“35207362 G31.9 Degenerative disease of nervous system, unspecified” does not have an equivalent SNOMED mapping. Although the suggested SNOMED concept of “80690008 Degenerative disease of the central nervous system” seems relevant, from a medical standpoint, it is much more granular that G31.9. Such an approach can lead to considerable data loss. According to the nervous system specification, G31.9 should include such compartments as central, peripheral, and autonomic nervous systems. Consequently, different types of degeneration can occur in different places. That is why G31.9 is mapped to the closest semantic ancestors, which as a combination can reflect its real meaning.
Also, we have empirically established, that (being made by some people) ICD’s Approximate Synonyms may be imprecise.
It should be an ‘Is a’ relationship, not a 'Maps to ’ relationship.
That is where you are totally right. Semantically these multiple “Maps to” mean “Is a”. But, currently, an “Is a” relationship is not allowed to use in mappings of the ICD9CM, ICD10 and ICD10CM vocabularies due to technical reasons. However, we do want to change this by the creation of the SNOMED Extension.