SMA seems to appear de novo (i.e., without any hereditary causes) in around 2–4% of cases.Spinal muscular atrophy affects individuals of all ethnic groups, unlike other well known autosomal recessive disorders, such as sickle cell disease and cystic fibrosis, which have significant differences in occurrence rate among ethnic groups. The overall prevalence of SMA, of all types and across all ethnic groups, is in the range of 1 per 10,000 individuals; the gene frequency is around 1:100, therefore, approximately one in 50 persons are carriers. There are no known health consequences of being a carrier. A person may learn carrier status only if ones child is affected by SMA or by having the SMN1 gene sequenced.Affected siblings usually have a very similar form of SMA. However, occurrences of different SMA types among siblings do exist – while rare, these cases might be due to additional de novo deletions of the SMN gene, not involving the NAIP gene, or the differences in SMN2 copy numbers. Diagnosis
SMA is diagnosed using genetic testing that detects homozygous deletion of the SMN1 gene in over 95% of cases, and a compound SMN1 mutation in the remaining patients.
The resection of the ileum leads to a malabsorption of vitamin B12, bile acids and the fat soluble vitamins A, D, E and K. Loss of the distal ileum also leads to loss of inhibitory hormones; leading to gastric hypersecretion, intestinal hypermotility (decreases in the intestinal transit time) leading to secretory diarrhea and macronutrient, micronutrient, vitamin and mineral deficiencies. Loss of the ileocecal valve leads to small intestinal bacterial overgrowth(SIBO) as bacterial flora normally found in the large intestines migrate proximally and colonize the small intestines leading to further malabsorption. SIBO leads to malabsorption as the bacteria colonizing the small intestine metabolize nutrients, directly competing with the intestinal absorption of nutrients. The bacteria colonizing the small intestines in SIBO may also cause bile acid deconjugation leading to malabsorption of lipids.In a process called intestinal adaptation, physiological changes to the remaining portion of the small intestine occur to increase its absorptive capacity. These changes usually take place over 1-2 years. These changes include:

Enlargement (increased diameter) and lengthening of the villi found in the lining
Increase in the diameter of the small intestine
Slow down in peristalsis or movement of food through the small intestine (an increase in the transit time) to increase the time available for nutrient absorptionOsteoporosis is a very common comorbidity in people with short bowel syndrome who are on parenteral nutrition, with an estimated prevalence of 57-67%. The contributing factors to the osteoporosis include malnutrition, vitamin D deficiency due to malabsorption and vitamin D deficiency due to scarce sunlight exposure due to chronic disability.