Amblyopia and Visual System Quiz

Visual Development & Amblyopia

• Amblyopia is reduction in visual acuity due to neurological deficits that impact visual output. It typically develops during critical periods of visual development.
• It's usually unilateral and is the most common cause of vision loss and monocular blindness in children.
• Amblyopia is categorized into various types, including:
  • Stimulus deprivation
  • Strabismic (eye misalignment)
  • Anisometropic (unequal refractive errors)
  • Ametropic (refractive errors like nearsightedness or farsightedness)
  • Meridional (astigmatism)
  • Idiopathic (unknown cause)
  • Hysterical/psychogenic (psychological factors)
  • Organic
  • Toxic
• The prevalence of amblyopia is reported between 1–5% worldwide. The incidence may be higher in medically underserved or low socioeconomic populations. These figures may vary between studies.
• The Greek word "amblyopia" means "dull sight" or "blunt sight."
• Visual function involves light sense, form sense, colour sense and motion sense.
• Visual acuity development has critical and sensitive periods
• Normal visual experience is needed for the rods and cones in the retina to synapse with ganglion cells
• Parvocellular (X) system needs a stable, well-focused image.
• 80-85% of visual cortex cells respond to vision from either eye.
• 15-17% of visual cortex cells respond to vision from an eye with strabismus.

Development of Visual Acuity

• Visual acuity develops from birth to 3-5 years old.
• A critical period for visual development exists from early childhood (few months to 7-8 years old).
• A sensitive period for visual development exists from deprivation until teenage years (some cases extend to adulthood).

Critical Period

• Deprivation during critical period results in loss of visual function.
• Neural plasticity makes vulnerable visual systems to abnormal experiences such as strabismus and other visual deprivation
• Severity of changes during these periods depends upon age when the defect develops.
• Amblyopia typically starts when an infant becomes binocular (2-4 months old).

Sensitive Period

• Amblyopia is treatable when treated during the sensitive period.
• Amblyopia is most successfully treated in children.
• The younger the amblyopic patient, the more rapid the response to treatment.
• Treatment is rarely successful after age 8.
• Exceptions include anisometropic amblyopia or those with no previous treatment.

Interfering factors in Normal Visual Development

• Cataract
• Unilateral ptosis (drooping eyelid)
• Injuries affecting the eye or the back of the head (e.g., occipital cortex)
• Tumours such as optic nerve tumours
• Anisometropia (unequal refractive errors causing different image sizes)
• Nystagmus (involuntary eye movements)

Mechanism of Amblyopia

• Stimulus deprivation results in a loss of form vision.
• Abnormal binocular interaction or competition results in incomplete images formed on the retina.

Amblyopia - Reduced Vision

• Loss of Snellen and grating acuity
• Loss of sensitivity to contrast
• Distortions in stimulus shape
• Uncertainty about stimulus position
• Motion deficits
• Crowding effect and separation difficulty

Crowding Phenomenon

• Line of letters of symbols identified less easily than single optotypes.
• A key characteristic of amblyopia, more prominent in amblyopic patients.

Types of Amblyopia

• Stimulus deprivation
  • Affects one or both eyes
  • Complete - little or no light entering the eye.
  • Partial - some light hits the retina, but poor quality image.
  • Conditions such as ptosis, corneal scar, or congenital cataracts may result in stimulus deprivation.
• Strabismic
  • Constant or near-constant unilateral strabismus
  • Onset usually in childhood.
• Anisometropic
  • Significant difference in refraction between the two eyes.
  • One eye receives a clearer image for all distances, while the other eye's image might be blurry.
• Ametropic
  • High bilateral hypermetropia (farsightedness) greater than 6.00DS frequently causes reduced visual acuity and cannot be compensated by the eye's accommodation
  • Bilateral occurs in severe myopia (shortsightedness).
• Meridional
  • Moderate or high degree of uncorrected astigmatism.
• Idiopathic
  • No refractive error.
  • Normal BSV.
  • Foveal suppression scotoma (4^ test).
• Hysterical and psychogenic
  • Psychogenic or psychosomatic disorders.
  • Malingerring.
• Organic
  • No lesion detectable but can be irreversible.
  • Conditions like achromatopsia, albinism, nystagmus, and MD may present.
• Toxic
  • Visual acuity loss occurs due to absorption or exposure to toxins such as cyanide, ibuprofen (some cases are reversible).
  • Specific examples include tobacco, alcoholics, extreme diets.

Investigation & Diagnosis of Amblyopia

• Case history
• Fundus and media examination
• Refraction test
• Visual acuity test
• Crowded visual acuity
• Grating acuity (careful interpretation)
• Pinhole test
• Contrast sensitivity test
• Strabismic & anisometropic amblyopes - may have marked losses of CS especially at higher spatial frequencies
• Cover test
• Prism test (10°prism)
• Uniocular fixation
• Neutral density filter test

Fixation Patterns and Amblyopia

• Strabismic amblyopia typically presents with constant unilateral deviation.
• Accuracy of fixation
• Ability to hold fixation (blink/versions)
• Objections to covering one eye.
• Cross fixation.

