A 52-year-old woman on long-term oral corticosteroids (prednisolone 15 mg daily for 8 years due to systemic lupus erythematosus) presents with sudden onset of glare and blurred vision in both eyes over the past 3 months. She reports that her vision is worse in dim light and bright light causes severe discomfort. On slit-lamp examination, fine granular opacities are seen at the posterior pole of the lens, just beneath the posterior capsule. Dilated fundoscopy is otherwise unremarkable. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Posterior Subcapsular Cataract (PSC) **Key Point:** Posterior subcapsular cataracts are **steroid-induced** opacities that develop at the posterior lens pole, beneath the posterior capsule. They are the most common iatrogenic cataract and present with **rapid vision loss** and **glare in dim light**—a distinctive clinical pattern. ### Epidemiology & Pathogenesis - **Incidence:** 1–2% risk with systemic corticosteroids; dose and duration dependent - **Mechanism:** Steroids impair lens epithelial cell metabolism and protein synthesis, causing posterior epithelial cell migration and vacuole formation - **Timeline:** Can develop within months of starting high-dose steroids (>15 mg prednisolone daily for >1 year) - **Reversibility:** Early PSC may regress if steroids are withdrawn; advanced PSC is irreversible ### Clinical Features of Posterior Subcapsular Cataract | Feature | Posterior Subcapsular Cataract | | --- | --- | | **Onset** | Rapid (weeks to months) | | **Location** | Posterior pole, just beneath posterior capsule | | **Appearance** | Fine granular, vacuolar opacities; "breadcrumb" pattern | | **Vision loss** | Disproportionate to opacity size (posterior location affects visual axis) | | **Glare** | Severe in **dim light** (opposite of cortical) | | **Monocular diplopia** | Present and prominent | | **Progression** | Rapid if steroids continued; may stabilize if stopped | | **Associated symptoms** | Photophobia, difficulty reading | ### Why Posterior Subcapsular Cataract Fits This Case 1. **Long-term high-dose corticosteroid use** — 15 mg prednisolone daily for 8 years is a major risk factor 2. **Rapid onset** — 3 months is typical for steroid-induced PSC 3. **Glare in dim light** — pathognomonic for posterior pole opacities 4. **Posterior pole granular opacities** — classic slit-lamp finding 5. **Bilateral presentation** — consistent with systemic steroid exposure 6. **Disproportionate vision loss** — posterior location affects visual axis despite small opacity size **Clinical Pearl:** Posterior subcapsular cataracts cause **glare and monocular diplopia in dim light** because the posterior opacities are in the visual axis and scatter light rays entering the eye. Patients often report difficulty reading or watching television in dim rooms. This is the **opposite** of cortical cataracts, which cause glare in **bright light**. **High-Yield:** Steroid-induced PSC is **dose- and duration-dependent**. Risk increases significantly with: - Cumulative dose >1 g prednisolone equivalent - Daily dose >15 mg for >1 year - Systemic (not topical) steroids **Mnemonic: PSC = Posterior Steroid Cataract; Glare in dim light (not bright); Rapid onset** ### Differential Diagnosis: Posterior Subcapsular vs. Cortical vs. Nuclear ```mermaid flowchart TD A[Cataract presentation]:::outcome --> B{Onset speed?}:::decision B -->|Rapid<br/>weeks-months| C{Glare in dim or bright light?}:::decision B -->|Gradual<br/>months-years| D{Location of opacity?}:::decision C -->|Dim light| E[Posterior subcapsular]:::action C -->|Bright light| F[Cortical]:::action D -->|Periphery to center| G[Cortical]:::action D -->|Central nucleus| H[Nuclear sclerotic]:::action E --> I[Risk: Steroids, trauma, diabetes]:::outcome F --> J[Risk: UV exposure]:::outcome H --> K[Risk: Age, smoking]:::outcome ``` | Feature | Posterior Subcapsular | Cortical | Nuclear | | --- | --- | --- | --- | | **Onset** | Rapid (weeks–months) | Gradual (years) | Gradual (years) | | **Location** | Posterior pole | Peripheral cortex | Central nucleus | | **Appearance** | Granular, vacuolar | Radial spokes, white | Brown/amber nucleus | | **Glare** | Dim light | Bright light | Minimal | | **Monocular diplopia** | Prominent | Present | Absent | | **Risk factors** | Steroids, trauma, diabetes | UV exposure | Age, smoking | | **Reversibility** | Early cases may regress | No | No | **Management Implications:** - **Steroid continuation:** If PSC is mild and steroids are essential (as in SLE), continue steroids but monitor for progression - **Steroid reduction:** If possible, taper steroids to lowest effective dose - **Surgical referral:** Cataract extraction indicated when vision falls below 6/12 or patient is symptomatic - **Prognosis:** Excellent visual outcome post-surgery (no associated posterior capsule opacification if surgery is timely)