In simple words: This code is used for problems with the arteries, smaller arterioles, and capillaries that are caused by other diagnosed conditions, like amyloidosis,but not by diabetes, syphilis, or tuberculosis related issues affecting blood vessels.

Other disorders of arteries, arterioles and capillaries in diseases classified elsewhere. Code first underlying disease, such as: amyloidosis (E85.-) Excludes1: diabetic (peripheral) angiopathy (E08-E13 with .51-.52) syphilitic endarteritis (A52.09) tuberculous endarteritis (A18.89)

Example 1: A patient with diagnosed amyloidosis presents with coldness and numbness in the extremities, indicating circulatory issues in the small vessels. I79.8 would be used along with the code for amyloidosis., A patient with rheumatoid arthritis experiences Raynaud's phenomenon, a condition affecting blood flow to the fingers and toes. I79.8 is used along with the rheumatoid arthritis code., A patient with systemic lupus erythematosus experiences skin ulcers due to vasculitis affecting small blood vessels in the legs.  I79.8 and the lupus code would be used.

Documentation should clearly link the circulatory disorder with the underlying disease. The type of circulatory disorder and the affected area should be documented. 

** This is a manifestation code, meaning it describes a symptom or a result of another condition.  Always code the underlying disease first.