2026 ICD-10-CM Diagnosis Code A81.9Atypical virus infection of central nervous system, unspecified

ICD-10-CM Codes›A00–B99›A80-A89›A81

ICD-10-CM A81.9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A81.9 is a billable ICD-10-CM diagnosis code for atypical virus infection of central nervous system, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 974 through 976. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other specified CNS infection and poliomyelitis and Viral infection.

Code Identity

ICD-10-CM Code
A81.9
Billable Status
Yes — Valid for Submission
Code Describes
Atypical virus infection of central nervous system, unspecified
Short Description
Atypical virus infection of central nervous system, unsp
Parent Code
A81
Atypical virus infections of central nervous system

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA80-A89Viral and prion infections of the central nervous system
CategoryA81Atypical virus infections of central nervous system
This CodeA81.9Atypical virus infection of central nervous system, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired prion disease
  • Autonomic disorder due to prion disease
  • Chorea due to prion disease
  • Dementia due to prion disease
  • Disorder of autonomic nervous system due to infectious disease
  • Familial Alzheimer-like prion disease
  • Parkinsonism due to prion disease
  • Prion disease
  • Secondary tic disorder
  • Slow viral central nervous system infection
  • Tic disorder due to prion disease

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Prion diseases of the central nervous system NOS

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Disease, diseased
      • prion, central nervous system
    • Infection, infected, infective(opportunistic)
      • atypical
        • virus
    • Infection, infected, infective(opportunistic)
      • slow virus
    • Infection, infected, infective(opportunistic)
      • virus, viral NOS
        • central nervous system
          • atypical
    • Infection, infected, infective(opportunistic)
      • virus, viral NOS
        • central nervous system
          • slow virus
    • Infection, infected, infective(opportunistic)
      • virus, viral NOS
        • slow

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS003
Other specified CNS infection and poliomyelitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR INF008
Viral infection
Default principal diagnosis: inpatient No · outpatient No

Patient EducationClinical

Neurologic Diseases

The brain, spinal cord, and nerves make up the nervous system. Together they control all the workings of the body. When something goes wrong with a part of your nervous system, you can have trouble moving, speaking, swallowing, breathing, or learning. You can also have problems with your memory, senses, or mood.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert A81.9 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
046.9 Cns slow virus infec NOS
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About A81.9Overview

Is A81.9 (Atypical virus infections of central nervous system) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report atypical virus infection of central nervous system, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does A81.9 group to?

When atypical virus infection of central nervous system, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 974, 975, 976, with relative weights from 0.8945 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of A81.9?

Under the General Equivalence Mappings, atypical virus infection of central nervous system, unspecified converts to ICD-9-CM 046.9 (cns slow virus infec NOS). The mapping is a direct match.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.