Ординатура / Офтальмология / Английские материалы / Pediatric Opthalmology_Mukherjee_2005

Colour defect can occur due to :

1.Absence of all cones responsible for colour vision i.e. achromatopsia.

2.All the three cone pigments are present but one of them is abnormal and the condition is called anomalous trichromacy. They are divided into -

(i) Protanomalous—When there is abnormal red sensitive cone. (ii) Deuteranomalous have abnormal green sensitive cone.

(iii) Tritanomalous have abnormal blue pigment. Former two are common, the last one is rare.

Distribution of pigment sensitive cones is not uniform, the central part is devoid of blue sensitive cones. Trichromatic vision extends up to 30° round the point of fixation.

Total absence of colour sensitive cone is suffixed with anopia that is protanopia deuteranopia and tritanopia respectively for red, green and blue blindness.8

3.Lesions of optic pathway also leads to acquired colour defect.

4.Changes in colour absorption by lens causes minor colour defect.

Sensitivity to colour depends on many factors either in isolation or in combination. They are—Brightness, hue this describes the name of the colour that the person sees, it refers to dominant wave length saturation. This is also called purity of colour and represent amount of white light present, lesser the white element purer is the colour.

Classification of colour blindness :

There are various methods to classify colour blindness :

According to type of cones involved9

1. Monochromatism

(i) Rod monochromatism (ii) Cone monochromatism

2. Dichromatism (i) Protanopia (ii) Deuteranopia (iii) Tritanopia

3. Anomalous trichromatism (i) Protanomaly

(ii) Deuteranomaly (iii) Tritanomaly

Colour blindness is very common among boys, about 8% boys suffer from some type of colour blindness or others. It is very rare in girls. Women are as often affected by acquired coloured blindness as men. In congenital colour blindness women are the carriers but

do not suffer from colour blindness. This is a classic example of x linked inheritance in red green blindness.10

As colour blindness is a dominantly male affliction without deterioration or improvement of vision in otherwise normal boys and is a bilaterally symmetrical in both eyes, all boys during their entry in primary school should be screened for presence of colour defect that may be mild to moderate. Total colour blindness where everything looks gray is very rare with normal vision.

Screening is necessary not for treatment but for planning the future profession of the boys. Persons with colour defect are not suitable for many jobs.

It is surprising that a colour blind does not find it difficult with traffic lights. This is most probably due to brightness of the traffic light and his experience. The boy learns to call leaves green and rose red by experience but when given to match different hues they fumble or fail. Hence ability of a boy to name a particular colour correctly does not exclude colour defect. To determine for colour vision there are various tests. None of them are perfect, some are simple and good for screening incidence of colour blindness and its probable type.

They are :

1. Pseudo isochromatic11 (polychromatic) charts

(i) Ishihara plates (only good for red-green deficiency) (ii) Stilling plates

(iii) Hardy-Rand-Ritler plates (Useful in blue blindness)

2.Lantern test (Edridge green lantern)—This is widely used test in railways and airways. It examines ability for a person to differentiate red from green signal under various conditions of brightness and size from a standard distance.

3.Fransworth hue discrimination test

(i) Frans worth D-15 test.

(ii) Frans worth Munsell 100 hue test.

4.Nagels anomaloscope—This fails to detect blue blindness.

5.Holmgrens wool test is a complicated time consuming test.

6.Inter Society Colour Council aptitude test—It has too many negative results.

7.Sloan achromatopsia test—This is useful in total colour blindness.

Common Clinical Types of Colour Blindness

1. Achromatopsia. This is an autosomal recessive condition. The children have poor vision, sensory nystagmus, photophobia and total colour blindness. They have either absent or have marked abnormal cone function. To them all colours are various shades of gray which is confirmed by Sloan achromatopsia test.

Photophobia in achromatopsia is not a true photophobia seen commonly in children due to irritation of trigeminal nerve. These children avoid bright light to keep the rods dark adapted12 to use them for better vision.

All children with nystagmus and poor vision should be examined for achromatopsia. Similarly all children with photophobia without evidence of irritation of fifth nerve should also be tested for achromatopsia. There is no treatment, however extra dark glasses with side covers give some improvement in vision. Any error of refraction should be corrected. Low vision aids do not have much value. The children have to be brought up as visually challenged.

2. Other congenital colour defects. These defects are present at birth but detected later. The child and his parents may not be aware of the defect unless pointed out. They have generally normal vision with normal eyes, do not suffer from any neurological deficit, lead a normal life. They generally have anomalous pigments in the cones.

There is no known treatment.

