What are the causes of strabismus?

Causes of Strabismus

Strabismus is caused by numerous factors affecting ocular alignment, including congenital disorders, neurological conditions, sensory deficits, mechanical restrictions, and iatrogenic causes, with the etiology determining the appropriate management approach. 1

Congenital and Childhood Causes

• Recurrent, decompensated, or unresolved childhood strabismus - Often persists into adulthood
• Genetic factors - Mutations in genes necessary for normal development of brainstem ocular motoneurons (PHOX2A, SALL4, KIF21A, ROBO3, HOXA1) 2
• Central nervous system abnormalities - Deficits in central neural pathways involved in visual perception and oculomotor control 3

Sensory Causes

• Sensory strabismus - Results from persistent visual deprivation affecting sensory fusion 4
• Severe anisometropia - Significant difference in refractive error between eyes
• Vision loss - From conditions like macular degeneration, epiretinal membranes, diabetic retinopathy
• Fixation switch diplopia - When previously suppressed eye gains better vision 1

Neurological and Muscular Causes

• Cranial nerve palsies - 3rd, 4th, and 6th nerve palsies affecting extraocular muscle function
• Myasthenia gravis - Neuromuscular junction disorder affecting eye muscle function
• Skew deviation - Vertical misalignment due to supranuclear pathway disruption

Age-Related Causes

• Sagging eye syndrome - Common cause of acquired binocular diplopia in patients 60-80 years old, more frequent in myopes 1
• Divergence insufficiency - Age-related esotropia at distance

Mechanical and Restrictive Causes

• Graves' disease - Thyroid eye disease causing restrictive strabismus
• Orbital trauma - Direct injury to extraocular muscles or their attachments
• High axial myopia - Including strabismus fixus

Iatrogenic Causes

1. Anesthetic myotoxicity

   • Following retrobulbar or peribulbar anesthesia
   • Initially presents as transient paresis followed by deviation
   • Superior and inferior rectus muscles most commonly affected 1

2. Surgical complications

   • Cataract/refractive surgery - Can unmask pre-existing strabismus or cause fixation switch diplopia
   • Glaucoma filtering surgery - Mass effect from implant/bleb, scarring, fat adhesion
   • Scleral buckling - Reported in up to 60% of patients after retinal detachment repair 5
   • Pterygium surgery - Direct injury to medial rectus or scarring
   • Blepharoplasty - Trochlear damage (upper lid) or inferior rectus damage (lower lid)
   • Ptosis repair - Can reveal previously unrecognized strabismus 1

3. Direct muscle/tissue damage

   • Bridle suture complications
   • Temporary muscle disinsertion during other procedures
   • Hardware implantation (scleral buckles, glaucoma plate reservoirs)

Binocular Vision Disorders

• Convergence insufficiency - Difficulty maintaining binocular fusion at near
• Foveal misregistration - Binocular central diplopia

Risk Factors for Iatrogenic Strabismus

• Injection by non-ophthalmologist
• Left eye injection
• Absence of hyaluronidase in anesthetic block 1
• Topical anesthesia has lower risk than regional anesthesia

Diagnostic Considerations

• Determining whether muscle/nerve damage is responsible
• Assessing whether paresis or restriction is the primary cause
• Evaluating for central versus peripheral origin of strabismus 3

Management Implications

• Success rates approximately 80% after one procedure, may exceed 95% with second procedure
• Complete elimination of diplopia may be difficult in cases with multifactorial etiology
• Multidisciplinary approach often required for complex cases 1

Understanding the specific cause of strabismus is crucial for determining appropriate treatment strategies and predicting outcomes, particularly in adult patients where quality of life can be significantly impacted by persistent diplopia and cosmetic concerns.