Central nervous system (CNS) damage is the primary feature of any Fetal Alcohol Spectrum Disorder (FASD) diagnosis. Prenatal exposure to alcohol — which is classified as a teratogen — can damage the brain across a continuum of gross to subtle impairments, depending on the amount, timing, and frequency of the exposure as well as genetic predispositions of the fetus and mother. While functional abnormalities are the behavioral and cognitive expressions of the FAS disability, CNS damage can be assessed in three areas: structural, neurological, and functional impairments.
All four diagnostic systems allow for assessment of CNS damage in these areas, but criteria vary. The IOM system requires structural or neurological impairment for a diagnosis of FAS. The "4-Digit Diagnostic Code" and CDC guidelines state that functional anomalies must measure at two standard deviations or worse in three or more functional domains for a diagnosis of FAS. The "4-Digit Diagnostic Code" further elaborates the degree of CNS damage according to four ranks:
Definite — Structural impairments or neurological impairments for FAS or static encephalopathy.
Probable — Significant dysfunction of two standard deviations or worse in three or more functional domains.
Possible — Mild to moderate dysfunction of two standard deviations or worse in one or two functional domains or by judgment of the clinical evaluation team that CNS damage cannot be dismissed.
Unlikely — No evidence of CNS damage.