Recallab › Question bank › Ophthalmology › #50198
Retinitis pigmentosa
A 22-year-old man has had difficulty seeing at dusk since childhood and now bumps into objects at the sides. Fundoscopy shows attenuated retinal arterioles, a waxy pale disc and mid-peripheral bone-spicule pigmentation. Which investigation finding would you expect?
- correctA. A markedly reduced or extinguished electroretinogram
- B. A normal electroretinogram and a normal visual field
- C. A normal electroretinogram with an abnormal electro-oculogram
- D. An enlarged blind spot with a normal peripheral field
Explanation
Key fact
Retinitis pigmentosa is a rod-cone dystrophy, so the electroretinogram is reduced or extinguished - and it is abnormal before the fundus is.
A markedly reduced or extinguished electroretinogram. The photoreceptors are degenerating, and the ERG measures exactly that.
Why
Rods are affected first, which is why night blindness comes first.
Rods are densest in the mid-periphery, so the field constricts from the sides.
The classical triad on fundoscopy is arteriolar attenuation, waxy disc pallor and bone-spicule pigmentation.
The electroretinogram records the mass response of the photoreceptors.
It becomes abnormal early and is eventually extinguished - the single most useful test.
The electroretinogram fails before the fundus looks abnormal
Why not the others
- B. A normal electroretinogram and a normal visual field — contradicts the history. The field is characteristically constricted to a ring scotoma and then to tunnel vision.
- C. A normal electroretinogram with an abnormal electro-oculogram — is the pattern of Best disease, a vitelliform macular dystrophy, not retinitis pigmentosa.
- D. An enlarged blind spot with a normal peripheral field — is seen in papilloedema and optic disc disorders, not in a photoreceptor dystrophy.
Remember
Night first, sides next, centre last. Central vision is preserved until late, which is why patients present with collisions rather than blurred vision.
Go deeper
Inheritance may be autosomal dominant, recessive or X-linked; the X-linked forms are the most severe. Rhodopsin gene mutations are the commonest dominant cause.
Look for syndromic associations: Usher syndrome with congenital deafness, Refsum disease with raised phytanic acid, Bardet-Biedl and Laurence-Moon syndromes.
Common complications are posterior subcapsular cataract and cystoid macular oedema, both of which are treatable and worth looking for.
Khurana AK. Comprehensive Ophthalmology, 10th ed. (Jaypee, 2025); Harrison's Principles of Internal Medicine, 22nd ed. (McGraw Hill, 2025).