Summary & Overview
HCPCS M1162: Encephalitis Due to Tetanus/Diphtheria/Pertussis Vaccine
HCPCS Level II code M1162 designates encephalitis caused by the tetanus, diphtheria, or pertussis vaccine occurring on or before a patient’s 13th birthday. As a diagnostic designation for a rare, serious pediatric vaccine-associated neurologic event, the code matters for clinical documentation, claims processing, adverse event surveillance, and payer adjudication across the country. Clear identification of this condition supports accurate clinical records and informs utilization and payment workflows.
Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise explanation of the code’s clinical meaning and service context, a review of typical sites of service for encounters (hospital inpatient, emergency department, and outpatient neurology follow-up), and notes on common billing modifiers when present in the input data. The publication outlines benchmarks and policy-relevant considerations for recording vaccine-related adverse events, highlights implications for claims processing and medical record coding, and points to areas where additional clinical or coding detail may be needed. Data not available in the input are explicitly noted to guide further documentation and payer-specific policy review.
Billing Code Overview
HCPCS Level II code M1162 indicates a diagnosis of encephalitis attributable to the tetanus, diphtheria, or pertussis vaccine occurring on or before the patient's 13th birthday. The code captures a vaccine-related neurologic adverse event with pediatric onset.
Service Type: Diagnostic and inpatient/outpatient evaluation and management related to vaccine-associated encephalitis
Typical Site of Service: Hospital inpatient or emergency department, with possible outpatient neurology follow-up
Data not available in the input.
Clinical & Coding Specifications
Clinical Context
A pediatric patient presents with acute neurologic symptoms after receipt of a tetanus, diphtheria, or pertussis-containing vaccine given prior to the 13th birthday. Typical presentation includes new-onset fever, altered mental status, seizures, focal neurologic deficits, or decreased responsiveness within days to weeks after immunization. The clinical workflow begins in the emergency department or urgent care: triage with vital signs and neurologic assessment, stabilization (airway, breathing, circulation), and seizure control if needed. Initial workup includes urgent neuroimaging (CT or MRI), lumbar puncture for cerebrospinal fluid (CSF) analysis, and laboratory studies (CBC, electrolytes, inflammatory markers, blood cultures). Empiric antimicrobial and antiviral therapy may be initiated while awaiting CSF results. Consultation with pediatric neurology and infectious disease is common. Hospital admission to a pediatric intensive care unit is frequent for monitoring and supportive care. Documentation must clearly note the temporal relationship of the encephalitis to the vaccine given on or before the 13th birthday and include diagnostic findings (CSF analysis, imaging, EEG) and consultant notes to support the HCPCS Level II code M1162 on the claim.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
22 | Increased procedural services | Use when work required is substantially greater than typically required for management of postvaccination encephalitis (extensive documentation required). |