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Questions and Answers
Question # 13
What is the incidence of NLD in the general school population and relative to other learning disabilities?
This question is often followed by a related one: "Is NLD too rare to be the focus of concerted research and clinical efforts?" Apart from the rather obvious rejoinder to this query--viz., that autism, a disorder much rarer than NLD, has been the subject of such concerted efforts--there is evidence that the "incidence" of NLD is increasing.
When we first carried out neuropsychological examinations of children whom we eventually came to characterize in terms of the NLD rubric (ca 1968), the incidence of NLD in children referred for neuropsychological assessment because of suspected learning disabilities was quite low. For every 20 or so children who exhibited a learning disability first noted in the academic milieu and of apparent phonological/linguistic origin, we would see only 1 (i.e., 5%) school-identified child who exhibited NLD. More recently, this ratio has been halved to approximately 10 of the more "standard" learning-disabled youngsters for every 1 who exhibits NLD (i.e., 10%), and there are indications that this trend toward a higher incidence of NLD among the population of children with LD is continuing.
Of course, this trend is also influenced by the increased quality of treatment that has been afforded to children with acute lymphocytic leukemia (ALL), those who suffer severe closed-head injuries (TBI), and children with the types of neurological disorders wherein the NLD syndrome is quite frequent (see NLD and Neurological Disease). Whereas, 25 years ago many --if not most--children with severe TBI and with ALL would have expired soon after the onset of their brain disorder, many--if not most--now survive. This increase in survival rate increases the incidence of children who present with the NLD syndrome when subjected to a comprehensive neuropsychological evaluation--a form of assessment that is being increasingly seen as a crucial aspect of treatment planning for such children (see Question #37).
It is always hazardous to speculate on "clinical" statistics because so many factors can impinge on them. For example, we may be assessing a higher proportion of children with the "developmental" manifestation of the NLD syndrome because school officials are becoming more adept at (a) dealing with children whose academic learning problems relate to one or more aspects of psycholinguistic skill development, and (b) identifying children with NLD as in need of assistance. Evidence in favour of the latter point is the fact that such children are now being referred to us at much younger ages than was, heretofore, the case. However, even this may be a function of our community situation and the availability of services for such children.
Other factors that may increase the referral of children who exhibit NLD characteristics may relate to an increased emphasis on physical education in the schools of our area over the past 25 years, a greater sensitivity to psychosocial and other adaptational problems on the part of school personnel, and more attention given to the anxious/withdrawn child in addition to his or her more obvious age-mates with hyperactivity/conduct disorder.
The factors mentioned in the last paragraph were phrased in a rather local or "provincial" fashion for a specific reason: Clinicians need to be sensitive to such dimensions in their own communities. Clinical incidence varies markedly between communities as a function of the quantity, quality, and "profile level" of the clinical services available, the cost of such services, and a host of other socio-political factors that can exert profound impacts on referral patterns. Be that as it may, it is felt that the factors that have been raised in conjunction with this question are important ones to consider, and that the more general questions that need to be posed in relation to this matter are suggested thereby.
Recently, we attempted to arrive at some consensus among psychologists and others familiar with referral patterns in North America. We were interested specially in their estimated incidence of major categories of conditions that are deemed to require some sort of "special education" assistance. These statistics represent that consensus.
(1) This "classification" is based upon the PRIMARY reason(s) for special attention.
(2) It is clear that children so classified may very well have adaptational difficulties in more than one of these realms. For example, it would come as no surprise to find that older children and adolescents with NLD are referred for professional assessment and intervention because of significant psychosocial difficulties. (As an aside, it is my experience that the role of NLD in the etiology of these adaptation problems is often overlooked. This has less than positive implications for the person with NLD.)
(3) The overall estimate of 10% incidence turns out to be that evident in one middle-class School District in Michigan, made up of a total population of some 12,000 students. The number receiving some form of "special" education is approximately 1,200.
(4) Recently, Professor Harry van der Vlugt of the University of Tilburg, The Netherlands, supervised a study involving the incidence of NLD in the Dutch population. He reports the following:
"In a representative Dutch school population (N=1,936) about 5% are classified as Learning Disabled, 1% as Mentally Retarded, and .75% as Psychosocially Disturbed. The clinical incidence of NLD in the regular school population is 5%. The incidence in the special school population (children with Learning Disabilities) is 10%". Thus, the incidence of NLD overall would be in the order of 5.5%.
It should be borne in mind that NLD can vary in severity, from quite mild to severe. In our experience, youngsters with mild manifestation of NLD are not often seen as in need of any "special" attention or services. That said, it is interesting to note that our finding of a 10% incidence rate of NLD in the more current referrals of children with LD is identical to that reported by Professor van der Vlugt for the Dutch population of children in special schools for children with LD.
Bottom Line. It would appear that an overall estimate of approximately 4%-5% incidence of NLD in the school-age populations of North America would be reasonable. Indeed, this estimate may be rather conservative. In any case, is clear that the vast majority of youngsters with NLD and who would benefit substantially from special services--in North America and in The Netherlands (which has an exceptionally well-developed special education system)--are not identified as in need of such services.
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