The Demographics of Acid Sphingomyelinase Deficiency
Limited information is available regarding the demographics of acid sphingomyelinase (ASM) deficiency. Two reports from the Netherlands and Australia examined the combined frequency of types A and B Niemann-Pick disease (NPD) among the total number of individuals diagnosed with lysosomal storage disorders, and estimated a birth rate of ~0.5 and ~0.4 per 100,000, respectively (Meikle et al, 1999; Weavers et al, 1999). However, since these numbers were based on the number of diagnosed cases only, they are likely to underrepresent the true frequencies because less severe patients often are not recognized by clinicians, and therefore are not referred to diagnostic laboratories. The only population in which DNA-based screening has taken place is the Ashkenazi Jewish community, where the carrier frequency of three common mutations causing type A NPD was found to be ~1:80 (Schuchman, Miranda, 1997), leading to an estimated birth rate of ~3 per 100,000 for this form of the disorder. However, due to the widespread use of DNA-based carrier testing and prenatal diagnosis in this community, the actual birth rate of type A NPD individuals in this group is likely to be lower.
Demographic data are currently available at the Mount Sinai School of Medicine for more than 600 individuals diagnosed with ASM deficiency. About 100 of these patients presented with a "classic" type A NPD phenotype; of these, ~66% were Ashkenazi Jewish. This is consistent with the historic literature suggesting that this form of NPD occurs more frequently in this group. The remaining patients derived from more than 30 different countries, and in contrast to the type A patients, very few of these individuals had Ashkenazi Jewish ancestry. The majority of these patients were from North America and Western Europe, although this may be reflective of enhanced awareness and diagnosis of lysosomal disorders in these regions, rather than a higher disease frequency. A significant number also were from Africa and the Middle East, suggesting that ASM deficiency may be more common in these regions than previously thought. In fact, more than 40% of the NPD patients referred to Mount Sinai from Europe were of North African or Arab ancestry. A significant number also were Turkish. Asia had the fewest number of reported NPD cases.
An interesting trend among the cases referred to Mount Sinai during the past 5 years has been the increased identification of adult NPD patients. Prior to 1998, the majority (~80%) of NPD patients referred to Mount Sinai were pediatric. Since that time this trend has slowly reversed; in 2001-2002, nearly 40% of the patients were adult (over age 18). This may be reflective of enhanced awareness and improved diagnosis of these less severe patients, although it is likely that many still remain undiagnosed. Further studies are needed to define the true frequency of ASM deficiency in different populations, in particular by using DNA- or enzyme-based screening procedures.