Acquired colour defect : The causes can be :

1.Lenticular (not seen in children)

2.Retinal —Macular

—Pan retinal photo coagulation 3. Optic pathway

Retinal :

1.Lesions of outer layers of retina give rise to rarer form of blue-yellow defect.

2.Lesions of inner layer give rise to red green defect.

The conditions that cause retinal colour defect are - pan photo coagulation, macular degeneration, high myopia and chorioretinitis.

Optic pathway. Optic pathway not only carries visual impulses but also colour sense to the central nervous system. Hence it is but natural that lesions of optic pathway should produce colour defect. In lesions of optic nerve, the colour sense is not totally abolished, it seems to be washed out13. Reduced colour sense is so consistent with lesion of optic nerve that diminished vision with normal colour senses excludes possibility of optic nerve lesion. In lesions of optic nerve, the coloured object when centrally fixed has duller colour than when shifted to periphery due to central relative scotoma.

Colour vision is diminished even before central vision is affected.14 Colour field may be lost in not only in central field but also in peripheral field also.

Inability to recognise colours when no colour blindness is present is called colour agnosia. Inability to recall the name of colour is called amenestic colour aphasia.14

Visual aphasia3 (word blindness). In this condition, the child has normal vision and colour sense but can not recognise words, letters or colours. The child can not copy written letters but can write spontaneously. It is generally seen in involvement of optic radiation.

Agraphia is inability to express meaning in written language while alexia means inability to read.

Dyslexia—15. Dyslexia is more serious and common disorder in children than alexia. In alexia the child can not comprehend written words. Incidence of dyslexia in developed countries is as high as 4 to 6 percent. No data is available for developing countries, most probably

because its presence is not appreciated and the child is considered to be dull. In fact dyslexic children have normal or higher intelligence than other children.

The condition is thought to be central in origin, exact location of the lesion is not known. It affects both the sexes equally. There may be positive history of learning difficulty in members of the same family.

The condition is congenital but detected only when the child is admitted to school. The child has difficulty in relating letters and groups of letters with appropriate sound15. The condition is permanent.

Diagnosis is not simple. It requires specially trained, multi disciplinary team to diagnose and train the child to lead as normal life as possible. Ophthalmologist role is to exclude other causes of reading difficulty i.e. error of refraction, muscle imbalance, bilateral amblyopia and treat them as far as possible.

Agnosia. Visual agnosia is a condition where a person can not recognise an object by sight but can recognise the object by touch or sound. This is caused due to lesions of Broadmann’s area 18.16

Visual hallucination. This is a condition where the patient complains of seeing an object that is not present. There are two types of hallucinations :

(i) Formed hallucination where the objects claimed to be seen are identifiable objects.

(ii) Unformed hallucination consists of lights, colours or amorphous images. There may be a mixture of both.

Many visual hallucination may be associated with auditory hallucination in psychological causes. Many of the drugs are known to produce visual hallucination.

Scotomas. Scotomas are defects in field of vision. They can be central i.e. round 30° of point of fixation or peripheral beyond 30°.

They are called absolute if the size and densities of scotoma does not change with size, colour of target and illumination. They are called relative if the size and density of scotoma changes with size and colour of target and illumination.

A scotoma is said to be steep if the margins of the field defect do not change with change in size of object. If the margins of the scotoma is different with different targets then the scotoma is called sloping.

A congruous field defect refers to similarity of scotoma to the contra lateral scotoma when they are superimposed on each other, they should match in outline.

The scotomas can be :

1.Choroioretinal

2.Retino neural

3.Optic nerve type

4.Chiasmal

5.Optic tract type

6.Geniculate

7.Optic radiation—Temporal, perital

8.Cortical

Chorioretinal scotomas can be isolated or multiple without any fixed pattern. They do not respect either and horizontal raphe of retina or vertical line of chiasmal lesion. They may be unilateral or bilateral.

Retino-neural scotomas can be :

Central Centrocecal or

Paracentral. They are due to involvement of papillo macular fibres. A scotoma is said to be central when it covers the point of fixation. When the central scotoma extends in to the blind spot it is called centrocecal scotoma. If some of the papilomacular fibres other then macular are involved, the scotoma is called para central. This does not involve either the point of fixation or blind spot.

The lesion when involve, the nerve fibre bundle cause : Bjerrum’s scotoma

Seidel scotoma Roenne’s step

The two Bjerrums scotoma may join to form an annular scotoma with Renne’s step.

These scotomas are monocular. The involvement of the other eye is independent of first

eye.

Scotomas due to lesions of optic nerve produce central scotoma that may be relative or absolute. They are unilateral unless the contra lateral nerve is also involved. The scotomas of optic nerve are associated with :

Diminished vision Diminished colour sense

Diminished contrast sensitivity and Afferent pupillary defect.

