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Amblyopia

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Title: Amblyopia—Pathophysiology 

 Author: Austin D. Bohner, MS4 University of Utah 

Date: 07/10/19 

Introduction/Definition: 

Brief description 

Amblyopia is caused by the dysfunction of the processing of visual information.1 This dysfunction results from the lack of visual stimulus on the retina and subsequent suppression of the development of cortical visual centers in the brain.2  

Epidemiology:  

Amblyopia is the most common cause of unilateral vision loss in the children with an estimated prevalence of 1-5%.3 

Reported categorical frequencies of amblyopia4 

  1. Strabismus – 50% 
  2. Refractive – 15 to 20% 
  3. Combined mechanism (strabismus and refractive) – 30% 
  4. Deprivation – Less than 5% 

 Pathophysiology: 

Overview— 

Amblyopia results from a disruption of development of the visual cortex early in life.1 After birth, a critical period of neuroplasticity in the development of the visual system occurs, during which, outside visual stimuli promote neuronal amplification and modification.2 At birth vison is roughly 20/400 and increases rapidly as cone density of the fovea increases nearly fivefold in the first few months and early years of life.5 An appropriately focused image on the retina is critical to create a normal visual stimulus for development. The developmentally sensitive period is thought to occur during the first few months of life until around the age of eight.6 Early visual disruption tends to result in more severe loss of vision/disruption to the development of the visual system.6 Treatments later in development have been less successful at restoring or improving vision, thus highlighting the need for early screening and intervention. The central suppression that leads to amblyopia is categorized into three causal processes: 

  1. Strabismus or ocular misalignment 
  2. Anisometropia or refractive error 
  3. Depravation or physical obstruction 

3 Major Classifications 

Strabismus, or eye misalignment, can cause amblyopia if this misalignment results in a loss of binocular vision (causing diplopia).The visual cortex is presented with two disparate images presented on the fovea and suppresses the neurological stimulus of one eye in order to prevent diplopia. This neuro-adaptation is a clever short-term solution to the diplopia but in the long-term can cause amblyopia. It is important to note that not all forms of strabismus result in amblyopia.  

Refractive amblyopia is caused by an uncorrected refractive error that causes image blur on the retina in one or both eyes.  The most common subtype of refractive amblyopia is unilateral and related to anisometropia, or a difference in the refractive error between the two eyes. In this subtype, one eye is able to see more clearly than the other eye.  The brain “ignores” neuronal information coming from the blurred eye, and preferentially focuses on the image from the clear eye.  This stimulates visual development of the good eye while neglecting development of the blurred eye, thus causing amblyopia.  

Bilateral refractive amblyopia, or ametropic/isometropia amblyopia, occurs in cases of bilateral severe uncorrected refractive error, and is less common.     

Deprivational amblyopia results for vison deprivation, because of a physical obstruction of the visual axis. Examples include congenital cataracts, ptosis, congenital corneal opacities, and vitreous hemorrhage. These physical obstructions block or severely distort the foveal image resulting in the most severe form of amblyopia. Deprivational amblyopia in infancy, if not corrected urgently, can result in permanent loss of visual development and vision.   

Keywords/Main Subjects: Amblyopia, Strabismus, Neuroplasticity 

Secondary CORE Category: Pediatric Ophthalmology and Strabismus 

Diagnosis: Amblyopia  

Faculty Approval by: Griffin Jardine, MD 

References: 

  1. Holmes, and Clarke. “Amblyopia.” The Lancet 367.9519 (2006): 1343-351. 
  1. Jefferis, Connor, and Clarke. “Amblyopia.” BMJ : British Medical Journal. 351.8033 (2015): H5811 
  1. Noorden, and Crawford. “The lateral geniculate nucleus in human strabismic amblyopia.” Investigative Ophthalmology & Visual Science. 33 (1992): 2729-2732 
  1. de Zárate, and Tejedor. Current concepts in the management of amblyopia. Clinical Ophthalmology. 1 (2007): 403 
  1. Hendrickson, Possin, Vajzovic, and Toth. Histologic development of the human fovea from midgestation to maturity. American Journal of Ophthalmology. 154 (2012): 767-778 
  1. Daw. Critical periods and amblyopia. Arch Ophthalmol 116 (1998): 502-505. 

Identifier: Moran_CORE_24066