Succinic semialdehyde (SSA) is a GABA metabolite.  It is formed from GABA by the action of GABA transaminase (4-aminobutyrate aminotransferase) and further oxidised to become succinic acid, which enters TCA cycle. SSA is oxidized into succinic acid by the enzyme succinic semialdehyde dehydrogenase, which uses NAD as a cofactor.   When the oxidation of succinic semialdehyde to succinic acid is impaired, accumulation of succinic semialdehyde takes place which leads to succinic semialdehyde dehydrogenase deficiency.