Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

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Fig. 15.5 Principles of electro-oculography

Dark adaptometry

1Principle. Dark adaptation (DA) is the phenomenon by which the visual system (pupil, retina and occipital cortex) adapts to decreased illumination. This test is particularly useful in the investigation of nyctalopia. The retina is exposed to an intense light for a time sufficient to bleach 25% or more of the rhodopsin in the retina. Following this, normal rods are insensitive to light and cones respond only to very bright stimuli. Subsequent recovery of light sensitivity can be monitored by placing the subject in the dark and periodically presenting spots of light of varying intensity in the visual field and asking the subject if they are perceived.

2Technique of Goldmann–Weekes adaptometry

aThe subject is exposed to an intense light that bleaches the photoreceptors and then is suddenly placed in the dark.

bThe threshold at which the subject just perceives a light is plotted.

c The flashes are repeated at regular intervals; the sensitivity of the eye to light gradually increases.

3The sensitivity curve is a bipartite plot of the light intensity of a minimally perceived spot versus time (Fig. 15.6).

aThe cone branch of the curve represents the initial 5–10 minutes of darkness during which cone sensitivity rapidly improves. The rod photoreceptors are also recovering but more slowly during this time.

bThe ‘rod-cone’ break normally occurs after 7–10 minutes when cones achieve their maximum sensitivity, and the rods become perceptibly more sensitive than cones.

cThe rod branch of the curve is slower and represents the continuation of improvement of rod sensitivity. After 15–30 minutes, the fully dark-adapted rods allow the subject to perceive a spot of light over 100 times dimmer than would be possible with cones alone. If the flashes are focused onto the foveola (where rods are absent), only a rapid segment corresponding to cone adaptation is recorded.

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Fig. 15.6 Dark adaptation curve

Colour vision tests

Colour vision (CV) testing is sometimes useful in the clinical evaluation of hereditary fundus dystrophies, where dyschromatopsia may be present prior to the development of impairment of visual acuity and visual field loss.

Principles

CV is a function of three populations of retinal cones each with its specific sensitivity; blue (tritan) at 414–424 nm, green (deuteran) 522 –539 nm and red (protan) at 549–570 nm.

•A normal person requires all these primary colours to match those within the spectrum. Any given cone pigment may be deficient (e.g. protanomaly – red weakness) or entirely absent (e.g. protanopia – red blindness). Trichromats possess all three types of cones (although not necessarily functioning perfectly), while absence of one or two types of cones renders an individual a dichromat or a monochromat respectively.

•Most individuals with congenital colour defects are anomalous trichromats and use abnormal proportions of the three primary colours to match those in the light spectrum.

•Those with red-green deficiency caused by abnormality of red-sensitive cones are protanomalous, those with abnormality of greensensitive cones are deuteranomalous and those with blue-green deficiency caused by abnormality of blue-sensitive cones are tritanomalous. Acquired macular disease tends to produce blue-yellow defects and optic nerve lesions red-green defects.

Colour vision tests

1Ishihara test is used mainly to screen for congenital protan and deuteran defects. It consists of a test plate followed by sixteen plates each with a matrix of dots arranged to show a central shape or number which the subject is asked to identify (Fig. 15.7A). A colour deficient person will only be able to identify some of the figures. Inability to identify the test plate (provided visual acuity is sufficient) indicates non-organic visual loss.

2Hardy–Rand–Rittler is similar to Ishihara but more sensitive since it can detect all three congenital colour defects (Fig. 15.7B).

3City University test consists of 10 plates each containing a central colour and four peripheral colours (Fig. 15.7C). The subject selects the peripheral colours which most closely matches the central colour.

4Farnsworth–Munsell 100-hue is the most sensitive for both congenital and acquired colour defects but is seldom used in clinical practice. Despite the name it consists of 85 hue caps contained in four separate racks in each of which the two end caps are fixed while the others are loose so they can be randomized by the examiner (Fig. 15.7D).

aThe subject is asked to rearrange the loose randomized caps ‘in their natural’ order in one box.

bThe box is then closed, turned upside down and then opened so that the markers on the inside of the caps become visible.

cThe findings are then recorded in a simple cumulative manner on a circular chart.

dEach of the three forms of dichromatism is characterized by failure in a specific meridian of the chart (Fig. 15.8).

