoderm of nasolacrimal furrow. It extends from the medial angle of eye to the region of developing mouth. The ectoderm gets buried to form a solid cord. The cord is later canalised. The upper part forms the lacrimal sac. The nasolacrimal duct is derived from the lower part as it forms a secondary connection with the nasal cavity. Some ectodermal buds arise from the medial margins of eyelids. These buds later canalise to form the canaliculi.

Extraocular muscles

All the extraocular muscles develop in a closely associated manner by mesodermally derived mesenchymal condensation. This probably corresponds to preotic myotomes, hence the triple nerve supply (III, IV and VI cranial nerves).

STRUCTURES DERIVED FROM THE EMBRYONIC LAYERS

Based on the above description, the various structures derived from the embryonic layers are given below :

1. Surface ectoderm

zThe crystalline lens

zEpithelium of the cornea

zEpithelium of the conjunctiva

zLacrimal gland

zEpithelium of eyelids and its derivatives viz., cilia, tarsal glands and conjunctival glands.

zEpithelium lining the lacrimal apparatus.

2. Neural ectoderm

zRetina with its pigment epithelium

zEpithelial layers of ciliary body

zEpithelial layers of iris

zSphincter and dilator pupillae muscles

zOptic nerve (neuroglia and nervous elements only)

zMelanocytes

zSecondary vitreous

zCiliary zonules (tertiary vitreous)

3.Associated paraxial mesenchyme

zBlood vessels of choroid, iris, ciliary vessels, central retinal artery, other vessels.

zPrimary vitreous

zSubstantia propria, Descemet's membrane and endothelium of cornea

zThe sclera

zStroma of iris

zCiliary muscle

zSheaths of optic nerve

zExtraocular muscles

zFat, ligaments and other connective tissue structures of the orbit

zUpper and medial walls of the orbit

zConnective tissue of the upper eyelid

4.Visceral mesoderm of maxillary process below the eye

zLower and lateral walls of orbit

zConnective tissue of the lower eyelid

IMPORTANT MILESTONES IN THE

DEVELOPMENT OF THE EYE

Embryonic and fetal period

Eye at birth

zAnteroposterior diameter of the eyeball is about 16.5 mm (70% of adult size which is attained by 7-8 years).

zCorneal diameter is about 10 mm. Adult size (11.7 mm) is attained by 2 years of age.

zAnterior chamber is shallow and angle is narrow.

zLens is spherical at birth. Infantile nucleus is present.

zRetina. Apart from macular area the retina is fully differentiated. Macula differentiates 4-6 months after birth.

zMyelination of optic nerve fibres has reached the lamina cribrosa.

zNewborn is usually hypermetropic by +2 to +3 D.

zOrbit is more divergent (50°) as compared to adult (45°).

zLacrimal gland is still underdeveloped and tears are not secreted.

Postnatal period

zFixation starts developing in first month and is completed in 6 months.

zMacula is fully developed by 4-6 months.

zFusional reflexes, stereopsis and accommodation is well developed by 4-6 months.

zCornea attains normal adult diameter by 2 years of age.

zLens grows throughout life.

MAINTENANCE OF CLEAR INTRODUCTION OCULAR MEDIA

Physiology of tears

Physiology of cornea

Physiology of crystalline lens

Physiology of aqueous humour and maintenance of intraocular pressure

PHYSIOLOGY OF VISION

Phototransduction

Processing and transmission of visual impulse

Visual perceptions

PHYSIOLOGY OF OCULAR MOTILITY AND BINOCULAR VISION

Ocular motility

Binocular single vision

INTRODUCTION

Sense of vision, the choicest gift from the Almighty to the humans and other animals, is a complex function of the two eyes and their central connections. The physiological activities involved in the normal functioning of the eyes are :

Maintenance of clear ocular media,

Maintenance of normal intraocular pressure,

The image forming mechanism,

Physiology of vision,

Physiology of binocular vision,

Physiology of pupil, and

Physiology of ocular motility.

MAINTENANCE OF CLEAR

OCULAR MEDIA

The main prerequiste for visual function is the maintenance of clear refractive media of the eye. The major factor responsible for transparency of the ocular media is their avascularity. The structures forming refractive media of the eye from anterior to posterior are :

Tear film,

Cornea,

Aqueous humour,

Crystalline lens, and

Vitreous humour

PHYSIOLOGY OF TEARS

Tear film plays a vital role in maintaining the transparency of cornea. The physiological apsects of the tears and tear film are described in the chapter on diseases of the lacrimal apparatus (see page 364).

PHYSIOLOGY OF CORNEA

The cornea forms the main refractive medium of the eye. Physiological aspects in relation to cornea include:

Transparency of cornea,

Nutrition and metabolism of cornea,

Permeability of cornea, and

Corneal wound healing.

