Correct Answer: A. ERG – normal
Retinitis pigmentosa (RP) is a progressive hereditary retinal dystrophy characterized by photoreceptor degeneration. The electroretinogram (ERG) is markedly abnormal, not normal, making this the correct answer to a "NOT true" question. ERG shows reduced or extinguished amplitudes with prolonged implicit times, reflecting severe loss of photoreceptor function. This is a hallmark diagnostic finding in RP. The ERG abnormality occurs early and is disproportionate to visual symptoms, making it the gold standard for diagnosis and monitoring disease progression. In Indian clinical practice, ERG is routinely used to confirm RP diagnosis and differentiate it from other retinal dystrophies. The normal ERG would suggest a functional retina, which is incompatible with RP pathophysiology where photoreceptors (rods initially, then cones) undergo progressive apoptosis.
Why the other options are wrong
B. Pale waxy disc — This is a TRUE and characteristic finding in RP. The optic disc appears pale and waxy due to gliosis and loss of neural tissue from photoreceptor degeneration. This is a classic ophthalmoscopic sign seen in advanced RP and is part of the diagnostic triad. NBE includes this as a distractor because it is indeed present in RP. C. Retinal pigmentation — This is TRUE and pathognomonic for RP. The characteristic 'bone spicule' pigmentation in the peripheral retina results from RPE migration and proliferation in response to photoreceptor loss. This pigmentary change is so typical that it defines the disease name. It appears early and progresses centripetally. D. Narrowing of vessels — This is TRUE in RP. Retinal arteriolar narrowing occurs due to loss of metabolic demand from degenerating photoreceptors and associated gliosis. Vessel attenuation is a consistent finding on fundoscopy and angiography. NBE pairs this with RP to test knowledge of vascular changes secondary to photoreceptor loss.
High-Yield Facts
- ERG is abnormal in RP — reduced/extinguished amplitudes with prolonged implicit times; this is the diagnostic hallmark, not normal
- Bone spicule pigmentation in peripheral retina is pathognomonic for RP and results from RPE migration
- Rod-cone dystrophy pattern — rods affected first (night blindness, peripheral vision loss), cones affected later (central vision loss)
- Pale waxy optic disc appears due to gliosis and neural tissue loss from photoreceptor degeneration
- Retinal arteriolar narrowing occurs secondary to reduced metabolic demand from photoreceptor loss
- Inheritance patterns — autosomal recessive (most common in India), autosomal dominant, X-linked; genetic testing increasingly available in Indian centers
Mnemonics
RP Fundus Triad PPV — Pale waxy disc, Pigmentation (bone spicules), Vessel narrowing. All three are present in RP; ERG is abnormal (not normal). ERG in RP Memory Hook 'RP = ERG Down' — Retinitis Pigmentosa = ERG Dramatically reduced/absent. When you see RP, think 'ERG is dead,' not normal.
NBE Trap
NBE exploits the fact that students may confuse ERG findings across retinal diseases. In RP, ERG is severely abnormal (the diagnostic gold standard), but in conditions like central serous retinopathy or early age-related macular degeneration, ERG may be relatively preserved. The trap is offering "ERG normal" as a plausible-sounding option for a progressive retinal disease.
Clinical Pearl
In Indian clinical practice, a young patient presenting with night blindness and progressive peripheral vision loss should raise suspicion for RP. ERG testing (increasingly available in tertiary centers across India) is the confirmatory test — an abnormal ERG with normal fundoscopy in early stages is classic. Genetic counseling is critical in Indian families due to high rates of consanguinity in autosomal recessive RP.
_Reference: Robbins Ch. 29 (Retina); Harrison Ch. 427 (Retinal Diseases); Boyd & Bansal Ophthalmology Ch. 8 (Retinal Dystrophies)_