Definition of Disability

The definition of disability in children used for the purposes of this report came from the SSA and was based on a definition passed by Congress in 1996. Under Title XVI, a child under age 18 years will be considered disabled if he or she has a medically determinable physical or mental impairment or combination of impairments that causes marked and severe functional limitations, and that can be expected to cause death or that has lasted or can be expected to last for a continuous period of not less than 12 months (Disability evaluation under Social Security, 1999).

Specific areas of functioning include: 1) acquiring and using information, 2) attending and completing tasks, 3) interacting and relating with others, 4) moving about and manipulating objects, 5) caring for yourself, 6) health and physical well-being. Disability is based on the degree to which the above functions are interfered with. Disability is established if there are marked limitations in at least two areas or there is an extreme limitation in one area of functioning. Where standardized tests of function exist, the regulations define a “marked” degree of functional limitation as more than two but less than three standard deviations below the mean and an “extreme” limitation as three or more standard deviations below the mean. (Disability evaluation under Social Security, 1999).

However, SSA's definition of disability is an administrative one, which is not commonly used by clinicians or researchers. In general, disability is defined differently by different researchers for different diseases. Furthermore, studies of children with short stature frequently focus on differences of ability, rather than on disability per se. Thus, any analysis of the literature to evaluate disability for SSA will be limited by the fact that few studies have primary analyses of disability that correspond to SSA's definition.

Evaluation of children with short stature due to a medically determinable cause

The evaluation of the association of short stature due to medically determinable causes with disability is complicated by the fact that it is often hard to distinguish whether the disability is due directly to short stature or to the underlying medical problem. As an example, Turner syndrome, which is caused by the absence of part or all of one X chromosome in a female, results in short stature in the majority of girls affected. These girls often have difficulties in visual-spatial relationships, but these difficulties are believed to be secondary to their genetic defect and not related to their short stature (Ross, Stefanatos, Roeltgen, et al., 1995). Similarly in Down syndrome, which is caused by chromosome 21 trisomy, children generally have short stature; however the significant mental retardation in these children is not thought to be secondary to their stature (Tolmie, 1997).

Evaluation of children with short stature may be done by a variety of medical specialties including endocrinology, gastroenterology, and genetics and depending on the specialty, different evaluations may be done. In general, research done on children with short stature has tended to focus on intelligence and psychological concerns, which fall into the first three areas of functioning discussed above (acquiring and using information, attending and completing tasks, and interacting and relating with others).

Evaluation of disability among children with short stature due to medically determinable causes is limited in that few studies examine disability per se, but instead compare different levels of ability (e.g. cognitive ability). In addition, direct associations between ability and height are often not reported.

Evaluation of children with short stature secondary to a skeletal dysplasia

Skeletal dysplasias are a relatively heterogeneous group of diseases that cause abnormalities in development of one or more parts of the skeleton. For the most part skeletal dysplasias tend to affect only bony development, specifically metaphyseal or epiphyseal development. However, certain skeletal dysplasias may also have other associated anomalies, such as severe myopia and cleft palate in diastrophic dysplasia, which may impact on a disability evaluation. Most, but not all, skeletal dysplasias will lead to stature below the third percentile. The most common skeletal dysplasias include achondroplasia, hypochondroplasia, osteogenesis imperfecta, and diastrophic dysplasia (Taybi and Lachman, 1996). The research on disability and skeletal dysplasia has tended to focus on structural abnormalities of the bones and short stature and has looked at problems such as difficulties in acquiring and using information, moving about and manipulating objects, caring for self, and health and physical well-being.

Evaluation of disability among children with skeletal dysplasia, as with children with short stature due to medically determinable causes, is limited in that few studies examine disability per se, but instead compare different levels of ability (e.g. cognitive ability). The main exception to this are studies of ambulation and mobility. In addition, direct associations between ability and height and between severity of short stature within a particular skeletal dysplasia are often not reported.

Evaluation of children with decreased growth velocity secondary to chronic disease

Some chronic diseases (e.g., inflammatory bowel disease, asthma, congenital heart disease) have been associated with poor growth in the affected child. The poor growth may lead to height less than the third percentile in some, or the height may stay within the normal range but with a decreased growth velocity (Mahoney, 1987). Of interest is whether decreased growth velocity can be used as a marker to indicate worsening severity of disease. This question does not directly relate to disability.

Certain chronic diseases, such as diabetes or chronic kidney disease, have clearly defined methods of measuring severity of disease (i.e., HgbA_1c and glomerular filtration rate, respectively). However, most chronic diseases have less well-defined measures of severity (e.g., asthma, congenital heart disease, sickle cell anemia). Therefore, different studies use different, non-standard definitions of severity, making comparisons across studies difficult. For some chronic diseases, severity of disease may just be a proxy for different categories or sub-types of the disease (e.g., congenital heart disease, juvenile rheumatoid arthritis). Growth may be affected differently by the different categories of the chronic disease, which may not be associated with the “severity” of the disease.

To properly evaluate the association between chronic disease severity and growth, studies must be longitudinal and measure height velocity. However, much of the research on severity of chronic disease has been cross-sectional. Therefore, height, rather than height velocity, has frequently been analyzed.