5 Farnsworth D15 hue discrimination test is similar to the Farnsworth–Munsell 100-hue test but utilizes only 15 caps.

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Fig. 15.7 Colour vision tests. (A) Ishihara; (B) Hardy–Rand–Rittler; (C) City University; (D) Farnsworth-Munsell 100-hue test

(Courtesy of T Waggoner – fig. B)

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Fig. 15.8 Farnsworth–Munsell test results of colour deficiencies. (A) Protan; (B) deuteran; (C) tritan

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Generalized photoreceptor dystrophies

Typical retinitis pigmentosa

Retinitis pigmentosa (RP) defines a clinically and genetically diverse group of diffuse retinal dystrophies initially predominantly affecting the rod photoreceptor cells with subsequent degeneration of cones (rod-cone dystrophy). It is the most commonly encountered hereditary fundus dystrophy with a prevalence of approximately 1 : 5000.

Inheritance

The age of onset, rate of progression, eventual visual loss and associated ocular features are frequently related to the mode of inheritance. RP may occur as an isolated sporadic disorder, or be inherited as AD, AR or XL. Many cases are due to mutation of the rhodopsin gene. XL is the least common but most severe form and may result in complete blindness by the third or fourth decades. Female carriers may have normal fundi or show a golden-metallic ('tapetal’) reflex at the macula (Fig. 15.9A) and/or small peripheral patches of bone-spicule pigmentation (Fig. 15.9B). RP, often atypical, may also be associated with systemic disorders which are usually AR (see below).

Fig. 15.9 Findings in carriers of XL retinitis pigmentosa. (A) ‘Tapetal’ reflex at the macula; (B) mild peripheral pigmentary changes

(Courtesy of D Taylor and CS Hoyt, from Pediatric Ophthalmology and Strabismus, Elsevier, Saunders, 2005 – fig. A)

Diagnosis

The diagnostic criteria for RP comprise bilateral involvement, with loss of peripheral and night vision. The classical triad of RP is: (a) arteriolar attenuation, (b) retinal bone-spicule pigmentation and (c) waxy disc pallor.

1 Presentation is with nyctalopia, often during the 2nd–3rd decades, though may be earlier or later, depending on the pedigree.

2Signs in chronological order:

•Subtle mid-peripheral RPE atrophy associated with mild arteriolar narrowing, and mid-peripheral intraretinal perivascular ‘bone-spicule’ pigmentary changes (Fig. 15.10A).

•Gradual increase in density of the pigmentary changes with anterior and posterior spread (Fig. 15.10B).

•Tessellated fundus appearance, due to RPE atrophy and unmasking of large choroidal vessels (Fig. 15.10C).

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•Severe arteriolar narrowing and gliotic ‘waxy pallor’ of the optic discs (Fig. 15.10D).

•The macula may show atrophy, epiretinal membrane formation and CMO; the latter may respond to systemic acetazolamide.

3ERG in early disease shows reduced scotopic rod and combined responses (Fig. 15.11); later photopic responses become reduced and eventually the ERG becomes extinguished.

4EOG is subnormal with an absence of the light rise.

5DA is prolonged and may be useful in early cases where the diagnosis is uncertain.

6Perimetry initially demonstrates small mid-peripheral scotomas that gradually coalesce to form the classical annular scotoma, which expands both peripherally and centrally. It ultimately leaves a tiny island of central vision which may eventually be extinguished. Perimetry is useful in monitoring the progression of disease.

7Prognosis is variable and tends to be associated with the mode of inheritance as follows:

•XL disease has the worst prognosis with severe visual loss by the 4th decade.

•AR disease and sporadic cases have a more favourable prognosis with retention of central vision until the 5th–6th decade or later.

