Microspherophakia: Characteristics and Genetics

Microspherophakia (MSP)

• Characterized by increased anteroposterior diameter and reduced equatorial diameter of the crystalline lens
• Rare condition, usually bilateral

Genetics

• Mutations in genes include:
  • LTBPs (family of proteins with structural homologies with fibrillins)
  • ADAMTS17 (part of the same family of metalloproteinases as ADAMTS10, associated with Weill-Marchesani syndrome)

Associations

• May occur as an isolated defect, or be associated with systemic/local anomalies
• Entities associated with microspherophakia include:
  • Weill-Marchesani syndrome
  • Homocysteinuria
• Familial microspherophakia is not associated with systemic abnormalities and has an autosomal recessive pattern of inheritance, but can also be autosomal dominant

General Pathology

• Zonular fibers may be abnormally long and somewhat rudimentary
• Abnormal development and distribution of secondary lens fibers
• Hyaloid degenerations occur due to changes in shape of lens affecting the lens fibers

Pathophysiology

• Mechanisms suggested include:
  • Zonular instability leading to lens dislocation/subluxation or secondary angle closure glaucoma

Diagnosis

• Thorough family history and history of systemic anomalies is required
• Symptoms:
  • Diminution of vision
  • Acute eye pain
• Signs:
  • Triad of angle closure glaucoma, shallow anterior chamber, and refractive myopia
  • Other findings include:
    • Lens dislocation/subluxation
    • Zonular instability
• Diagnostic procedures:
  • UBM (ultrasound biomicroscopy) for morphological details of anterior chamber angle, iris, ciliary body, zonules, and lens
  • Screening tests for associated systemic conditions (e.g. serum homocysteine levels, urine screening test for homocysteinuria)

Complications

• Glaucoma is common in microspherophakia (up to 51% of patients)
• Secondary angle closure glaucoma is the most common sight-threatening complication
• Progressive microspherophakia leads to severe and progressive lenticular myopia
• Zonular instability can lead to lens dislocation or subluxation

Management

• Medical and surgical management of complications can be difficult
• Cycloplegics are the treatment of choice
• Miotics may lead to pupillary block glaucoma
• Stepwise protocol for treatment of glaucoma in microspherophakia:
  • Medications
  • Laser trabeculoplasty
  • Trabeculectomy
• Indications for clear lens extraction in patients with microspherophakia:
  • Uncontrolled glaucoma
  • Progressive myopia
  • Zonular instability
• Intraoperative complications associated with clear lens extraction in these patients include:
  • Difficulty in performing capsulorhexis
  • Difficulty in implanting IOL due to smaller bag size