2026 ICD-10-CM Diagnosis Code I40.1Isolated myocarditis

ICD-10-CM Codes›I00–I99›I30-I5A›I40

ICD-10-CM I40.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I40.1 is a billable ICD-10-CM diagnosis code for isolated myocarditis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 314 through 316, 974 through 976. Coders also document this condition as eosinophilic myocarditis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Myocarditis and cardiomyopathy.

Code Identity

ICD-10-CM Code
I40.1
Billable Status
Yes — Valid for Submission
Code Describes
Isolated myocarditis
Short Description
Isolated myocarditis
Same as the full description in the CMS dataset.
Parent Code
I40
Acute myocarditis

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI30-I5AOther forms of heart disease
CategoryI40Acute myocarditis
This CodeI40.1Isolated myocarditis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Eosinophilic myocarditis
  • Giant cell myocarditis
  • Idiopathic myocarditis
  • Isolated myocarditis
  • Isolated diffuse granulomatous myocarditis

Tabular List NotesGuidance

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

Inclusion Terms

  • Fiedler's myocarditis
  • Giant cell myocarditis
  • Idiopathic myocarditis

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.

    • Abramov-Fiedler myocarditis(acute isolated myocarditis)
    • Fiedler's
      • myocarditis (acute)
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • acute or subacute (interstitial)
        • idiopathic
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • eosinophilic
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • Fiedler's (acute) (isolated)
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • giant cell (acute) (subacute)
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • granulomatous (idiopathic) (isolated) (nonspecific)
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • idiopathic (granulomatous)
    • Myocarditis(with arteriosclerosis)(chronic)(fibroid) (interstitial) (old) (progressive) (senile)
      • isolated (acute)
    • Syndrome
      • Fiedler's

Clinical ClassificationClinical

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

CCSR CIR005
Myocarditis and cardiomyopathy
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Giant Cell Myocarditis

    an often fatal inflammatory disorder that affects the myocardium. morphologically, it is characterized by the presence of an inflammatory infiltrate in the myocardial tissue that includes multinucleated giant cells. it is often associated with the presence of an autoimmune disorder. patients present with arrhythmias and/or heart failure. heart transplantation is the only treatment option available.

Patient EducationClinical

Cardiomyopathy

Cardiomyopathy is the name for diseases of the heart muscle. These diseases enlarge your heart muscle or make it thicker and more rigid than normal. In rare cases, scar tissue replaces the muscle tissue.

Read the full article at MedlinePlus

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

Convert I40.1 to ICD-9-CMHistory

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

ICD-9-CM
422.91 Idiopathic myocarditis
Approximate The match is approximate rather than exact.

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 I40.1Overview

Is I40.1 (Acute myocarditis) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report isolated myocarditis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I40.1 group to?

When isolated myocarditis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 314, 315, 316, 974, 975, 976, with relative weights from 0.6821 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I40.1?

Under the General Equivalence Mappings, isolated myocarditis converts to ICD-9-CM 422.91 (idiopathic myocarditis). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.