Amblyopia

Amblyopia, also known as lazy eye, is a sensory vision impairment that occurs in the central visual pathways without any structural eye disease. The primary causes of amblyopia are classified into three main types:

1. Anisometropic Amblyopia
2. Strabismic Amblyopia
3. Deprivation Amblyopia

Anisometropic Amblyopia

• Occurs when one eye has a significantly higher refractive error than the other, leading to insufficient visual stimulation.
• A difference of two lines or more in visual acuity between the two eyes is considered amblyopia.
• Contrast sensitivity curve is primarily affected at high spatial frequencies.
• Amblyopia is less common in myopia below -3.0 D, while in hyperopia, a difference of +1.0 to +2.0 D between eyes may lead to amblyopia.

Strabismic Amblyopia

• Develops in children with strabismus, where one eye is used for fixation while the other deviates.
• Alternating fixation prevents strabismic amblyopia.
• Esotropia (inward deviation) is more likely to cause amblyopia than exotropia (outward deviation).
• Can develop from birth to seven years old.
• Successfully treated with occlusion therapy, but may recur until around 9-10 years of age.
• Studies show that even in middle-aged and adult patients, vision in the amblyopic eye can improve if the dominant eye is lost.

Deprivation Amblyopia

• Caused by obstructions in the visual axis, such as congenital corneal opacities, cataracts, eyelid tumors, or intraocular pathologies that significantly reduce visual input.
• Contrast sensitivity and visual acuity are equally affected across all spatial frequencies.

Diagnosis & Auxiliary Tests

• Amblyopia should be suspected in children with strabismus or a strong fixation preference for one eye.
• It should also be considered in cases where:
  • Hyperopic difference >2.0 D
  • Myopic difference >4.0 D
  • Astigmatism difference >1.25 D
• Afferent pupillary defects may be present in severe amblyopia.
• Color vision remains normal in amblyopic patients who have sufficient vision to detect colors.
• Fixation tests:
  • If a child resists covering one eye, amblyopia may be present in the uncovered eye.
  • If both eyes alternate fixation equally, amblyopia is unlikely.
  • In severe amblyopia, the affected eye will drift away after a short period of fixation.
• Crowding phenomenon:
  • Amblyopic children may identify single letters better than grouped letters on a vision chart.
• Eccentric fixation:
  • Severe amblyopia can cause the patient to fixate with a non-foveal retinal area instead of the central fovea.

Critical Periods

• Greatest risk for amblyopia occurs when one eye is occluded for 4-6 weeks in early life.
• Studies show that only three days of monocular occlusion during this period can lead to a dominance shift in cortical neurons.
• Sensitivity gradually decreases but remains until 3-9 months of age.

Treatment

• Correction of refractive errors is the first and most crucial step.
• Any strabismus should be corrected with glasses or surgery.
• Main treatment options include:
  1. Occlusion Therapy
  2. Penalization Therapy
  3. Pleoptic Therapy
  4. CAM Treatment

Occlusion Therapy

• The gold standard treatment, forcing the use of the amblyopic eye by covering the dominant eye.
• Different occlusion methods:
  • Full-time occlusion of the healthy eye.
  • Part-time occlusion (5-6 hours per day) until vision equalization is achieved, followed by maintenance occlusion (1-2 hours daily) until 9 years old.
  • Alternating occlusion (covering the good eye for 2-7 days and the amblyopic eye for 1 day).
• Important considerations:
  • Occlusion should not cause amblyopia in the healthy eye.
  • Children may resist occlusion, requiring parental engagement, interactive activities, and modern digital tools to maintain compliance.
  • In strabismic children, the uncovered eye may start to deviate, which is considered a sign of successful treatment.
  • If no improvement is observed after six months of consistent occlusion, treatment should be discontinued.
  • Treatment goal: Equal or near-equal vision in both eyes.

Penalization Therapy

• Uses atropine drops to blur the vision in the dominant eye, forcing the brain to use the amblyopic eye.
• Mainly used for mild to moderate amblyopia.

Pleoptic Therapy

• Uses a modified direct ophthalmoscope (pleoptophor) to stimulate the peripheral retina with bright light, followed by foveal stimulation to reinforce central vision.

CAM Treatment

• Particularly effective in anisometropic amblyopia but also useful for strabismic amblyopia.
• Uses rotating high-contrast black and white grating patterns to stimulate vision.
• Treatment typically involves 7-minute sessions per eye with increasing complexity.
• Discontinuation may lead to regression, so occlusion therapy should be continued in parallel for long-term stability.