Pathophysiology

Amblyopia is primarily a defect of central vision; the peripheral visual field is usually normal. In early postnatal development, there are critical periods of cortical development during which neural circuits display a heightened sensitivity to environmental stimuli and are dependent on natural sensory experience for proper formation. (See Chapter 6 for additional discussion of the neural basis of amblyopia.) During these periods, the child’s developing visual system is vulnerable to abnormal input due to visual deprivation, strabismus, or significant uncorrected refractive errors. Also during these periods, the visual system’s plasticity allows the greatest opportunity for reversal of amblyopia. In general, the critical period for development of visual deprivation amblyopia is earlier than that of strabismic or anisometropic amblyopia (see the Classification section). Furthermore, amblyopia due to visual deprivation develops more rapidly and is deeper than that due to strabismus or anisometropia.

Abnormal early visual experience can also result in profound disturbances of neuron function within the visual system, resulting in the vision loss of amblyopia. Cells of the primary visual cortex can lose their innate ability to respond to stimulation of 1 or both eyes, and cells that remain responsive can show significant functional deficiencies. Abnormalities also occur in neurons within the lateral geniculate body. However, very little change has been found in the retinas of subjects with amblyopia.

The receptive fields of neurons in the amblyopic visual system are abnormally large. This may account for the crowding phenomenon (also known as contour interaction), a characteristic of amblyopia in which optotypes of a given size are easier to recognize when presented singly than when closely surrounded by similar forms, such as a full line of letters (see Chapter 1).

Classification

Amblyopia can be categorized by the disorders responsible for its occurrence.

Strabismic Amblyopia

One of the most common forms of amblyopia, strabismic amblyopia develops in the deviating eye of a child with strabismus. Constant, nonalternating heterotropias are the type most likely to cause significant amblyopia. Strabismic amblyopia is thought to result from competitive or inhibitory interaction between neurons carrying nonfusible input from the 2 eyes. This interaction leads to domination of cortical vision centers by input from the fixating eye and also results in reduced responsiveness to input from the nonfixating eye. In young children with strabismus, suppression develops rapidly. This visual adaptation occurs to avoid diplopia and visual confusion (see Chapter 6). Amblyopia does not always prevent diplopia, however. For example, older patients with longstanding strabismus have a small risk of developing diplopia after strabismus surgery, despite the presence of significant amblyopia.

Several features that are typical of strabismic amblyopia are less common in other forms of amblyopia. In strabismic amblyopia, grating acuity, the ability to detect patterns composed of uniformly spaced stripes, is often reduced considerably less than recognition acuity measured with optotype charts. This discrepancy must be considered when the results of tests based on grating detection, such as Teller Acuity Cards II (Stereo Optical Co, Inc, Chicago, IL) and the LEA Grating Acuity Test (Good-Lite Co, Elgin, IL), are interpreted.

When visual acuity is measured through a neutral density filter, the acuity of an eye with strabismic

amblyopia tends to decline less sharply than that of an eye with organic disease. This phenomenon is called the neutral density filter effect.

Eccentric fixation is the consistent use of a nonfoveal region of the retina for monocular viewing. Minor degrees of eccentric fixation, detectable only with special tests such as visuscopy, are present in many patients with strabismic amblyopia and relatively mild vision loss. Clinically evident eccentric fixation can be detected by observing a decentered position of the corneal light reflex from the fixating amblyopic eye while the dominant eye is covered. It implies visual acuity of 20/200 or worse and a poorer prognosis for visual recovery with treatment.

Refractive Amblyopia

Refractive amblyopia is another common form of amblyopia, the etiology of which is consistent defocus of the retinal image in 1 or both eyes. There are 2 types: anisometropic and isoametropic.

Anisometropic amblyopia

In anisometropic amblyopia, dissimilar refractive errors in the 2 eyes cause the image on 1 retina to be chronically defocused. Considered more prevalent than strabismic amblyopia in some recent studies, this condition is thought to result partly from the direct effect of image blur and partly from interocular competition or inhibition similar (but not identical) to that responsible for strabismic amblyopia. Levels of anisometropia that can lead to amblyopia are greater than 1.50 D of anisohyperopia, 2.00 D of anisoastigmatism, and 3.00 D of anisomyopia. Higher levels are associated with greater risk. The eyes of a child with anisometropic amblyopia usually appear normal to the family and primary care physician, which may cause a delay in detection and treatment.

Isoametropic amblyopia

Isoametropic amblyopia (bilateral ametropic amblyopia) is a bilateral decrease in visual acuity that results from large, approximately equal, uncorrected refractive errors in the 2 eyes of a young child. The mechanism of this form of amblyopia involves the deleterious effect of blurred retinal images on the immature visual system. Hyperopia exceeding 4.00–5.00 D and myopia exceeding 5.00–6.00 D carry a risk of inducing isoametropic amblyopia. Uncorrected bilateral astigmatism in early childhood may result in loss of resolving ability limited to the chronically blurred meridians (meridional amblyopia). The degree of cylindrical isoametropia that produces meridional amblyopia is not known, but most ophthalmologists recommend correction when there is more than 2.00–3.00 D of cylinder.

Visual Deprivation Amblyopia

The least common but most severe and difficult to treat of the forms of amblyopia, visual deprivation amblyopia occurs because of an eye abnormality that obstructs the visual axis or otherwise interferes with central vision. The most common cause of visual deprivation amblyopia (also known as stimulus deprivation amblyopia, deprivation amblyopia, visual stimulus deprivation amblyopia, and formvision deprivation amblyopia) is a congenital or early-acquired cataract, but blepharoptosis, periocular lesions that obstruct the visual axis, corneal opacities, and vitreous hemorrhage may also be causal. Because amblyopia is responsible for permanent vision loss in many ocular abnormalities of early childhood, there is greater urgency in their management compared to similar conditions in adults. Unilateral visual deprivation amblyopia tends to be worse than amblyopia produced by bilateral deprivation of similar degree, because interocular competition adds to the direct developmental impact of severe image degradation (see Chapter 6). Even in bilateral cases, however, visual acuity can be 20/200 or worse.