ID has a standardized assessment called the Supports Intensity Scale (SIS); this measures severity on a system built around how much support an individual will need. While ASD also classifies severity by support needed, there is no standard assessment; clinicians are free to diagnose severity at their own judgment. Causes
Among children, the cause of intellectual disability is unknown for one-third to one-half of cases. About 5% of cases are inherited from a persons parents. Genetic defects that cause intellectual disability, but are not inherited, can be caused by accidents or mutations in genetic development. Examples of such accidents are development of an extra chromosome 18 (trisomy 18) and Down syndrome, which is the most common genetic cause. Velocardiofacial syndrome and fetal alcohol spectrum disorders are the two next most common causes. However, there are many other causes. The most common are:

Genetic conditions. Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons like de novo mutations in genes associated with intellectual disability. The most prevalent genetic conditions include Down syndrome, Klinefelter syndrome, Fragile X syndrome (common among boys), neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria (PKU), and Prader–Willi syndrome. Other genetic conditions include Phelan-McDermid syndrome (22q13del), Mowat–Wilson syndrome, genetic ciliopathy, and Siderius type X-linked intellectual disability (OMIM: 300263) as caused by mutations in the PHF8 gene (OMIM: 300560). In the rarest of cases, abnormalities with the X or Y chromosome may also cause disability.
Non-occlusive disease (NOD) or Non-occlusive mesenteric ischaemia (NOMI) is a life-threatening condition including all types of mesenteric ischemia without mesenteric obstruction. It affects mainly elderly patients above 50 years of age who suffer from cardiovascular disease (myocardial infarction, congestive heart failure or aortic regurgitation), hepatic, chronic kidney disease or diabetes mellitus. It can be triggered also by a previous cardiac surgery with a consequent heart shock. It represents around 20% of cases of acute mesenteric ischaemia. Pathophysiology
Non-occlusive mesenteric ischemia occurs due to severe vasoconstriction of mesenteric vessels supplying the intestine. Acute abdominal pain is the only early acute symptom in those patients, which makes early diagnosis difficult. Diagnosis
CT angiography would be helpful in differentiating occlusive from non-occlusive causes of mesenteric ischaemia. Prognosis
Non-occlusive disease has a poor prognosis with survival rate between 40-50%. == References ==