If the lesion is nearer the chiasma, it will produce ipsilateral diminished vision and central scotoma with absolute or relative contra lateral hemianopia.

Scotomas due to chiasmal lesions :

The scotoma contains uncrossed temporal fibre of ipsilateral eye and crossed nasal fibres of contra lateral eye. The ratio of crossed to uncrossed fibres is 3:217. Crossing of nasal fibres is not straight, the inferior nasal fibres form a curve that encroaches the posterior end of the contra lateral optic nerve. The superior nasal fibre similarly form a bend into the anterior end of ipsilateral optic tract. These arrangements result in various types of scotomas due to location of the lesion, size of the lesion and number of lesions.

1.The anterior chiasmal lesion on one side only produces ipsilateral central scotoma and contra lateral hemianopia.

The scotomas are congruous, homonymous and hemianopic. The scotomas may involve macula or macula may be spared. The scotomas respect vertical meridian can respect horizontal meridian as well. Sometimes a temporal crescent may be present. Probable causes of macular sparing are :

1.Dual blood supply to the occipital cortex.

2.The macular area is spread over a large area hence it is possible that enough macular fibres are spared to retain macular vision.

3.It is presumed that there is double innervation of macular area.

Some specific type of field defects :

1. Enlargement of blind spot is not always diagnostic. It is seen in : (i) papilledema

(ii) Chronic simple glaucoma (iii) Opaque nerve fibres

(iv) Drusen of optic nerve (v) Coloboma of optic disk

(vi) Peripapillary edema of retina (vii) Juxtapapillary chroiditis.

2. Constriction of peripheral field :

(i) Onset of optic atrophy—Post papilledematous or post neurotic. (ii) Retinitis pigmentosa

(iii) Advanced glaucoma (iv) Hysteria

(v) Malingeing

3. Altitudinal field defect :

(i) Acute ischaemic optic neuropathy

(ii) Occlusion of central retinal artery branch (iii) Trauma to superior occipital lobe

(iv) Optic tract syndrome

Cortical blindness. In these cases the patient is blind but denies blindness. The pupillary reaction is normal. There are no fundal findings that can be attributed to blindness. The lesion is bilateral, occipito cortical.

Bilateral homonymous hemianopia. These scotomas are difficult to chart, have poor prognosis. Vision may not improve beyond hand movement, is caused due to ischaemic lesion of both the occipital cortexes. There may be some gap between involvement of two cortexes.

Amblyopia22, 23, 24, 25, 26

Amblyopia is a common cause of diminished vision in children which if not treated early results in permanent loss of vision generally in one eye but on occasions it may be bilateral.

It has been defined variously from time to time.

Most accepted description states it to be a symptomatic disturbance of vision in children due to deprivation of vision or abnormal binocular interaction.24 The cause of which can not be explained by presence of disorders of ocular media or visual pathway. When amblyopia is present without any detectable cause, is called pure amblyopia.23

Prevalence of amblyopia ranges between 0.5 to 3.5 percent in school going children.25 It is seen equally in boys and girls. No race is immune. There is no fixed hereditary factor, however, the predisposing causes like error of refraction or concomitant squint may have genetic background.

Amblyopia develops due to failure of visual pathway development, which may start at birth or soon after. Development of amblyopia after full maturation of visual pathway is rare. Onset after six to seven years is almost unknown.

For development of visual pathway it is essential to have a clearly formed image on the macula. The macular development is stretched up to 6 to 7 years post natal. Any derangement of formation of vision during this period is bound to cause amblyopia. Amblyopia developing early is more difficult to manage than those developing later. Anything that prevents formation of equally clear image on both the maculae i.e. physical obstruction like ptosis, corneal or lenticular opacity, anisometropia or squint will result in amblyopia.

Characteristics of amblyopic eyes :

1.Amblyopia starts in the critical period of development of visual pathway.

2.It is seen in children under six to seven years of age.

3.If amblyopia is not treated before eight years of age it becomes permanent.

4.Amblyopia is mostly uniocular disorder however in a very small percentage of cases it can be bilateral.

5.It is not possible for the parents to know that the child is amblyopic unless the child undergoes examination of the vision. This happens more often in children who have amblyopia in spite of straight eyes.

6.In contrast to this a child may be brought with squint and found to have amblyopia. This is also known as amblyopia with squint.

7.Diminished distant vision - The amblyopic eye always has poorer vision than the normal fellow eye. A difference of two lines on Snellen’s chart after best correction is diagnostic.

8.Vision in amblyopic eye may be as low as perception of light in congenital squint. Generally vision in amblyopic eye ranges between 6/12-6/18. A child may have 6/36 vision before starting treatment and end up with 6/12 vision. This eye will still be considered to be amblyopic.

