2027 ICD-10-CM Diagnosis Code A81.1Subacute sclerosing panencephalitis

ICD-10-CM Codes›A00–B99›A80-A89›A81

ICD-10-CM A81.1
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

A81.1 is a billable ICD-10-CM diagnosis code for subacute sclerosing panencephalitis. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 56 through 57, 791, 793. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 5 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.

For Medicare Advantage risk adjustment, A81.1 maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified) under the V28 model, adding a risk factor of about 0.341 for a community, non-dual, aged beneficiary in payment year 2027.

Code Identity

ICD-10-CM Code
A81.1
Billable Status
Yes — Valid for Submission
Code Describes
Subacute sclerosing panencephalitis
Short Description
Subacute sclerosing panencephalitis
Same as the full description in the CMS dataset.
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 CodeA81.1Subacute sclerosing panencephalitis

Medicare Risk Adjustment (HCC)Billing

A81.1 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.

CMS-HCC V28 Category (Payment Model)
HCC 127— Dementia, Mild or Unspecified
Payment HCC · PY 2027 one of 65 ICD-10-CM codes in this category
Risk Adjustment Factor (RAF) Weight
+0.341
community, non-dual, aged · ranges 0.000–0.438 across segments
Hierarchy
Superseded by HCC 125 and HCC 126
a more severe related category takes the payment when both are reported
Prior Model (CMS-HCC V24)
HCC 52
V24 retired V28 pays 100% of MA risk scores since PY 2026
Other CMS Models
ESRD (V21): HCC 52 · ESRD (V24): HCC 52
ESRD V21 weights: 0.045 dialysis, 0.000–0.328 functioning graft · ESRD V24 weights: 0.042 dialysis, 0.000–0.399 functioning graft
Part D (RxHCC)
RxHCC 112 — Dementia, Except Alzheimer's Disease
also risk-adjusts in the Part D prescription drug model (V08)

Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Chronic infectious disease of central nervous system
  • Dementia associated with viral encephalitis
  • Dementia due to subacute sclerosing panencephalitis
  • Measles of the central nervous system
  • Morbillivirus encephalitis
  • Paramyxoviridae encephalitis
  • Post measles encephalitis
  • Post-infectious encephalitis
  • Sequela of infection caused by Morbillivirus
  • Subacute sclerosing panencephalitis
  • Van Bogaert's sclerosing leukoencephalitis
  • White matter disorder caused by infection

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to A81.1: its own notes plus those printed at A81, block A80-A89, and Chapter 1. A note printed at a category, block or chapter applies to every code under it.

Includes

Further defines, or gives examples of, the content of the category.

From A81 Atypical virus infections of central nervous system applies to 10 codes
  • diseases of the central nervous system caused by prions
From Chapter 1 (A00-B99) Certain infectious and parasitic diseases applies to 1,068 codes
  • diseases generally recognized as communicable or transmissible

Applicable To

Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.

  • Dawson's inclusion body encephalitis
  • Van Bogaert's sclerosing leukoencephalopathy

Excludes1

Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.

From Block A80-A89 Viral and prion infections of the central nervous system applies to 48 codes
  • postpolio syndrome (G14)
  • sequelae of poliomyelitis (B91)
  • sequelae of viral encephalitis (B94.1)

Excludes2

Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.

From Chapter 1 (A00-B99) Certain infectious and parasitic diseases applies to 1,068 codes
  • carrier or suspected carrier of infectious disease (Z22.-)
  • certain localized infections - see body system-related chapters
  • infectious and parasitic diseases complicating pregnancy, childbirth and the puerperium (O98.-)
  • infectious and parasitic diseases specific to the perinatal period (P35-P39)
  • influenza and other acute respiratory infections (J00-J22)

Use Additional Code

Add a second code, after this one, to fully describe the condition.

From A81 Atypical virus infections of central nervous system applies to 10 codes
From Chapter 1 (A00-B99) Certain infectious and parasitic diseases applies to 1,068 codes
  • code to identify resistance to antimicrobial drugs (Z16.-)

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name A81.1, its category, or a range that includes it.

Excludes1 1

These codes carry an Excludes1 note naming A81.1: they are not reported together with it, unless the two conditions are unrelated.

  • B05 Measles
    subacute sclerosing panencephalitis (A81.1)

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

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

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR INF008
Viral infection
Default principal diagnosis: inpatient No · outpatient No

Clinical InformationClinical

  • Subacute Sclerosing Panencephalitis

    a rare, slowly progressive encephalitis caused by chronic infection with the measles virus. the condition occurs primarily in children and young adults, approximately 2-8 years after the initial infection. a gradual decline in intellectual abilities and behavioral alterations are followed by progressive myoclonus; muscle spasticity; seizures; dementia; autonomic dysfunction; and ataxia. death usually occurs 1-3 years after disease onset. pathologic features include perivascular cuffing, eosinophilic cytoplasmic inclusions, neurophagia, and fibrous gliosis. it is caused by the sspe virus, which is a defective variant of measles virus. (from adams et al., principles of neurology, 6th ed, pp767-8)

Patient EducationClinical

Degenerative Nerve Diseases

Degenerative nerve diseases affect many of your body's activities, such as balance, movement, talking, breathing, and heart function. Many of these diseases are genetic. Sometimes the cause is a medical condition such as alcoholism, a tumor, or a stroke. Other causes may include toxins, chemicals, and viruses.

Read the full article at MedlinePlus

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

Convert A81.1 to ICD-9-CMHistory

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

ICD-9-CM
046.2 Subac scleros panenceph
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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About A81.1Overview

What is the ICD-10 code for subacute sclerosing panencephalitis?

The ICD-10-CM code for subacute sclerosing panencephalitis is A81.1 (sometimes written as A811). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is A81.1 (Atypical virus infections of central nervous system) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report subacute sclerosing panencephalitis on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does A81.1 group to?

When subacute sclerosing panencephalitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 793, 791, 57, 56, with relative weights from 1.2835 to 4.2192 depending on complications. Higher weights mean higher Medicare reimbursement.

Is A81.1 a CC or MCC?

CMS lists A81.1 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 5 closely related codes in its exclusion list.

Can A81.1 be reported with G14?

Not as a rule. The Excludes1 note at block A80-A89, which applies to A81.1, lists "postpolio syndrome (G14)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).

What HCC is A81.1?

A81.1 (subacute sclerosing panencephalitis) maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified), commonly written as HCC 127, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 52 under the retired V24 model. It also maps in the ESRD (V21) and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 112.

Does A81.1 risk-adjust for Medicare Advantage payment?

Yes. When documented and reported on a Medicare Advantage encounter, A81.1 adds a risk adjustment factor of about 0.341 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.000 to 0.438 depending on the payment segment). A more severe related category (HCC 125 and HCC 126) supersedes it when both are reported. See the full factor table on the HCC 127 category page.