Q04.5 ICD-10-CM Code: Megalencephaly
Q04.5 maps to CMS-HCC V28 182. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC coding software
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FY 2026 Apr update / Congenital malformations, deformations and chromosomal abnormalities (Q00-QA0) / Congenital malformations of the nervous system (Q00-Q07)
Q04.5
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMegalencephaly
A birth defect where the brain is abnormally large due to excessive brain tissue development.
Buddy Insight
Megalencephaly involves abnormally enlarged brain tissue that can cause increased intracranial pressure, seizures, and developmental delays.
CMS-HCC V28
MappedHCC 182
Coefficient HCC 182: 0.478 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 114
Code-level coefficient reference
ESRD/PACE
MappedHCC 72
Code-level coefficient reference
RXHCC
MappedHCC 155
Code-level coefficient reference
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for Q04.5. Check the code and parent instructions in the Code Book.
Excludes 2
Official- inborn errors of metabolism (E70-E88)Inherited from Q00-QA0
Related Codes
Includes
OfficialNo Includes notes are included in this display for Q04.5. Check the code and parent instructions in the Code Book.
Excludes 1
Official- cyclopia (Q87.0)Inherited from Q04
- macrocephaly (Q75.3)Inherited from Q04
Code First
OfficialNo Code First sequencing instructions are included in this display for Q04.5. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for Q04.5. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for Q04.5. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is Q04.5 an HCC code?
Yes. Q04.5 (Megalencephaly) maps to HCC 182, Spinal Cord Disorders/Injuries under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.478. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: Q04.5 is billable and maps to V28 HCC 182, Spinal Cord Disorders/Injuries. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- Q04.5
- Description
- Megalencephaly
- HCC (V28)
- HCC 182 — Spinal Cord Disorders/Injuries
- RAF reference coefficient
- 0.478
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work Q04.5 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for Q04.5
For Q04.5, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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What This Code Means
Q04.5 is the ICD-10-CM diagnosis code for megalencephaly. A birth defect where the brain is abnormally large due to excessive brain tissue development. Q04.5 sits in the ICD-10-CM chapter for congenital malformations, deformations and chromosomal abnormalities (q00-qa0), within the section covering congenital malformations of the nervous system (q00-q07).
Under the CMS-HCC V28 risk adjustment model, Q04.5 maps to Spinal Cord Disorders/Injuries (HCC 182) with a source-labeled community, non-dual, aged reference coefficient of 0.478. No V24 mapping is shown for Q04.5; use the applicable model and payment year when reviewing the V28 mapping. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
Distinguish between megalencephaly and hydrocephalus, which may coexist. For Q04.5, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, and MEAT documentation criteria for Q04.5 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Distinguish between megalencephaly and hydrocephalus, which may coexist
- •Document whether this is associated with other syndromes or malformations
Clinical Significance
Megalencephaly involves abnormally enlarged brain tissue that can cause increased intracranial pressure, seizures, and developmental delays. This condition requires ongoing neurological monitoring and management of complications including potential need for surgical intervention.
Documentation Requirements
- ✓Documentation of abnormally enlarged brain on neuroimaging
- ✓Head circumference measurements showing macrocephaly
- ✓Neuroimaging findings confirming increased brain tissue volume
- ✓Neurological examination and developmental assessment
- ✓Assessment for increased intracranial pressure signs
- ✓Seizure history and neurological complications
- ✓Genetic evaluation if indicated for underlying causes
- ✓Growth and developmental monitoring plans
Commonly Confused Codes
- •Q75.3: Macrocephaly involves enlarged head but may not have increased brain tissue
- •G93.1: Brain swelling from anoxia is acquired, not congenital
- •Q03.9: Hydrocephalus causes head enlargement but from fluid, not tissue
- •Q04.6: Congenital cerebral cysts cause space-occupying lesions differently
- •R94.02: Abnormal brain scan findings are test results, not diagnoses