A81.83
BillableFatal familial insomnia
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A81.83 an HCC code?
Yes. A81.83 maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model.
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for A81.83
For A81.83 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed A81.83 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
A81.83 is the ICD-10-CM diagnosis code for fatal familial insomnia. Fatal familial insomnia is a rare inherited brain disorder that causes progressive and severe sleep disturbances, leading to deterioration of physical and mental function and ultimately death. It is caused by a genetic mutation that affects prion proteins in the brain. A81.83 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering viral and prion infections of the central nervous system (a80-a89).
Under the CMS-HCC V28 risk adjustment model, A81.83 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. A81.83 was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This is a rare genetic disorder—verify the diagnosis is documented by a neurologist or specialist and confirm the familial/hereditary nature is established before coding. Because A81.83 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A81.83 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a rare genetic disorder—verify the diagnosis is documented by a neurologist or specialist and confirm the familial/hereditary nature is established before coding
- •Fatal familial insomnia is reportable in many jurisdictions due to its prion disease classification; check state and facility reporting requirements and consider adding a secondary code for the genetic predisposition if applicable
Clinical Significance
Fatal familial insomnia is an extremely rare hereditary prion disease caused by a specific PRNP gene mutation (D178N with methionine at codon 129), presenting with progressive intractable insomnia, autonomic dysfunction, and eventual dementia. It affects approximately 40 families worldwide and is uniformly fatal.
Documentation Requirements
- ✓Genetic testing confirming PRNP D178N mutation with 129M haplotype
- ✓Documented family history consistent with autosomal dominant inheritance
- ✓Sleep study results showing progressive insomnia and disrupted sleep architecture
- ✓Autonomic dysfunction documentation: hypertension, tachycardia, hyperthermia, hyperhidrosis
- ✓Neurological progression: cognitive decline, hallucinations, motor dysfunction