A 52-year-old woman with newly diagnosed primary open-angle glaucoma is being evaluated for structural damage assessment and baseline documentation. Her IOP is 28 mmHg, and automated perimetry shows early arcuate defect. Which investigation is most appropriate for quantifying optic nerve head damage and establishing baseline for monitoring?

Explanation

## OCT in POAG: Structural Assessment and Monitoring **Key Point:** OCT of the optic nerve head and RNFL is the gold standard imaging modality for quantifying structural glaucomatous damage and establishing baseline measurements for longitudinal monitoring of disease progression. ### Why OCT Over Other Imaging Modalities? | Modality | Strength | Limitation | Role in POAG | |----------|----------|-----------|---------------| | **OCT (RNFL + ONH)** | High resolution, reproducible, quantitative, detects early loss | Operator-dependent signal quality | **Gold standard for baseline & monitoring** | | **HRT (Confocal SLO)** | 3D topography, good for disc analysis | Less sensitive for RNFL, older technology | Complementary (rarely first-line) | | **Fundus autofluorescence** | Metabolic imaging of RPE | Poor specificity for glaucoma | Research use, not diagnostic | | **B-scan ultrasound** | Posterior segment imaging when media opaque | No structural glaucoma assessment | Not applicable here | **High-Yield:** OCT provides **quantitative RNFL thickness** and **optic disc cup-to-disc ratios**, which are reproducible and allow detection of progression (typically >5% annual RNFL loss indicates significant progression). ### OCT Parameters in POAG 1. **RNFL thickness** — normally 100–150 μm; thinning indicates axonal loss 2. **Optic disc cup-to-disc ratio** — increased cupping (>0.6) suggests glaucomatous damage 3. **Neuroretinal rim area** — loss of rim tissue is a hallmark of glaucoma 4. **Ganglion cell complex (GCC)** — emerging parameter for early detection **Clinical Pearl:** OCT can detect structural loss **before** visual field defects become apparent — making it valuable for early detection and monitoring in borderline cases. ### Timeline of Glaucomatous Damage Detection ```mermaid flowchart LR A[Elevated IOP]:::outcome --> B[Structural loss on OCT]:::outcome B --> C[Visual field defect on perimetry]:::outcome C --> D[Clinically visible optic atrophy]:::outcome style A fill:#fff4e6 style B fill:#e8f5e9 style C fill:#e3f2fd style D fill:#ffebee ``` **Mnemonic:** **RNFL** = **R**etinal **N**erve **F**iber **L**ayer (the structure OCT measures to detect glaucomatous damage). **Warning:** ~~Fundus photography alone cannot quantify RNFL thickness~~ — it is qualitative only. OCT provides objective, reproducible measurements essential for monitoring.