9.Crowding phenomenon. An amblyopic eye has better vision when single optotypes are shown but when letters of same size in a line are shown, the child is unable to read them. Greater the difference between the single letter vision and linear vision, poorer is the prognosis.

10.Vision with neutral density filter. In non amblyopic eye if neutral density filter is put in front of the eye, the vision is reduced by one to two lines but not in amblyopic eye when neutral density filter of increasing strength is put in front of the eye the vision either remains same or even improves.27 In case of organic amblyopia i.e. toxic amblyopia vision is markedly reduced with neutral density filter.26 The explanation for the phenomenon is—The scotopic and mesopic vision of an amblyopic eye is equal to non amblyopic eye but less in photopic vision. Placing a neutral density film simulates scotopic vision, thus it is not affected.

11.Colour vision and dark adaptation are normal in amblyopia.

12.Contrast sensitivity, grating acuity, Vernier acuity and spatial localisation are lowered.

13.Visual evoked response to pattern stimuli is poorer in amblyopic eye.

14.There may be milder degree of relative afferent pupillary response.

15.The amblyopic eye may have mobility defect in the form of pursuit, fixation and saccades.

16.There may be latent nystagmus.

17.There may be poor accommodation.

18.Severe degree of amblyopia may have eccentric fixation.

Classification of amblyopia. There is no unanimity in classification of amblyopia. Various classification have been done on the basis of presumed physiopathology of the condition. Many of the terms used in the past are either avoided22 or altogether discarded.

Various terms used in amblyopia are :

1.Strabismic amblyopia. Amblyopia due to abnormal interaction between two eyes due to squint.

2.Anisometropic amblyopia. Amblyopia due to uncorrected unequal refraction into eye.

3.Stimulus deprivation amblyopia is caused due to blurring of retinal image due to opacity in the media, i.e. congenital or traumatic cataract, corneal opacity, ptosis. It was formerly called amblyopia exanopsia. The term is no more in vogue22, 24.

4.Ametropic amblyopia is caused due to high uncorrected error of refraction.

5.Meridional amblyopia due to high astigmatism in a particular meridian.

6.Amblyopia of arrest due to arrest of development of vision.

7.Amblyopia of extinction due to secondary loss of vision.

The last two terms are also not used any more.

The toxic amblyopia, nutritional amblyopia and amblyopia in various neuropathies like retrobulbar neuritis do not fit into the definition of amblyopia hence they are not included in this chapter.

One of the widely used classification is to divide amblyopia into :

(1) Organic amblyopia and (2) Functional amblyopia. The latter is used to denote psychological disturbance and does not fulfil the criteria of amblyopia.

Organic amblyopia is also known as developmental amblyopia and can be unilateral or less commonly bilateral. The bilateral developmental amblyopia is generally stimulus deprivation amblyopia and the causes are - Bilateral cataract of equal density, high uncorrected hypermetropia and motor nystagmus.

The uniocular developmental amblyopia can be strabismic or anisometropic separately or in combination.

For practical purposes the best way is to classify amblyopia is amblyopia with squint and amblyopia without squint. The latter is called straight eye amblyopia.

Amblyopia with squint : Characteristics :

1.The child constantly uses one eye for fixation. The deviating eye becomes amblyopic, patients with alternate fixation do not develop amblyopia.

2.Esotropes develop more amblyopia than exotropes. Congenital esotropes do not develop amblyopia due to cross fixation.

3.Hypertropes generally do not develop amblyopia as they manage to maintain fusion by abnormal head posture.26

4.Micro strabismus causes more amblyopia when compared to large degree squint27.

5.Strabismus causes more amblyopia than anisometropia.

6.Strabismic amblyopia is caused due to suppression of image in the squinting eye.

7.Amblyopia with squint report for checking earlier than straight eye amblyopia, not because they are aware of diminished vision but for cosmetic blemish of squint.

8.Degree of amblyopia is influenced by duration of squint rather than age of onset of squint.

9.Earlier the squint develops, deeper is the amblyopia and more difficult to treat. Thus in case of congenital squint where inhibition is present at birth, the vision in the amblyopic eye will not improve beyond perception of light.25

10.The suppression is at the level of cortex.27

11.The condition is less amenable to treatment than straight eye amblyopia.

12.No correlation exists between severity of amblyopia and angle of squint. Micro tropias are known to cause severe amblyopia.

Straight eye amblyopia : Characteristics :

1. Error pf refraction :

(i) High degree of hypermetropia in one eye, the amblyopic eye called anisometropic amblyopia.

(ii) Both eyes have equal errors of refraction mostly of high degree. (iii) No significant error of refraction.