Despite the increasing prominence of body composition and obesity in both the scientific and lay press, nutritional goals remain sparsely described and poorly defined for patients with neuromuscular diseases. This is especially true with regards to spinal muscular atrophy[2
]. Recently Messina and colleagues described the results of a large survey of Italian SMA type 2 patients, reporting a high incidence of markedly underweight patients (compared to healthy age and gender norms) in this group and a concurrent high incidence of swallowing dysfunction[3
]. The authors, in reporting the latter finding, concluded that an integrated, multidisciplinary approach to identify feeding difficulties, malnutrition, and the need for supplementation with enteric feeding should become standard of care in the management of patients with SMA type 2[3
]. We recently described increased adiposity and risk of overweight status among a cohort comprised largely of SMA type 2 and 3 children[18
], using conventional definitions[17
] applied to modern body composition imaging techniques (DXA)[15
]. The present study supports the seemingly contradictory findings of these two prior studies, specifically that patients with SMA may be at risk for both complications: malnutrition due to feeding and swallowing dysfunction and obesity due to nutritional inputs markedly greater than the modest energy needs in this largely inactive population; non-ambulatory patients with relatively high levels of clinical function may be at particular risk of one, the other, or both of these nutritional complications depending on the degree of severity of the underlying disease.
In this study, relatively high functioning but non-ambulatory subjects were observed to have significantly higher fat mass index compared to both lower functioning and ambulant peers, with a trend toward higher likelihood of FMI percentile above the 85th, connoting “at risk for overweight” using conventional definitions. This population, comprising both mild type 2 patients as well as type 3 patients who have lost ambulation, may be at especial risk of obesity resulting from a combination of low metabolic demand relative to higher functioning peers and high caloric inputs relative to those with poorer motor function.
Consistent with the report by Messina et al (2008), reported swallowing or feeding dysfunction was widespread among the subjects with SMA type 2 surveyed in this study, with high prevalence among both low and high functioning subjects with SMA type 2[3
]. While a survey of the type applied is likely insensitive to subtle differences in swallowing function (particularly in comparison to formalized evaluations of swallowing function such as a video esophagram), this study failed to demonstrate any notable differences between high and low functioning subjects with SMA type 2 despite large differences in clinical function. Importantly, this approach does not address differences in food portions and independent access to calories between the two groups. It may be that relative ease of access to excess calories may in part underlay the increased adiposity seen among high functioning non-ambulatory subjects (Cohort 2) compared to their lower functioning peers. At the same time, fat mass index was also significantly higher among these subjects than among those with retained ambulation, reflecting, perhaps, the effect of diminished metabolic demand among the non-ambulatory. In this context, just as with otherwise healthy individuals, caloric input above metabolic demand will result in excess adiposity.
There are several limitations to this study. The rarity of SMA limits the available size of the study cohort; the age requirement for inclusion (>5 years), a necessity given the ethical (radiation exposure) and technical (the need to remain still) constraints of DXA, further limits the applicability of these observations, particularly to younger children. While body composition and nutrition is neither a primary endpoint nor a significant focus of the natural history study from which subjects were selected, the co-administration of extensive medical care, including ongoing management by a nutritionist, may alter the patient population studied in unexpected ways and confound the reported results. Furthermore, whether the observed incidence of obesity is reflective of societal factors specific to this cohort of American patients with SMA or can be extrapolated to other populations is unknown. As noted above, while the stratification of subjects with SMA type II mirrors the work of other groups[31
], the choice of a somewhat arbitrary stratification raises the possibility of observer bias into the reported result. While the survey chosen for this study has been applied previously in studies of SMA[3
] and Duchenne Muscular Dystrophy[32
], observations using an indirect, survey-based method must be approached with considerable caution. Importantly, body composition and swallowing function are at best secondary markers for dietary imbalance; caloric intake was not addressed in this study. Clarification of the caloric and nutritional requirements of patients with SMA is an important and largely unaddressed area for future study.
In conclusion, despite widespread recognition of the importance of nutritional optimization in the care and management of patients with this disease[2
] it is far from clear what comprises optimum nutritional management and body composition in SMA. In this study we describe body composition using DXA and the presence of feeding and swallowing dysfunction, among a well described, genetically confirmed cohort of patients subdivided by clinical status and motor function on a standardized rating scale (the HFMSE). We observe that non-ambulatory SMA subjects with relatively high function have increased fat mass and are likely at particular risk of becoming overweight when compared to their lower functioning or ambulator peers. We speculate that the increased adiposity seen in this particular subset of SMA reflects the specific challenges faced by these patients who may be especially predisposed, despite limitations in mobility and the possible presence of swallowing dysfunction, to excess caloric intake. For this subset of the SMA population, increased adiposity may represent a significant, modifiable factor contributing to the overall morbidity of the disease. This study further emphasizes and elaborates previous recommendations[2
] regarding the role of a dedicated and experienced nutritionist in the care of these children and highlights the importance of individualized dietary care and management in SMA.