Low vision (amblyopia) is defined as a reduction in visual acuity, with the best vision correction, that is not a direct consequence of any abnormality in the structure of the eyeball or the visual pathways. Amblyopia most commonly affects one eye, although it can also occur bilaterally.

There are several types of visual impairment:

• strabismic ophthalmia,
• anisometropic binocular optics,
• isometropic blindness,
• deprivation blindness (due to lack of stimuli)

Low vision is the most common cause of unilateral reduced visual acuity in early childhood. It is very important to note that almost all vision loss associated with visual impairment can be prevented or influenced to regress. The only prerequisites are an early diagnosis and appropriate treatment.

Strabismic optic impairment is the most common form of visual impairment. It is caused by permanent monocular suppression of the strabismic eye. Suppression is an adaptive mechanism in children whereby the vision in the strabismic eye is 'inhibited'. In strabismic optic nystagmus, the strabismic eye has a low visual acuity, the difference between the healthy eye and the strabismic eye being at least two rows on the visual acuity test board (Snellen tables).

The second most common type of visual impairment is anisometropic visual impairment. It involves a difference in the refraction of the two eyes, most commonly hyperopia, but can also involve unilateral high myopia (from -6 Dsph).

Isometropic myopia, is caused by a binaural high refractive error. It occurs in myopia of more than 10 Dsph, and in hyperopia of more than 5 Dsph. Such defects indicate the risk of developing bilateral visual impairment. Most ophthalmologists also recommend correction of astigmatism over 2 Dsph cylindrical, although the size of the cylindrical defect causing the visual impairment is not precisely known.

Deprivation amaurosis, formerly known as disuse amaurosis, is caused by congenital or acquired opacities of the optic centres. It is the rarest form of visual impairment, the worst prognosis and the most difficult to treat.