Introduction
Amblyopia is a Greek word literally meaning bluntness of vision. In the clinical field it is used to denote a reduction in vision in the absence of any ophthalmoscopically detectable retinal anomaly, and in the absence of any disorder of the afferent visual pathways (Duke‐Elder 1973). The reduction in vision is not immediately ameliorated with the correction of any refractive error.

Amblyopia is usually classified according to the cause:

• strabismic, as a result of squint;

• anisometropic, where the refractive error is unequal;

• meridional, where the vision is blurred by the presence of astigmatism (irregular corneal curvature);

• ammetropic, where a high refractive error results in both eyes receiving a blurred image; and

• stimulus deprivation, where reduced vision occurs secondary to an obstacle in the anterior visual pathway.

Amblyopia can be bilateral but is most commonly unilateral. It is not uncommon for amblyopia to be of mixed aetiology.

Pathophysiology
The organization of the adult visual cortex (brain) is determined by early visual experiences (Wiesel 1963). The period within which abnormal visual input can lead to a disruption of the normal pattern of development is called the 'critical period' (Hockfield 1998). There are several critical periods, each associated with different visual functions (Harwerth 1990) that probably reflect the development of different parts of the brain. These critical periods do not end abruptly and can be considered as a continuum from extreme sensitivity to almost no sensitivity to external stimuli. Amblyopia finds its roots in these critical periods at young ages when the brain and visual system are immature and connections between neurons are still being formed and stabilized. During the critical period amblyopia is reversible.
The focus of this review will be unilateral stimulus deprivation amblyopia: interventions for other types of amblyopia are being evaluated in a series of Cochrane reviews which are underway.

Etiology
Stimulus deprivation amblyopia, also known as amblyopia ex anopsia, refers to the type of amblyopia where loss of vision results from disuse or lack of formation of retinal images, most commonly as a result of one of the following:
(1) uncorrected infantile cataracts (opacity of the lens);
(2) ptosis (droopy lid) (Dray 2002; Gusek 2000);
(3) hemangioma (blood rich swelling on the lid) (Schulz 1982);
(4) vitreous hemorrhages (bleeding into the clear gel that fills the eye) (Ferrone 1994);
(5) aphakia (absence of the natural lens);
(6) occlusion (patching prescribed to treat amblyopia of the other eye) (Awaya 1973; Von Noorden 1973; Von Noorden 1981).
In some cases the eye will be otherwise healthy; in others there is co‐existing pathology such as microphthalmus (small eye), coloboma (incomplete formation of the eye) or retinal dystrophy (malformation of the retina) that limits the visual prognosis.
A common type of stimulus deprivation amblyopia is that resulting from a congenital or infantile unilateral cataract. The affected eye is subjected to stimulus deprivation secondary to the cataract until the cataract is removed and then severe aphakic defocus until optical correction is given. The aphakic eye continues to be subjected to anisometropia and aniseikonia even after optical correction (Enoch 1983). The early insult to the visual system seems to make this type of amblyopia particularly dense and resistant to treatment, and the visual prognosis is generally reported to be poor (Kanski 1994;Taylor 1997).

Epidemiology
The prevalence of amblyopia in the general population is uncertain, with estimates ranging from 1% to 5% (Brown 2000; Hillis 1983). In European children, the prevalence ranges from 1% to 2.5% (Kvarnstrom 2001; Newman 2000). Amblyopia accounts for 8% to 29% of blindness in different populations; Copenhagen (Buch 2001), India (Dandona 2003), and Indonesia (Saw 2003). Stimulus deprivation amblyopia is a rare type of amblyopia, probably seen in less than 3% of amblyopic patients (Hillis 1983). There are no known age, gender, race, or developing‐developed country differences.

Presentation
Routine health checks of babies and toddlers provide an opportunity for the causative signs associated with stimulus deprivation amblyopia, such as ptosis or cataract, to be detected. Post‐natal screening may be carried out by a variety of personnel, e.g. pediatricians, nurses, or family doctors although provision of such screening is not universal. Access to health care professionals and services may be limited, especially in rural areas of developing countries.
Parents may notice signs associated with stimulus deprivation amblyopia such as leucocoria (whitish pupils) with congenital cataracts, strabismus (cross‐eyes) or nystagmus (abnormal eye movements) secondary to the visual loss, leading them to seek referral. In either case most patients presenting for treatment of stimulus deprivation amblyopia will be under a year old (Mein 1991).

Diagnosis
There are four main steps in the diagnosis of amblyopia.
(1) Visual acuity testing using an age appropriate test. Vision testing in young children presents challenges. Qualitative methods such as assessing fixation preference in squint and observation of eye movements directed to a moving target such as the Catford drum are in use. However, quantitative tests of acuity, such as the square wave black and white gratings used in preferential looking tests, are more precise. Preferential looking tests rely on the observation that infants prefer to look at patterned rather than plain surfaces (Fanz 1958). If the child can discern the striped panel on the card presented, it will look at it. The degree of visual angle subtended by the stripes is known and therefore a Snellen equivalent can be calculated. In older children testing methods are less objective, relying on identification of pictures or letter optotypes in Snellen or LogMAR notation.
(2) External and internal eye examination to identify any pathology.
(3) Cycloplegic refraction and, if necessary, prescription of glasses.
(4) Rechecking visual acuity with any necessary refractive correction in place. Some improvement in visual acuity can be expected with glasses alone. There should therefore be a period of adjustment into glasses before retesting. It is likely that there will be variation in the time allowed for adjustment into glasses. Where possible the adjustment period will be reported, and outcomes will be discussed in this context.
In the absence of clear evidence regarding what constitutes a significant interocular difference in acuity, and in the light of the poor evidence available as to what constitutes normal vision on many commonly used tests, at different ages, we have elected to define amblyopia as vision of worse than 6/9 on a Snellen based test, or 0.2 LogMAR (or equivalent).

