abnormal myelination dysmyelination MP:0013438 Dysmyelination refers to malformed and defective myelin sheath as opposed to the destruction of previously normal myelin that is seen in demyelination. Dysmyelination disorders often arise from hereditary mutations that affect the synthesis and formation of myelin. hypomyelination reduced myelination reduced amount of myelin present in the form of a myelin sheath surrounding an axon due to defects in the synthesis and formation of myelin MPheno.ontology