• Normal and equal visual input is necessary (trophic) for proper cell growth and synaptogenesis of the postchiasmal visual pathways in the brain (lateral geniculate, visual cortex).
• Visual system cell growth and synaptogenesis (“cortical vision”) is initiated at birth and is finished by 9 years of age. If, during this period, one eye (or both) is not capable of normal and equal visual input, synaptogenesis and cell growth is disturbed, resulting in deficient vision.
• The younger the cortical vision system the more sensitive it is to abnormal input.
• Amblyopia will develop after only 1 week of abnormal visual input in an infant less than 1 year old.
• The clinical rule is that a week of abnormal visual input per year of life is amblyogenic. The period of cortical visual development between birth and 17 weeks of life is the “sensitive, or critical period.”
• If a congenital or neonatal amblyogenic stimulus is not treated before the end of the sensitive period it is impossible to recover full and normal vision. A congenital or neonatal amblyogenic stimulus is more potent and leads to irreversible neuronal changes if not treated by about 4 months of age.

• Image-degradation amblyopia: there is usually either a structural problem involving the lids and periocular tissues (ptosis, hemangioma, orbital tumor, orbital inflammation), or poor image processing (retinal edema and inflammation, optic nerve dysplasia and atrophy).
• Strabismic amblyopia: infants and children (up to 9 years of age) with strabismus develop a neuro-adaptive mechanism to prevent diplopia. This is called “suppression” and is a facultative scotoma (blind spot) centered on the fovea of the non-fixing eye. This scotoma switches from one eye to the other when the patient changes fixation. When one eye becomes dominant the other is suppressed, leading to amblyopia (50% of patients with strabismus). In patients who regularly switch fixation between eyes amblyopia does not occur (50% of patients with strabismus).
• Ametropic amblyopia: This type of amblyopia results from abnormal cortical visual input due to uncorrected significant refractive errors. Children with significant myopia (nearsightedness), hyperopia (farsightedness), or astigmatism are unable to position themselves or the world so it is in focus on the retina. Amblyopia occurs when the retina transmits only a blurred image to the brain. A potent amblyogenic stimulus occurs when only one eye has a refractive error or there is a significant difference between the eyes (anisometropic amblyopia).

• Other causes of cortical visual loss
  • Structural disease involving the visual cortex
    • Tumors
    • Vascular malformations
    • Demyelination
    • Trauma
• Clinically silent ocular disorders
  • Retinal degenerations and dystrophies
  • Posterior optic atrophy
  • Glaucoma
  • Early corneal dystrophy
• Hysterical visual loss or conversion reaction

Question: How, when, and where was visual loss first noticed?
Significance: Age of onset and duration of vision loss is crucial in reversibility of amblyopia.

Question: How has this changed?
Significance: Worsening acuity may imply an organic component to the amblyopic vision loss.

Question: Other ocular abnormalities noted or treated?
Significance: These may contribute to the etiology of the amblyopic vision loss.

Question: Familial ocular history?
Significance: There are forms of amblyopia with higher prevalence in families.

Question: The presence of any trauma, prenatal, perinatal, or postnatal medical or surgical problems?
Significance: These questions will aid with determining the etiology of the amblyopia.

Question: Current and past medications and known allergies?
Significance: These routine questions may be important in the medical management of the strabismus, e.g., use of certain topical eye drops.

Question: Exposures to toxins or new climates/travel/day care, or recent systemic illness?
Significance: These historical questions also aid in determining organic causes of vision loss in addition to amblyopia, e.g., toxoplasmosis.

Finding: Accurate monocular and binocular visual acuities are the most sensitive indicators of amblyopia. Snellen (letter), Allen (picture), or HOTV charts are the most accurate testing methods.
Significance: The ultimate diagnosis of amblyopia is determined by an accurate measurement of visual acuity.

Finding: Other important diagnostic signs are the presence of head posture, strabismus, nystagmus, abnormal pupillary responses, photophobia, and absent or asymmetric red fundus reflexes.
Significance: All of these physical signs can help with differentiating organic vision loss from amblyopia.

Finding: A general developmental and neurological examination
Significance: Helps rule out obvious central nervous system pathology.

Test: Hematologic
Significance: There are no hematologic tests or blood chemistries that will help with diagnosis. These are ordered as indicated by the diagnosed amblyogenic stimulus (e.g., uveitisblood analysis).

Test: Radiologic
Significance: No imaging is helpful for diagnosis, but may be used to define associated condition (trauma, developmental or acquired orbital and lid abnormalities).

Test: Electrophysiologic
Significance: Visual evoked responses and electroretinography (see Strabismus chapter) are useful to diagnose and characterize any associated organic disease of the afferent visual system (retinal degeneration, optic nerve dysplasia, anoxic encephalopathy). The presence of both amblyopia and organic disease affects the prognosis of visual recovery from amblyopia treatment.

• Amblyogenic condition: Once the diagnosis of amblyopia is made, treatment is first directed toward reversing or decreasing the amblyogenic stimulus. Image-degradation amblyopia is treated by medically or surgically clearing the ocular media (lid hemangioma reduction, cataract extraction, ptosis surgery, corneal transplantation). Strabismic amblyopia is treated prior to surgery as part of the general treatment. This usually consists of a combination of spectacles and penalization. Ametropic amblyopia is first treated by giving the patient full correction of the optical error either with spectacles or contact lenses full time. As long as vision improves no additional treatment is needed.
• Penalization: If treating the amblyogenic condition alone does not restore normal vision then penalization is added. Occlusion of the good eye with an adhesive patch on the eye is the most effective method. This is started for all but 1 hour a day (to prevent amblyopia in the good eye). The patient is followed closely, 1 week for every year of age to a maximum of once a month. Patching is then tapered based on the patient’s response. Many children will not tolerate a patch, although alternatives (contact lenses, tinted glasses, optical blurring, plaster casts) all prove inadequate. It has been shown that pharmacologic cycloplegia of the good eye (atropine drops daily) is effective at stabilizing vision and improving vision in some cases. Atropine cycloplegia of the good eye results in poor near vision, forcing the patient to use the amblyopic eye for close vision. This is effectively a “near patch.” Experimental treatment of amblyopia in older children and adults with a systemic dopamine agonist (L-dopa) resulted in improved visual function, thus opening a new avenue of research for treatment after visual maturation.

• The effectiveness of treatment should be closely monitored, as the most common reason for a poor response with penalization is non-compliance. Frequent visits provide positive reinforcement, reassurance, and encouragement needed by both the family and the patient.
• Patients are followed at intervals according to their age. A general rule is 1 week for each year of life up to 4 to 6 weeks for children over 4. Once vision is stabilized, visits and penalization can be tapered to maintain best vision until 8 to 9 years of age.
• Contact with social services for blind and visually handicapped individuals must be made for children even if they are only suspected of being visually impaired. Encourage families to make the contact even when the child may be too young to provide objective data of the extent of handicap.

• Screening children as soon as possible, testing visual acuity in school and by the primary care physician, is currently the best method of identifying possible amblyopia and improving the treatment prognosis.
• The general compliance rate with penalization is 50%, effectively reducing the prognosis for attainment of normal vision.

Q: What will be the final vision of an amblyopic eye?
A: The greater the structural or visual difference between the two eyes the less the chance of achieving normal vision. Obtaining 20/200 vision from counting fingers vision with treatment gives the patient a functional eye.

Q: Will the eye see normal after successful treatment?
A: An amblyopic eye which is 20/20 after treatment remains subjectively different from the normal eye to the patient for his or her lifetime.

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