Treatment options
The visual loss attributable to stimulus deprivation amblyopia can be severe and when untreated can have an impact in adult life. For individuals with amblyopia the lifetime risk of serious visual impairment, due to loss or damage of the better‐seeing eye, is estimated to be between 1.2% and 3.3% (Rahi 2002). In addition there are implications for employment prospects and therefore income. The number of jobs barred to individuals with reduced vision increases with the severity of the deficit (Adams 1999).

Stages of treatment
(1) The first step in treating stimulus deprivation amblyopia is to attempt to correct the causative factor that is degrading the quality of the visual image, e.g. removing infantile cataracts. Most studies, and text books agree that in cases of early unilateral form deprivation correction must be undertaken in the first eight to twelve weeks of life if good visual acuity is to be obtained (Birch 1986; Birch 1988; Gregg 1992; Kanski 1994; McCulloch 1994; Taylor 1997)
(2) The next step is to prescribe any necessary glasses to make the quality of visual stimulation received by the child's amblyopic eye as good as possible. Where cataract surgery has been performed intraocular implants may be used or contact lenses may be prescribed.
(3) Occlusion therapy may then be initiated. Occlusion forces use of the amblyopic eye, stimulating the formation of functional connections in the brain (Boothe 2000).

Occlusion regimens
Protocols and practices vary from centre to centre and practitioner to practitioner. Duration of occlusion therapy varies from full‐time to as short as one hour. Factors affecting the amount prescribed include the density of the amblyopia, the age of the child and the likely waiting time to the next appointment. Follow‐up is recommended at intervals of one week per year of age during periods of aggressive patching (Simon 1987). Occlusion can be gradually stopped when visual acuity becomes equal in the two eyes or if no progress has been made after three months of good compliance with occlusion (Pratt‐Johnson 2001). It is recommended that children in this situation are monitored up to the age of visual maturity to ensure that amblyopia does not recur. Some periods of maintenance occlusion may be required during that time (Mein 1991).

Additions to occlusion therapy
(1) CAM visual stimulator: uses rotating high‐contrast square wave gratings to stimulate the amblyopic eye.
(2) Pleoptics: employs after‐images to encourage foveal fixation with normal projection in the amblyopic eye

Atropine penalization and optical penalization (use of lenses to reduce the acuity) give occlusion to form favoring use of the amblyopic eye. These treatments for amblyopia are being evaluated in other Cochrane reviews which are in preparation.
This review will examine the role of total occlusion to form and light as an intervention for stimulus deprivation amblyopia. Total occlusion, also known as conventional occlusion, is usually achieved by means of an opaque, adhesive patch. Less commonly, occlusive contact lenses are employed. Bilateral stimulus deprivation amblyopia is not usually treated with occlusion therapy and will therefore be excluded from this review.

Measuring outcomes
Traditional clinical methods for assessing the degree of amblyopia in preverbal children have been based on the observation of fixation patterns. These methods require highly trained examiners and are not always without error (Wright 1986; Zipf 1976). The primary outcome measure of amblyopia treatment is change in visual acuity assessed using an age‐appropriate test (Fulton 1978; Sebris 1987) however crowded vision tests are more sensitive to amblyopia than uncrowded tests. Crowding, or contour interaction, occurs when interference from surrounding stimuli prevent the visual target from being distinguished. A sensitivity analysis is planned, if appropriate to look at the impact of measuring outcomes from treatment using uncrowded tests on the size and direction of effect.
Developmental improvements in visual acuity complicate evaluating the pure treatment effect. Stewart 2003 suggested an approach to measuring outcomes that takes account of this. Other methods that may be used to measure improvement of amblyopia are binocular function tests such as stereoacuity (Schmidt 1994).
The goal in amblyopia treatment is the improvement of visual acuity to better than or equal to 6/9 on Snellen or 0.2 LogMAR (or equivalent), i.e. restoration of normal visual acuity.

Factors affecting outcome
Compliance with therapy is critical for successful treatment but can often be very difficult to achieve. Young children in particular can become distressed by being restricted to reduced visual acuity. In addition wearing an occlusive patch can cause discomfort. It has been suggested that, if possible, compliance should be monitored in order to more effectively measure response to treatment. Other factors that are thought to affect the success of treatment are the duration of stimulus deprivation and age at the onset of therapy (Maurer 1989): the earlier the onset and the later treatment is commenced the worse the visual prognosis.

Risks from occlusion therapy
Potential adverse effects from occlusion therapy include inducing amblyopia in the occluded eye, skin allergies, infections or corneal abrasions (or both) from contact lens wear, intractable diplopia, and psychological factors such as distress.

Rationale for a systematic review
There are reports in the literature of patients who have achieved good levels of vision following early treatment for stimulus deprivation amblyopia (Gregg 1992; McCulloch 1994). There is however a lack of standardization and poor agreement among experts as to the optimum amount of occlusion needed for the effective management of stimulus deprivation amblyopia. Commencing occlusion therapy in infants with very poor vision can be harrowing for the parents and stressful for the child. It is therefore necessary to examine existing data in order to establish what degree of improvement can realistically be expected and the most effective occlusion regime for achieving this.