Parentheses are used in both the Alphabetic Index and Tabular List to
enclose supplementary words that may be present or absent in the
statement of a disease or procedure without affecting the code number to
which it is assigned. The terms within the parentheses are referred to as
nonessential modifiers. The nonessential modifiers in the Alphabetic
Index to Diseases apply to subterms following a main term except when a
nonessential modifier and a subentry are mutually exclusive, the subentry
takes precedence. For example, in the ICD-10-CM Alphabetic Index
under the main term Enteritis, “acute” is a nonessential modifier and
“chronic” is a subentry. In this case, the nonessential modifier “acute”
does not apply to the subentry “chronic”.