For details see page 90

PHYSIOLOGY OF CRYSTALLINE LENS

The crystalline lens is a transparent structure playing main role in the focussing mechanism for vision. Its physiological aspects include :

Lens transparency

Metabolic activities of the lens

Accommodation.

For details see page 39 and 168

PHYSIOLOGY OF AQUEOUS HUMOUR AND MAINTENANCE OF INTRAOCULAR PRESSURE

The aqueous humour is a clear watery fluid filling the anterior chamber (0.25ml) and the posterior chamber (0.06ml) of the eyeball. In addition to its role in maintaining a proper intraocular pressure it also plays an important metabolic role by providing substrates and removing metabolities from the avascular cornea and the crystalline lens. For details see page 207.

PHYSIOLOGY OF VISION

Physiology of vision is a complex phenomenon which is still poorly understood. The main mechanisms involved in physiology of vision are :

Initiation of vision (Phototransduction), a function of photoreceptors (rods and cones),

Processing and transmission of visual sensation, a function of image processing cells of retina and visual pathway, and

Visual perception, a function of visual cortex and related areas of cerebral cortex.

PHOTOTRANSDUCTION

The rods and cones serve as sensory nerve endings for visual sensation. Light falling upon the retina causes photochemical changes which in turn trigger a cascade of biochemical reactions that result in generation of electrical changes. Photochemical changes occuring in the rods and cones are essentially similar but the changes in rod pigment (rhodopsin or visual purple) have been studied in more detail. This whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction.

Photochemical changes

The photochemical changes include :

Rhodopsin bleaching. Rhodopsin refers to the visual pigment present in the rods – the receptors for night (scotopic) vision. Its maximum absorption spectrum is around 500 nm. Rhodopsin consists of a colourless protein called opsin coupled with a carotenoid called retinine (Vitamin A aldehyde or II-cis-retinal). Light falling on the rods converts 11-cis-retinal component of rhodopsin into all-trans-retinal through various

stages (Fig. 2.1). The all trans-retinal so formed is soon separated from the opsin. This process of separation is called photodecomposition and the rhodopsin is said to be bleached by the action of light.

Rhodopsin regeneration. The 11-cis-retinal is regenerated from the all-trans-retinal separated from the opsin (as described above) and vitamin-A (retinal) supplied from the blood. The 11-cis-retinal then reunits with opsin in the rod outer segment to form the rhodopsin. This whole process is called rhodopsin regeneration (Fig. 2.1). Thus, the bleaching of the rhodopsin occurs under the influence of light, whereas the regeneration process is independent of light, proceeding equally well in light and darkness.

Fig. 2.1. Light induced changes in rhodopsin.

Visual cycle. In the retina of living animals, under constant light stimulation, a steady state must exist under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated. This equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle (Fig. 2.2).

Fig. 2.2. Visual cycle.

Electrical changes

The activated rhodopsin, following exposure to light, triggers a cascade of complex biochemical reactions which ultimately result in the generation of receptor potential in the photoreceptors. In this way, the light energy is converted into electrical energy which is further processed and transmitted via visual pathway.

PROCESSING AND TRANSMISSION OF VISUAL IMPULSE

The receptor potential generated in the photoreceptors is transmitted by electrotonic conduction (i.e., direct flow of electric current, and not as action potential) to other cells of the retina viz. horizontal cells, amacrine cells, and ganglion cells. However, the ganglion cells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex.

The phenomenon of processing of visual impulse is very complicated. It is now clear that visual image is deciphered and analyzed in both serial and parallel fashion.

Serial processing. The successive cells in the visual pathway starting from the photoreceptors to the cells of lateral geniculate body are involved in increasingly complex analysis of image. This is called sequential or serial processing of visual information.

Parallel processing. Two kinds of cells can be distinguished in the visual pathway starting from the ganglion cells of retina including neurons of the lateral geniculate body, striate cortex, and extrastriate cortex. These are large cells (magno or M cells) and small cells (parvo or P cells). There are strikinging differences between the sensitivity of M and P cells to stimulus features (Table 2.1).

Table 2.1. Differences in the sensitivity of M and P cells to stimulus features

The visual pathway is now being considered to be made of two lanes: one made of the large cells is called magnocellular pathway and the other of small cells is called parvocellular pathway. These can be compared to two-lanes of a road. The M pathway and P pathway are involved in the parallel processing of the image i.e., analysis of different features of the image.

VISUAL PERCEPTION

It is a complex integration of light sense, form sense, sense of contrast and colour sense. The receptive field organization of the retina and cortex are used to encode this information about a visual image.

1. The light sense

It is awareness of the light. The minimum brightness required to evoke a sensation of light is called the light minimum. It should be measured when the eye is dark adapted for at least 20-30 minutes.

