A 28-year-old woman with a family history of progressive night blindness and peripheral vision loss presents with nyctalopia and constricted visual fields. Fundoscopy shows bone-spicule pigmentation and waxy optic disc pallor. Which investigation would best confirm the diagnosis of retinitis pigmentosa?

## Clinical Context The patient presents with classic features of retinitis pigmentosa (RP): progressive nyctalopia, peripheral vision loss, bone-spicule pigmentation, and optic disc pallor. The investigation must confirm the diagnosis by demonstrating the characteristic functional defect in photoreceptors. ## Why ERG is the Gold Standard for RP Diagnosis **Key Point:** Electroretinography (ERG) is the gold standard investigation for retinitis pigmentosa because it objectively demonstrates the characteristic photoreceptor dysfunction that defines the disease. ### ERG Findings in Retinitis Pigmentosa **Mnemonic:** **DARK** — Diminished A-wave and b-wave, Absent in advanced disease, Reduced amplitude, Kinetic (progressive) - **Rod dysfunction first:** Scotopic (dark-adapted) ERG shows severely reduced or extinguished b-wave with relatively preserved a-wave (rod-cone dystrophy pattern) - **Progressive cone involvement:** Photopic (light-adapted) ERG becomes abnormal as disease advances - **Diagnostic sensitivity:** Detects functional abnormality before significant ophthalmoscopic changes - **Objective measure:** Quantifies disease severity and monitors progression - **Genetic counseling:** Helps establish diagnosis in family members with minimal symptoms **High-Yield:** ERG is pathognomonic for RP and is essential for diagnosis, especially in early stages when fundoscopic findings may be subtle. ## Comparison of Investigations in Retinitis Pigmentosa | Investigation | Role in RP | Findings in RP | Diagnostic Value | |---|---|---|---| | **ERG** | Gold standard for diagnosis | Reduced/extinguished scotopic b-wave; progressive cone involvement | Pathognomonic; detects early disease | | **OCT** | Structural assessment; monitors photoreceptor loss | Thinning of outer nuclear layer; disrupted photoreceptor layer | Supports diagnosis; assesses macular involvement | | **FAF** | Assesses RPE integrity and disease progression | Speckled or ring-pattern autofluorescence | Prognostic; identifies areas of RPE degeneration | | **VEP** | Assesses optic nerve function | May be normal or mildly reduced | Non-specific; not diagnostic for RP | **Clinical Pearl:** In RP, the ERG is abnormal while the visual evoked potential (VEP) remains relatively normal, distinguishing retinal from optic nerve pathology. ## Diagnostic Algorithm for Suspected RP ```mermaid flowchart TD A[Nyctalopia + peripheral vision loss + bone-spicule pigmentation]:::outcome --> B{Confirm diagnosis}:::decision B -->|Objective functional test| C[ERG - Gold Standard]:::action C --> D{ERG findings}:::decision D -->|Reduced scotopic b-wave| E[Retinitis Pigmentosa Confirmed]:::outcome D -->|Normal ERG| F[Consider other diagnosis]:::outcome B -->|Structural assessment| G[OCT + FAF]:::action G --> H[Supports diagnosis; assess macular involvement]:::outcome ``` ## When to Use Supplementary Tests - **OCT:** To assess macular involvement and photoreceptor layer integrity; helps predict central vision prognosis - **FAF:** To identify areas of active RPE degeneration and predict progression; useful for counseling - **Visual field testing:** To document peripheral vision loss and monitor progression - **Genetic testing:** For family members and to guide potential gene therapy trials