Lesser Known Disorders
Each issue of this series contains at least three lesser known disorders. Some of these disorders may contain subtypes which will also be presented. You will also notice that each disorder has a code. These codes represent the coding system for all disabilities and disorders listed in the Educator’s Diagnostic Manual(EDM) Wiley Publications.
Disorders in this issue:
VI 3.13-Optic Nerve Hypoplasia
Disability Category: Visual Impairment
Definition
Hypoplasia” means smaller than normal. Optic nerve hypoplasia (ONH) refers to small or underdevelopment of the nerve which transmits vision signals from the eye to the brain. This is usually associated with permanent vision loss, which may be mild or severe. ONH may affect either one or both eyes (American Association for Pediatric Ophthalmology and Strabismus, 2005a).
Explanation
Vision can range from near-normal levels to severe impairment. Children with ONH may have precisely difficulty locating objects, due to a constricted visual field or impaired depth perception. Some children have light sensitivity.
If vision is significantly affected in both eyes, an infant will present with poor vision or shaking of the eyes (nystagmus). If only one optic nerve is small, the eye may cross in or drift out (American Association for Pediatric Ophthalmology and Strabismus, 2005a).
VI 3.14-Retinitis Pigmentosa (RP)
Disability Category: Visual Impairment
Definition
Retinitis pigmentosa is a progressive degeneration of the retina (part of the eye) which affects night vision and peripheral vision (U.S. National Library of Medicine, 2004).
Explanation
Retinitis pigmentosa commonly runs in families. The disorder can be caused by defects in a number of different genes which have recently been identified.
The cells controlling night vision, called rods, are most likely to be affected. However, in some cases, retinal cone cells are most damaged. The hallmark of the disease is the presence of dark pigmented spots in the retina (U.S. National Library of Medicine, 2004).
VI 3.14a -Autosomal dominant inheritance
In families with the autosomal dominant RP, an affected parent can have both affected and unaffected children. The patient has one gene for retinitis pigmentosa paired with one normal gene and has a 50% chance of passing the disease to their child, even if their partner is unaffected.
VI 3.14b -Autosomal recessive inheritance
In families with autosomal recessive Retinitis Pigmentosa, unaffected parents can have both affected and unaffected children. There may not be a known family history of the disorder. Both parents have normal retinas and carry a defective gene. There is a 25% chance that their child will be afflicted.
VI 3.14c -X-linked inheritance
In families with the X-linked type, only males are affected, while females carry the genetic trait but do not experience serious vision loss.
VI 3.15-Retinoblastoma
Disability Category: Visual Impairment
Definition
Retinoblastoma is a malignant (cancerous) tumor of the retina. The retina is the thin nerve tissue that lines the back of the eye that senses light and forms images (National Cancer Institute, 2004).
Explanation
Although retinoblastoma may occur at any age, it most often occurs in younger children, usually before the age of 5 years. The tumor may be in one eye only or in both eyes. Retinoblastoma is usually confined to the eye and does not spread to nearby tissue or other parts of the body.
This disease is caused by mutations in a gene called the Retinoblastoma-1 (RB1) gene. These mutations are either inherited (passed from the parents to the children) or new (not passed from the parents to the children) mutations. Some new mutations may become “inherited” (the new mutation is passed from the parents to the children). Tumors caused by inherited mutations are called hereditary retinoblastomas. Tumors caused by new mutations are called sporadic retinoblastomas. Hereditary retinoblastomas may form in one or both eyes, and they are generally found in younger children (National Cancer Institute, 2004).
VI 3.15a-Bilateral-Retinoblastoma
When the tumors are present in both eyes it is referred to as bilateral retinoblastoma
VI 3.15b-Unilateral-Retinoblastoma
When the tumors are present in one eye, it is referred to as unilateral retinoblastoma
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