The human eye in its ordinary use throughout the day is capable of functioning normally over an exceedingly wide range of illumination by a highly complex phenomenon termed as the visual adaptation. The process of visual adaptation primarily involves :

Dark adaptation (adjustment in dim illumination), and

Light adaptation (adjustment to bright illumination).

Dark adaptation

It is the ability of the eye to adapt itself to decreasing illumination. When one goes from bright sunshine into a dimly-lit room, one cannot perceive the objects in the room until some time has elapsed. During this period, eye is adapting to low illumination. The time taken to see in dim illumination is called ‘dark adaptation time’.

The rods are much more sensitive to low illumination than the cones. Therefore, rods are used

more in dim light (scotopic vision) and cones in bright light (photopic vision).

Dark adaptation curve (Fig. 2.3) plotted with illumination of test object in vertical axis and duration of dark adaptation along the horizontal axis shows that visual threshold falls progressively in the darkened room for about half an hour until a relative constant value is reached. Further, the dark adaptation curve consists of two parts: the initial small curve represents the adaptation of cones and the remainder of the curve represents the adaptation of rods.

where there are maximum number of cones and decreases very rapidly towards the periphery (Fig. 2.4). Visual acuity recorded by Snellen's test chart is a measure of the form sense.

Fig. 2.3. Dark adaptation curve plotted with illumination of test object in vertical axis and duration of dark adaptation along the horizontal axis.

When fully dark adapted, the retina is about one lakh times more sensitive to light than when bleached. Delayed dark adaptation occurs in diseases of rods e.g., retinitis pigmentosa and vitamin A deficiency.

Light adaptation

When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises. The process by means of which retina adapts itself to bright light is called light adaptation. Unlike dark adaptation, the process of light adaptation is very quick and occurs over a period of 5 minutes. Strictly speaking, light adaptation is merely the disappearance of dark adaptation.

2. The form sense

It is the ability to discriminate between the shapes of the objects. Cones play a major role in this faculty. Therefore, form sense is most acute at the fovea,

Fig. 2.4. Visual acuity (form sense) in relation to the regions of the retina: N, nasal retina; B, blind spot; F, foveal region; and T, temporal retina.

Components of visual acuity. In clinical practice, measurement of the threshold of discrimination of two spatially-separated targets (a function of the fovea centralis) is termed visual acuity. However, in theory, visual acuity is a highly complex function that consists of the following components :

Minimum visible. It is the ability to determine whether an object is present or not.

Resolution (ordinary visual acuity). Discrimination of two spatially separated targets is termed resolution. The minimum separation between the two points, which can be discriminated as two, is known as minimum resolvable. Measurement of the threshold of discrimination is essentially an assessment of the function of the fovea centralis and is termed ordinary visual acuity. Histologically, the diameter of a cone in the foveal region is 0.004 mm and this, therefore, represents the smallest distance between two cones. It is reported that in order to produce an image of minimum size of 0.004mm (resolving power of the eye) the object must subtend a visual angle of 1 minute at the nodal point of the eye. It is called the minimum angle of resolution (MAR).

The clinical tests determining visual acuity measure the form sense or reading ability of the eye. Thus, broadly, resolution refers to the ability to identify the spatial characteristics of a test figure. The test targets

in these tests may either consist of letters (Snellen’s chart) or broken circle (Landolt’s ring). More complex targets include gratings and checker board patterns. Recognition. It is that faculty by virtue of which an individual not only discriminates the spatial characteristics of the test pattern but also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive components in addition to spatial resolution. For recognition, the individual should be familiar with the set of test figures employed in addition to being able to resolve them. The most common example of recognition phenomenon is identification of faces. The average adult can recognize thousands of faces.

Thus, the form sense is not purely a retinal function, as, the perception of its composite form (e.g., letters) is largely psychological.

Minimum discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. The best example of minimum discriminable is vernier acuity, which refers to the ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.

3. Sense of contrast

It is the ability of the eye to perceive slight changes in the luminance between regions which are not separated by definite borders. Loss of contrast sensitivity results in mild fogginess of the vision.

Contrast sensitivity is affected by various factors like age, refractive errors, glaucoma, amblyopia, diabetes, optic nerve diseases and lenticular changes. Further, contrast sensitivity may be impaired even in the presence of normal visual acuity.

4. Colour sense

It is the ability of the eye to discriminate between different colours excited by light of different wavelengths. Colour vision is a function of the cones and thus better appreciated in photopic vision. In dim light (scotopic vision), all colours are seen grey and this phenomenon is called Purkinje shift.