•AD disease generally has the best prognosis with retention of central vision beyond the 6th decade.

Fig. 15.10 Progression of retinitis pigmentosa. (A) Early changes; (B) advanced changes; (C) unmasking of choroidal vessels; (D) end-stage disease

(Courtesy of P Saine – fig. A)

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Fig. 15.11 ERGin early retinitis pigmentosa shows reduced scotopic rod and combined responses

Ocular associations

Regular follow-up of patients with RP is essential to detect other vision-threatening complications, some of which may be amenable to treatment.

1 Posterior subcapsular cataracts are common in all forms of RP; surgery is often beneficial.

2 Open-angle glaucoma occurs in 3% of cases.

3Myopia is common.

4Keratoconus is uncommon.

5 Vitreous changes, which are common, consist of posterior vitreous detachment and occasionally intermediate uveitis.

6Optic disc drusen occur more frequently in patients with RP.

7Coats-like disease with lipid deposition in the peripheral retina and exudative retinal detachment (Fig. 15.12) may occasionally develop in adult life.

Fig. 15.12 Coats-like features associated with retinitis pigmentosa

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Atypical retinitis pigmentosa

Atypical RP describes conditions that are closely related to typical RP or represent incomplete forms of the disease.

1Cone-rod dystrophy in which the cone component is affected earlier and more severely than in typical RP. Presentation is with impairment of central vision and not with nyctalopia. Examination may show a macular lesion with or without peripheral changes.

2RP sine pigmento is characterized by absence or paucity of pigment accumulation (Fig. 15.13A), which may subsequently appear with time.

3Retinitis punctata albescens is an AR variant characterized by scattered whitish-yellow spots, most numerous at the equator, usually sparing the macula, and associated with arteriolar attenuation (Fig. 15.13B). They are similar to the spots in fundus albipunctatus but often have a more radial pattern.

4Sector RP is an AD variant characterized by involvement of inferior quadrants (Fig. 15.13C). Progression is slow and many cases are stationary.

Fig. 15.13 Atypical retinitis pigmentosa. (A) Sine pigmento; (B) retinitis punctata albescens; (C) sector

Important systemic associations

Bassen–Kornzweig syndrome (abetalipoproteinaemia)

1Inheritance is AR.

2Pathogenesis. Deficiency of chylomycrons and low-density lipoproteins results in malabsorption of fat-soluble vitamins A, E and occasionally K.

3Systemic features

•Failure to thrive and steatorrhea in infancy followed by severe spinocerebellar ataxia.

•Blood shows ‘thorny’ red cells (acanthocytosis) and low plasma cholesterol and triglycerides.

4Fundus shows scattered white dots followed by RP-like changes developing towards the end of the 1st decade. Treatment with large doses of vitamin A and E may prevent visual loss.

5Other ocular features include ptosis, ophthalmoplegia, strabismus and nystagmus.

Refsum disease

1 Inheritance is AR. The infantile and adult forms are genetically distinct.

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2Pathogenesis. Deficiency in phytanic acid alpha-hydrolase results in accumulation of phytanic acid throughout the body. Early detection and treatment with a diet low in phytanic acid can arrest disease progression.

3Systemic features

aInfantile disease is characterized by dysmorphic facies, mental handicap, hepatomegaly and deafness.

bAdult disease is characterized by cerebellar ataxia, polyneuropathy, anosmia, deafness, cardiomyopathy and ichthyosis (Fig. 15.14B).

4Fundus appearance may be similar to RP or merely show salt-and-pepper changes.

5Other ocular features include cataract, prominent corneal nerves, optic atrophy, nystagmus and poorly dilating pupils.

