Ectopia Lentis: Zonule Status as the Key Differentiator

GROUP 1: Lens Subluxated WITH Intact Zonules

1. Marfan Syndrome (FBN1 gene - AD)

• Direction: Superotemporal (most characteristic) - upward and outward
• Zonule status: Zonules are intact but elongated due to fibrillin-1 deficiency; accommodation is often retained
• Degree: 80% have bilateral ectopia lentis; subluxation is the rule; outright dislocation into AC or vitreous is rare
• Key point: The subluxated lens tends to deviate toward the meridian of intact/less-stretched zonules

Fig. 10.29 Marfan syndrome: (A) arachnodactyly; (B) high-arched palate; (C) superotemporal subluxation with intact zonules (slit-lamp view showing stretched but present zonule fibers); (D) rare complete dislocation into vitreous

2. Familial Ectopia Lentis (AD)

• Bilateral symmetrical superotemporal displacement
• May manifest congenitally or later in life
• Isolated ocular condition - no systemic features
• Zonules present but weak/elongated

3. Ectopia Lentis et Pupillae (AR, rare)

• Congenital bilateral: lens and pupil displaced in opposite directions (lens temporal, pupil nasal, or vice versa)
• Small pupils that dilate poorly
• May be associated with microspherophakia
• Zonules are present, zonular dehiscence is partial

4. Weill-Marchesani Syndrome (AR or AD, FBN1)

• Conceptually the converse of Marfan syndrome - short stature, brachydactyly, learning difficulties
• Direction: Inferior subluxation, late childhood/early adulthood
• Zonule status: Early - zonules present but abnormal, leading to subluxation; later can progress to dislocation
• Microspherophakia (small, spherical lens) is characteristic
• High risk of angle closure from pupillary block due to microspherophakia - the spherical lens blocks aqueous flow

5. Aniridia

• Occasionally associated with ectopia lentis (inferior subluxation)
• Partial zonule weakness accompanies the iris hypoplasia

6. Sulfite Oxidase Deficiency

• Ectopia lentis is universal
• Zonule status varies; can have intact but functionally deficient zonules

GROUP 2: Lens Subluxated or Dislocated WITH Broken/Disintegrated Zonules

1. Homocystinuria (AR - cystathionine beta-synthase deficiency)

• Direction: Inferonasal - downward and inward (opposite of Marfan)
• Zonule status: Zonules disintegrate because they are rich in cysteine, which is deficient in homocystinuria. This is the most distinctive feature from Marfan
• Almost universal by age 25 in untreated cases
• Accommodation is lost
• Systemic: coarse blond hair, blue irises, Marfanoid habitus, neurodevelopmental delay, severe thrombotic predisposition (very important perioperatively - general anesthesia carries risk of thrombosis)
• Treatment: pyridoxine (B6), folic acid, vitamin B12 to reduce plasma homocysteine

Fig. 10.28: (A) Ectopia lentis et pupillae; (B) inferior subluxation in aniridia

2. Trauma (Blunt)

• The most common acquired cause
• Blunt force tears the suspensory ligament focally (partial) or completely (360° rupture → complete dislocation)
• Partial zonule rupture → subluxation; the lens tilts toward the meridian of intact remaining zonules
• Complete 360° rupture → dislocation into vitreous (most common) or anterior chamber (less common)
• Signs: phakodonesis (lens tremor), iridodonesis (iris tremor), deepened AC over dehiscent area, lens edge visible under mydriasis, uniocular diplopia if pupil partly aphakic
• An underlying predisposing condition (e.g., pseudoexfoliation) should always be considered when zonular rupture occurs with seemingly minor trauma

3. Pseudoexfoliation Syndrome

• Most common cause of acquired zonule weakness worldwide
• Exfoliation material deposits on and weakens the zonules progressively
• Leads to subluxation; complete dislocation can occur spontaneously or with minimal trauma (e.g., during cataract surgery)
• The zonule fibers are structurally abnormal and fragile - not just stretched
• High surgical risk: capsule/zonule complex is weak even if frank subluxation is not yet present

4. Chronic Inflammation / Cyclitis

• Prolonged uveitis (chronic cyclitis, syphilis) causes zonule dissolution through inflammatory mediators
• Zonules are directly destroyed by the inflammatory process
• Lens may subluxate or dislocate

5. Hypermature Cataract

• Liquefaction of the lens cortex with reduction in lens volume and shrinkage causes progressive zonule relaxation and eventual rupture
• The bag-zonule complex loses tension and the lens can subluxate or dislocate

6. High Myopia / Buphthalmos (Large Axial Length)

• Mechanical stretching of the zonules due to globe enlargement
• Zonules become thin, elongated, and may break, permitting lens subluxation
• More common in high myopia (>6D) and congenital glaucoma with buphthalmos

Summary Comparison Table

Clinical Pearls

• A subluxated lens always tilts toward the meridian of intact zonules - this helps localize the site of zonular weakness on examination
• Marfan vs. Homocystinuria: Both cause bilateral ectopia lentis, but Marfan goes up (intact zonules), homocystinuria goes down and in (zonules disintegrated). This is the classic exam distinction
• Phakodonesis/iridodonesis are signs of zonule instability - always look for these before cataract surgery
• Pseudoexfoliation can show phakodonesis without visible subluxation - always examine zonules carefully with retroillumination before surgery
• In homocystinuria, avoid general anesthesia if possible due to extreme thrombotic risk; if surgery is needed, anticoagulation and amino acid normalization are required preoperatively