Theories of colour vision

The process of colour analysis begins in the retina and is not entirely a function of brain. Many theories have been put forward to explain the colour perception, but two have been particularly influential:

1. Trichromatic theory. The trichromacy of colour vision was originally suggested by Young and subsequently modified by Helmholtz. Hence it is called Young-Helmholtz theory. It postulates the existence of three kinds of cones, each containing a different photopigment which is maximally sensitive to one of the three primary colours viz. red, green and blue. The sensation of any given colour is determined by the relative frequency of the impulse from each of the three cone systems. In other words, a given colour consists of admixture of the three primary colours in different proportion. The correctness of the YoungHelmholtz’s trichromacy theory of colour vision has now been demonstrated by the identification and chemical characterization of each of the three pigments by recombinant DNA technique, each having different absorption spectrum as below (Fig. 2.5):

Red sensitive cone pigment, also known as erythrolabe or long wave length sensitive (LWS) cone pigment, absorbs maximally in a yellow portion with a peak at 565 mm. But its spectrum extends far enough into the long wavelength to sense red.

Green sensitive cone pigment, also known as chlorolabe or medium wavelength sensitive (MWS) cone pigment, absorbs maximally in the green portion with a peak at 535 nm.

Blue sensitive cone pigment, also known as cyanolabe or short wavelength sensitive (SWS) cone pigment, absorbs maximally in the blue-violet portion of the spectrum with a peak at 440 nm.

Fig. 2.5. Absorption spectrum of three cone pigments.

Thus, the Young-Helmholtz theory concludes that blue, green and red are primary colours, but the cones with their maximal sensitivity in the yellow portion of the spectrum are light at a lower threshold than green.

It has been studied that the gene for human rhodopsin is located on chromosome 3, and the gene for the blue-sensitive cone is located on chromosome 7. The genes for the red and green sensitive cones are arranged in tandem array on the q arm of the X chromosomes.

2. Opponent colour theory of Hering. The opponent colour theory of Hering points out that some colours appear to be ‘mutually exclusive’. There is no such colour as ‘reddish-green’, and such phenomenon can be difficult to explain on the basis of trichromatic theory alone. In fact, it seems that both theories are useful in that:

The colour vision is trichromatic at the level of photoreceptors, and

Colour apponency occurs at ganglion cell onward. According to apponent colour theory, there are

two main types of colour opponent ganglion cells:

Red-green opponent colour cells use signals from red and green cones to detect red/green contrast within their receptive field.

Blue-yellow opponent colour cells obtain a yellow signal from the summed output of red and green cones, which is contrasted with the output from blue cones within the receptive field.

PHYSIOLOGY OF OCULAR

MOTILITY AND BINOCULAR VISION

PHYSIOLOGY OF OCULAL MOTILITY

See page 313.

PHYSIOLOGY OF BINOCULAR SINGLE VISION

See page 318.

OPTICS

Light

Geometrical optics

Optics of the eye (visual optics)

ERRORS OF REFRACTION

Hypermetropia

Myopia

Astigmatism

Anisometropia

Aniseikonia

ACCOMMODATION AND ITS ANOMALIES Accommodation

Mechanism

Far point and near point

Range and amplitude

Anomalies of accommodation

Presbyopia

Insufficiency of accommodation

Paralysis of accommodation

Spasm of accommodation

DETERMINATION OF ERRORS OF REFRACTION

Objective refraction

Subjective refraction

SPECTACLES AND CONTACT LENSES

Spectacles

Contact lenses

REFRACTIVE SURGERY

OPTICS

LIGHT

Light is the visible portion of the electromagnetic radiation spectrum. It lies between ultraviolet and infrared portions, from 400 nm at the violet end of the spectrum to 700 nm at the red end. The white light consists of seven colours denoted by VIBGYOR (violet, indigo, blue, green, yellow, orange and red). Light ray is the term used to describe the radius of the concentric wave forms. A group of parallel rays of light is called a beam of light.

Important facts to remember about light rays are :

The media of the eye are uniformally permeable to the visible rays between 600 nm and 390 nm.

Cornea absorbs rays shorter than 295 nm. Therefore, rays between 600 nm and 295 nm only can reach the lens.

Lens absorbs rays shorter than 350 nm. Therefore, rays between 600 and 350 nm can reach the retina

in phakic eye; and those between 600 nm and 295 nm in aphakic eyes.

GEOMETRICAL OPTICS

The behaviour of light rays is determined by rayoptics. A ray of light is the straight line path followed by light in going from one point to another. The rayoptics, therefore, uses the geometry of straight lines to account for the macroscopic phenomena like rectilinear propagation, reflection and refraction. That is why the ray-optics is also called geometrical optics.

The knowledge of geometrical optics is essential to understand the optics of eye, errors of refraction and their correction. Therefore, some of its important aspects are described in the following text.

Reflection of light

Reflection of light is a phenomenon of change in the path of light rays without any change in the medium (Fig. 3.1). The light rays falling on a reflecting surface are called incident rays and those reflected by it are