Forms of retinitis pigmentosa

1. Rod-cone dystrophy (classic retinitis pigmentosa, by far the most frequent form).

2. Cone-rod dystrophy (inverse retinitis pigmentosa).

3. Sectoral retinitis pigmentosa.

4. Retinitis pigmentosa sine pigmento (form without pigment).

5. Unilateral retinitis pigmentosa.

6. Leber's amaurosis (form occurring in early childhood).

7. Retinopathy punctata albescens (punctate retinitis).

8. In combination with other disorders in syndromes and metabolic disorders such as mucopolysaccharidoses, Fanconi's syndrome, mucolipidosis IV, peroxisomal disorders, Cockayne's syndrome, mitochondrial myopathies, Usher's syndrome, neuronal and ceroid lipofuscinoses, renal tubular defect syndromes, etc.

Retinitis pigmentosa occurs almost exclusively as rod-cone dystrophy. Therefore, the other extremely rare forms are not discussed here except for the inverse form of classic retinitis pigmentosa, which is presented for purposes of comparison.

Inheritance: Individual genetic forms may be identified from among the heterogeneous group of disorders comprising retinitis pigmentosa. This group of disorders can involve various genotypes as well as variable phenotypic expression or different stages of a disorder with one specific genotype. There are over 15 purely ocular forms of retinitis pigmentosa. The most common form of inheritance is autosomal recessive (60%), followed by autosomal dominant (up to 25%), and X-linked (15%). Rhodopsin gene mutations (chromosome 3) and "retinal degeneration slow" (RTS) gene mutations (chromosome 6) have also been described.

Symptoms: Initial symptoms of retinitis pigmentosa include glare, night blindness, progressive visual field defects, loss of visual acuity, and color vision defects. The age of manifestation depends on the type of inheritance.

Findings and diagnostic considerations: The diagnosis is made by ophthalmoscopy on the basis of a classic picture.

Rod-cone dystrophy (primarily the rods are affected first). "Bone-spicule" proliferation of retinal pigment epithelium is observed in the middle periphery of the retina. This will gradually spread toward the center and farther peripherally (Fig. 12.31). Early deficits include color vision defects and disturbed contrast perception. Atrophy of the optic nerve, discernible as a waxy yellow appearance of the optic disk, will occur in the advanced stages. The arteries will appear narrowed, and the fundus reflex will be extremely muted. The patient will typically have a "gun-barrel" visual field with good visual acuity for a surprisingly long time but with progressive loss of the peripheral visual field.

Cone-rod dystrophy (primarily the cones are affected first). Here, there is early loss of visual acuity with gradual progressive loss of visual field. In both forms of retinitis pigmentosa, the diagnosis is confirmed by electroretinogra-phy. Light response in the electroretinogram will be sharply reduced or absent early in the clinical course of the disease.

Differential diagnosis: Differential diagnosis should consider changes collectively referred to as pseudoretinitis pigmentosa because they simulate the clinical picture of retinitis pigmentosa. The most common causes that should be excluded in this context are:

— Advanced retinitis pigmentosa.

— Advanced retinitis pigmentosa.

Fig. 12.31

Typical signs include narrowed retinal vessels, waxy yellow appearance of the optic disk due to atrophy of the optic nerve, and "bone-spicule" proliferation of retinal pigment epithelium.

❖ Posttraumatic changes.

❖ Postinflammatory or postinfectious changes. These may include degenerative retinal pigment epithelial disease secondary to rubella with "salt and pepper" fundus of punctate areas of atrophy and proliferation of retinal pigment epithelium. Other causes include syphilis, which may present with placoid lesions of pigment epithelial atrophy and proliferations.

❖ Medications, such as chloroquine, Myambutol (ethambutol), and thioridazine.

Treatment: The causes of the disorder cannot be treated. Edge-filtered eyeglasses (eyeglasses with orange or blue colored lenses that filter out certain wavelengths) and magnifying near vision aids can help make better use of the patient's remaining vision.

Prophylaxis: No prophylaxis is possible.

Clinical course and prognosis: Retinitis pigmentosa is chronically progressive. The clinical course depends on the specific form of the disorder; severe forms lead to blindness.

12.6 Toxic Retinopathy Definition

Retinal changes resulting from use of medications.

Epidemiology: Toxic retinopathy is rare.

Pathogenesis: Toxic retinopathy can remain asymptomatic for a long time. Loss of visual acuity occurs if the macula is affected.

Chloroquine in doses exceeding 250 g causes retinal damage. Macular edema can occur initially. Later, punctate pigment epithelial changes develop, which may progress to bull's eye maculopathy with concentric rings of hypopigmentation and hyperpigmentation in the macular region (Fig. 12.32). These findings are usually bilateral and symmetrical. Other toxic retinal changes are listed in the appendix.

Diagnostic considerations: The diagnosis is made by binocular ophthalmos-copy with the pupil dilated and confirmed by electrophysiologic studies that include an electroretinogram, electro-oculogram, and visual evoked potentials (see Fig. 12.2a).

Differential diagnosis: Retinal pigment epithelium or retinal bleeding can result from many other retinal disorders, and may also be associated with the underlying disease for which the medication was prescribed.

— Chloroquine toxicity (bull's eye maculopathy).

— Chloroquine toxicity (bull's eye maculopathy).

Retinal Neovascularization
Fig. 12.32 Chronic use of this medication causes concentric rings of atrophy and proliferation of retinal pigment epithelium (arrows).

Treatment: The medication should be discontinued if possible.

Prophylaxis: Regular ophthalmologic follow-up examinations are indicated before and during treatment that involves medications with known ocular side effects.

Clinical course and prognosis: The clinical course depends on the specific medication and dose. Findings may improve after the medication is discontinued. However, with chloroquine in particular, findings may continue to worsen even years later.

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