10° Fixation Test

• Allows observation of fixation preference in straight or small angle strabismus.
• 10° vertical prism is held over one eye (BD).
• Induces vertical deviation.
• Abnormal patterns include spontaneous alternation, delayed re-fixation, limited fixation durations, re-fixation delays not induced by blinks or pursuit movement.

Direct Ophthalmoscope

• Viewing the inside portion of the eye
• Filter switch, aperture dial, diopter dial, and rheostat are used to adjust viewing conditions.

Uniocular Fixation

• Used as a diagnostic tool to test fixation with occlusion of the fellow eye.
• Normal fixation is directed toward the fovea.

Direct Ophthalmoscope Procedure

• Patient positioning and ophthalmoscope adjustments.
• Darkening of the room.
• Target fixation and finding the red reflex.
• Identifying the optic nerve and retinal blood vessels.
• Assessing fixation using graticule target and occluding fellow eye.
• Identifying macula and fovea.

Retina

• Macula, Fovea, Optic Cup and Optic Disc are central parts of the eye.
• Bright spot of light is seen in the fovea when the patient looks directly into the light

Central and Eccentric Fixation

• Central fixation - fovea used to receive the image (lying in the principle visual direction)
• Eccentric fixation - fixation on a point on the retina other than the fovea.

Eccentric and Wandering Fixation

• Fixation further away from fovea, worse vision.
• Area of retina used for fixation, visual acuity indication.
• Wandering fixation is a uniocular condition where the fovea has lost its superiority and fixation wanders around.

Neutral Density Filter (NDF)

• Reduces all wavelengths of light equally.
• Amblyopic eyes perform well in low lighting.
• Differentiation between strabismic amblyopia and reduced VA due to organic factors.

Neutral Density Filter (NDF) Test

• Patient wears refractive correction.
• Record VA distance uniocularly.
• Occlude the abnormal eye and place NDF in front of the normal eye.
• Increase NDF density until the point where VA drops by 2 lines.
• Occlude the normal eye and hold the same filter over the abnormal eye.
• Record VA.

SBISA Bar

• 17 filters ranging from palest pink to dark red.
• Used to assess density of suppression.
• Used to judge the risk of intractable diplopia during occlusion treatment.
• Method: Patient fixes on a spotlight at near. Lightest filter is placed over fixing eye. Density of filter is increased until the patient reports diplopia (one red, one white light), light changes from red to white.

Management of Amblyopia

• Aim is to achieve and maintain maximum visual acuity.

• Remove any cause of stimulus deprivation and correct the refractive erros.

• Choose appropriate form of treatment (eg: occlusion, atropine).

• Discontinue treatment when maximum acuity is achieved or watch for stabilization.

• Go back to Step 2 (if VA isn't maintained).

• Refractive Adaptation (if glasses required).

• Occlusion

• Atropine

• Optical penalization.

Types of Treatment

• Total light occlusion
• Partial form occlusion
• Cycloplegic occlusion
• Optical penalization
• Total form occlusion

Management Considerations

• Age
• General health and vision
• Fixation type
• School status
• Duration and type of strabismus
• Nystagmus
• Density of suppression

Advice to Parents

• Involve patient in choices.
• Encourage compliance (especially in lower-income families, with education as a motivator).
• Previous admittance to children's ward due to poor compliance.

Contraindications

• Patch/atropine allergy.
• Eye infection.
• Pathological causes of reduced VA.
• Low density of suppression (high risk of intractable diplopia)
• Previous treatment failure.
• Child with multiple diagnoses/syndromes
• Terminal diagnosis
• Emotional upset by treatment
• Social problems.

When to Stop Treatment

• When visual acuity (VA) is equal to the fellow eye on a linear test.
• When free alternation occurs
• When there is no further improvement, or the density of suppression reduces with a high risk of intractable diplopia.

Prognosis

• Successful outcomes in 63-83% of patients
• Success is dependent on Treatment age, Patient severity of amblyopia, Treatment compliance , Type of amblyopia, Number of amblyogenic factors
• (E.g., strabismic and anisometropic factors).
• The treatment is more cost-effective than cataract and macular hole treatments.

Extra Reading

• Stewart, CE, Moseley MJ, Fielder AR, et al. Refractive adaptation in amblyopia: quantification of effect and implications for practice. British Journal of Ophthalmology, 2004;88:1552–1556.
• Rowe, Fiona J. Clinical Orthoptics. 3rd ed. Hoboken: John Wiley & Sons, 2012.
• Ansons, Alec M, and Helen Davis. Diagnosis and Management of Ocular Motility Disorders. Hoboken: Wiley, 2013.
• Levi DM. Rethinking amblyopia 2020. Vision Res. 2020 Nov;176:118-129. doi: 10.1016/j.visres.2020.07.014. Epub 2020 Aug 28. PMID: 32866759; PMCID: PMC7487000.