I66 ICD-10-CM Code: Occlusion and stenosis of cerebral arteries, not resulting in cerebral infarction

I66 is the ICD-10-CM diagnosis code for occlusion and stenosis of cerebral arteries, not resulting in cerebral infarction. I66 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering cerebrovascular diseases (i60-i69).

Header codes like I66 cannot be reported on claims directly, they organize child codes that share clinical context but the actual diagnosis must be coded to the highest level of specificity supported by the documentation. Coders should look at I66's child codes and select the one that matches the patient's documented presentation, since payers reject header codes submitted as the primary diagnosis. For risk adjustment workflows, header codes never contribute to a Medicare Advantage member's RAF score on their own; only billable child codes that happen to map to a payment HCC affect risk-adjusted plan payments.