Optic nerve hypoplasia is a congenital defect that describes the underdevelopment of the optic nerve during pregnancy. ONH may be the exaggeration of the natural dying back of fibers as the child develops in utero. It can occur in one or both eyes, but more commonly in both.
ONH is not a progressive disease or inherited, and it cannot be cured. It is one of the three most common causes of visual impairment in children.
What causes ONH?
There is no known cause of ONH but it has been associated with maternal alcohol abuse, maternal diabetes, use of anti-epileptic drugs and young maternal age of 20 years or less. However, these incidences account for very few of the total cases.
It appears that all races and socio-economic groups are affected by optic nerve hypoplasia.
Certain characteristics are noted as being associated with the disorder:
- The spectrum of visual acuity ranges from normal to no light perception.
- Optic nerve hypoplasia is a stable condition. Visual acuity will not deteriorate over time.
- Sometimes mild light sensitivity (photophobia) has been noted.
- ONH may occur alone or in conjunction with neurological or hormonal abnormalities. Some hormonal problems may appear only in later life.
- Nystagmus is present in a large percentage of ONH children and, in most cases, is associated with significant reduced visual acuity.
- Depth perception can be severe if there is substantial vision loss.
Conditions associated with Optic Nerve Hypoplasia
Brain and hormonal abnormalities are common in children suffering with nystagmus (involuntary eye movement) and severe bilateral vision loss. These conditions are less common in those with mild vision loss.
Abnormalities can include encephaloceles (neural tube defect), ventricle anomalies, cerebral atrophy and (rarely) tumors. And hormonal insufficiencies linked to the thyroid, growth hormone, pituitary, adrenal, and anti-diuretic hormone (ADH).
The associated brain anomalies can be identified by an MRI or CT scan. Hormonal insufficiencies will need an examination by a hormonal disorders specialist (pediatric endocrinologist).
Current research has found certain myths that are prevalent in connection with the disorder:
- All mothers of children with ONH were drug users during pregnancy.
- ONH occurs in clusters due to use of pesticides in the environment.
- The associated brain anomalies have a profound effect on the visual outcome/or spatial orientation of these patients.
Proposed Teaching Strategies for ONH children
- The contrast, size and lighting of materials for a child with nystagmus and severe bilateral vision loss needs to be increased because of the child’s generally depressed fields.
- Fine and gross motor activities, such as nesting and stacking, container play, pouring activities, ball tossing and rolling, cardboard box play and practice with stairs need to be given an ONH child to develop learned aspects of depth perception.
- Other conditions need to be considered when developing a teaching plan for children with ONH. A distracted and impulsive child can be helped by dependable daily routines and limited distractions. Slowed activity pace and predictable routines may help irritable behaviour and frequent snacks can help hypoglycaemic children.
- In the absence of functional vision, a multi-sensory learning approach is needed.
- Each child needs constant medical and educational evaluation.
- If a specific field loss is identified, materials should be adapted to the child’s field of vision and they should be encouraged to turn their heads to identify persons outside their vision field.
- Light sensitivity can be dealt with by wearing tinted lenses, adjusting lighting levels and reducing surface glare.
- Due to a loss of detail vision and vision field loss, a comprehensive evaluation by an instructor in Orientation and Mobility is necessary to meet the child’s needs. 0