Amblyopia and strabismus are both visual disorders. Eyes, ocular nerve pathways, and brain centres need to function correctly for us to see well. Strabismus is a disorder of the extra ocular muscle or the supplying motor nerves. Amblyopia is a brain developmental disorder. This article will talk about both Amblyopia and Strabismus in detail and also the differences between both, highlighting their clinical features, causes, and treatment methods.
Amblyopia
Amblyopia is a disorder of the brain. It is not due to any eye disorder. However, in some cases, an early onset eye disorder can lead to amblyopia which persists even after the eye disorder resolves. Amblyopia is a developmental disorder where the part of brain receiving signals from the affected eye does not develop properly as it does not get stimulated to its full potential during the critical period. The critical period is the time duration from birth to two years in humans, where the visual cortex of the brain develops exponentially due to the magnitude of visual information it receives. When there is a lack of visual stimulation the visual cortex fails to develop properly as demonstrated in vision deprived kittens by Dr. David H Hubel. He won the Nobel Prize for physiology due to his work in this field.
Many people are unaware of their amblyopia because it is mild enough to go unnoticed. Routine tests may pick those people up. Visual disorders like impaired depth perception, poor special acuity, low contrast sensitivity, and reduced motion sensitivity are commonly seen in amblyopic individuals. There are three types of amblyopias. Strabismus amblyopia is due to early onset strabismus, or misalignment of eyes. Adult onset strabismus results in double vision because the relevant brain areas develop early in life. Strabismus usually means normal vision in the preferred eye and abnormal vision in the deviated eye. Early onset strabismus send altered signals to the brain area receiving signals from the deviated eye and this interrupts normal development of the visual cortex. If untreated, this results in abnormal vision when strabismus is corrected later. Refractive amblyopia is due to refractive errors. When there is a difference between the refraction of the two eyes, the signal send to the brain gets skewed. When there is a refractive error which is not corrected during the critical period, amblyopia results. Occlusion amblyopia is the abnormal development of the visual cortex due to early opacification of ocular media (lens, vitreous, aqueous).
Treatment of amblyopia consists of correction of the underlying visual deficit and mono-ocular improvement therapies.
Strabismus
Strabismus is a misalignment of the two eyes. It is mostly due to uncoordinated movement of extra ocular muscles. There are many types of and presentation of strabismus. If there is a deviation when looking with both eyes, it is called heterotropia. This includes horizontal deviation (outward and inward) as well as vertical (one eye is slightly higher or lower than the other) deviation. Horizontal outward deviation is also known as divergent squint, and horizontal inward deviation is also known as convergent squint. If there is deviation only when looking with one eye or the other, it is known as heterophoria. This also includes two horizontal and two vertical deviations. Misalignment of eyes may or may not be due to extra ocular muscle paralysis. If it is due to muscle paralysis it is called paretic and if it is not, non-paretic. Paretic misalignment may be due to cranial nerve palsies, opthamloplegia and Kearn-Sayre Syndrome.
Diagnosis of strabismus is clinical, with cover test. Prism lenses, Botulinum toxin, and surgery are the common treatment methods for strabismus.
What is the difference between Amblyopia and Strabismus?
• Strabismus is misalignment of eyes while amblyopia is an abnormal development of the visual areas of the brain.
• Strabismus is a primary eye disorder while amblyopia is a consequence.
• Strabismus may come at any age while amblyopia always starts during the critical period.