Fig. 15.14 Selected systemic associations of retinitis pigmentosa. (A) Acanthocytosis in Bassen–Kornzweig syndrome; (B) ichthyosis in adult Refsumdisease; (C) ptosis in Kearns–Sayre syndrome; (D) polydactyly in Bardet–Biedl syndrome

Kearns–Sayre syndrome

Kearns–Sayre syndrome is characterized by chronic progressive external ophthalmoplegia (Fig. 15.14C) associated with other systemic problems, which are described in Chapter 19. The fundus usually has a ‘salt and pepper’ appearance most striking at the macula; less frequently findings are typical RP or choroidal atrophy similar to choroideremia.

Bardet–Biedl syndrome

1Inheritance is genetically heterogeneous.

2Systemic features include hypogonadism in males, polydactyly (Fig. 15.14D), truncal obesity, renal anomalies and mental handicap.

3Fundus typically shows a bull's eye maculopathy due to cone-rod dystrophy. Less frequently findings are typical RP, RP sine pigmento and retinitis punctata albescens. Although only 15% of patients show retinopathy by 10 years of age, almost 80% are blind by the age of 20 years.

Usher syndrome

Usher syndrome is a distressing condition which accounts for about 5% of all cases of profound deafness in children, and is responsible for about half of all cases of combined deafness and blindness. There are three major types in which sensorineural deafness is associated with typical RP with or without vestibular dysfunction.

1Inheritance is AR (genetically heterogeneous).

2Classification

aType I (75% of patients) – congenital, profound deafness with vestibular dysfunction; visual loss due to RP with extinguished ERG occurs in the 1st decade.

bType II (23%) – congenital, moderate to severe deafness with normal vestibular function; visual loss occurs in the 2nd decade.

cType III (2%) – progressive hearing loss and progressive vestibular dysfunction and relatively late-onset pigmentary retinopathy.

3Systemic features include premature ageing beginning in infancy, dwarfism, skeletal anomalies, deafness, photosensitivity, mental handicap and early demise.

4Fundus shows salt and pepper pigmentation and optic atrophy.

5 Other ocular features are miosis, cataract and orbital fat atrophy.

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Progressive cone dystrophy

Progressive cone dystrophy predominantly affects the cone system. In some cases there is no evidence of rod dysfunction whilst in others rod dysfunction subsequently develops but cone deficiency still predominates (cone-rod dystrophy).

1Inheritance. Most cases are sporadic; the remainder are AD or XL.

2Presentation is in the 2nd–4th decades with gradual bilateral impairment of central and colour vision which may be followed by photophobia.

3Signs in chronological order:

•The macula may be virtually normal or show non-specific pigmentary changes (Fig. 15.15A).

•A golden sheen may be seen in XL disease (Fig. 15.15B).

•A bull's eye maculopathy is classically described but is not universal (Fig. 15.15C); other causes of a bull's eye appearance are given in Table 15.1.

•Progressive RPE atrophy at the macula with eventual geographic atrophy (Fig. 15.15D).

4ERG. Photopic responses are subnormal or non-recordable and flicker fusion frequency is reduced, but rod responses are preserved until late (Fig. 15.16).

5EOG is normal to subnormal.

6 DA. The cone segment is abnormal. The rod segment is initially normal but may become subnormal later.

7CV shows a severe deuteran-tritan defect out of proportion to visual acuity.

8 FA of bull's eye maculopathy shows a round hyperfluorescent window defect with a hypofluorescent centre (Fig. 15.17).

9Prognosis is poor with eventual severe loss of central vision to the level of 6/60 or CF.

Fig. 15.15 Progressive cone dystrophy. (A) Early pigment mottling; (B) Golden sheen in XL disease; (C) ‘bull's eye’ maculopathy; (D) geographic atrophy

(Courtesy of Moorfields Eye Hospital – fig. A)

Table 15.1 -- Other causes of bull's eye macula

1In adults

•Chloroquine maculopathy

•Advanced Stargardt disease

•Fenestrated sheen macular dystrophy

•Benign concentric annular macular dystrophy

•Clofazimine retinopathy

2In children

•Bardet–Biedl syndrome

•Hallervorden-Spatz syndrome

•Leber congenital amaurosis

•Lipofuscinosis

•AD cerebellar ataxia

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