Common causes of
visual impairment

Refractive Errors
 

Myopia (Nearsightedness): The image of distant objects is not focused on the retina but rather in front of it, making it appear blurry. The child can see objects that are near but not at a distance. Myopia can result from an elongated eyeball, a lens that is too strong, or a cornea that is excessively curved.

Hyperopia (Farsightedness): The focusing point is behind the retina resulting in straining to focus correctly, particularly at close distances. Therefore the child can see well at a distance but not at near. Hyperopia can result from shortness of the eyeball, a lens that is weak, or a cornea that is relatively flat.

Astigmatism: A cylindrical curvature of the cornea which prevents light rays from focusing on one point on the retina. The result is both near and far objects may appear blurry. Astigmatism often occurs in combination with myopia and hyperopia.

Eye Conditions
 

Albinism: Inherited condition resulting in decreased pigment which causes abnormal optic nerve development. Nystagmus (see below) and refractive errors are also often present with this condition. In addition to a decreased visual acuity, children with albinism may be sensitive to light. Tinted lenses can relieve light sensitivity and glasses or low vision aids can help maximize vision.

Amblyopia (“lazy eye”): The suppression of the image of one eye usually due to that eye having a significantly poorer acuity or being turned in/out. Children with amblyopia can have some functional field loss and poor or absent depth perception. Patching of the stronger eye and/or the use of glasses may be prescribed.

Cataracts: Opacity or cloudiness of the lens. Because light cannot pass through the lens, vision is affected. Some types of cataracts progressively worsen, while others remain unchanged. Cataracts can be found in one eye (unilateral) or both eyes (bilateral). Children with cataracts may have reduced visual acuity, blurred vision, poor color vision, light sensitivity, or nystagmus. Depending on the size and severity of the cataract, surgery to remove it may be recommended.

Coloboma: A birth defect which causes a cleft in the pupil, iris, lens, retina, choroid, or optic nerve. It can result in reduced acuity and field loss if the damage extends to the retina.

Glaucoma: Increased pressure in the eye due to blockage of normal flow of fluid in the eye. The vision of children with glaucoma can fluctuate based on changes in pressure. A child with glaucoma may also have peripheral field loss, poor night vision, and light sensitivity. If not treated, damage to the optic nerve can result.

Nystagmus: Involuntary movement of the eye. This can be horizontal, vertical, circular or mixed. Because the eyes are moving, a child with nystagmus has difficulty maintaining fixation on objects resulting in reduced visual acuity and fatigue. Nystagmus can be minimized by turning the head or eyes in a certain position, called the “null point.” The null point differs from person to person, but is often discovered by the child.

Optic Nerve Atrophy: Damage or degeneration to the optic nerve which carries visual signals to the brain. Vision loss will be dependent on the amount of damage, but may include blurred vision, poor color and night vision, and light sensitivity.

Optic Nerve Hypoplasia: Underdevelopment of the optic nerve in utero, resulting in a small optic nerve and visual impairment. The degree of visual impairment varies significantly but there is usually an acuity loss. Optic nerve hypoplasia may be associated with other conditions.

Retinitis Pigmentosa (RP): a hereditary, degenerative condition of the retina which results in loss of peripheral vision or “tunnel vision”. Initially starts with difficulty in seeing in dimly lit settings and progresses to a significant visual impairment.

Retinoblastoma: A cancerous tumor of the retina which requires vigorous treatment of all tumors through laser, radiation, and/or chemotherapy. Progression of retinoblastoma may result in enucleation (removal) of the eye. If one eye is removed, the child will not have depth perception.

Retinopathy of Prematurity (ROP): Disruption in the normal development of blood vessels of the retina in premature infants which can result in scarring and detachment of the retina. Children with ROP may have a decreased visual acuity and refractive errors.

Strabismus: A muscle imbalance resulting in the inability of both eyes to look directly at an object at the same time. Types of strabismus include: esotropia (an inward turn), exotropia (an outward turn), hypertropia (an upward turn), and hypotropia (a downward turn).

Cortical Visual Impairment
 

Unlike refractive errors and structural impairments, cortical visual impairment is not caused by any condition of the eye. Rather, it is due to damage to the visual cortex of the brain or the visual pathways which results in the brain not adequately receiving or interpreting visual information.

Children with cortical visual impairment often also have cerebral palsy, seizure disorder, and developmental delays as a result of the damage to the brain. They may exhibit inattention to visual stimuli, preference for touch over vision when exploring objects, and difficulty visually discriminating objects that are placed close together or in front of a visually complex background.

Because this visual impairment is due to the neurological processing of visual information, visual performance may fluctuate slightly or significantly from day to day, or even from moment to moment depending upon the environment and the seizure activity, motor position, general health, and mood of the child.