Which of the following is the PRIMARY anatomical defect in involutional (senile) ectropion?

Explanation

## Anatomical Basis of Involutional Ectropion **Key Point:** Involutional (senile) ectropion results from **horizontal lid laxity** due to age-related attenuation and dehiscence of the medial canthal tendon and lateral canthal ligament, combined with horizontal lid stretching. ### Pathophysiology of Involutional Ectropion Involutional ectropion develops through a cascade of age-related changes: 1. **Horizontal lid laxity** — The primary defect - Attenuation of the medial canthal tendon (MCT) - Stretching of the lateral canthal ligament - Loss of elastic fibers in the tarsal plate - Dehiscence of the MCT from the lacrimal crest 2. **Loss of lid support** - Inferior rectus overaction (age-related) - Gravity and chronic traction - Chronic blepharospasm or rubbing 3. **Cicatricial changes** (secondary) - Conjunctival scarring from chronic exposure - Fibrosis of the lower lid retractors **High-Yield:** The **medial canthal tendon dehiscence** is the hallmark finding on examination; the lid can be pulled away from the globe easily (positive distraction test). ### Clinical Features - Outward turning of the lower lid margin - Exposure of the palpebral conjunctiva - Epiphora (paradoxically, due to lacrimal punctum eversion and poor drainage) - Conjunctival injection and chemosis ### Comparison with Other Lid Laxity Causes | Feature | Involutional | Paralytic | Mechanical | |---------|-------------|-----------|----------| | **Primary defect** | Horizontal laxity + MCT dehiscence | CN VII palsy | Scar contracture | | **Medial canthal tendon** | Attenuation, dehiscence | Intact | Intact | | **Facial nerve function** | Normal | Absent | Normal | | **Cicatricial component** | Secondary | None | Primary | **Clinical Pearl:** The **snap test** (lid recoil after manual depression) and **distraction test** (horizontal lid mobility) help quantify laxity and confirm the diagnosis.