What is ICD-10 code G93.42?

G93.42 is the ICD-10-CM diagnosis code for Megalencephalic leukoencephalopathy with subcortical cysts. A rare inherited brain disorder characterized by abnormal brain enlargement and white matter changes with fluid-filled cysts. It belongs to the ICD-10-CM chapter for diseases of the nervous system (g00-g99). G93.42 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G93.42 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. G93.42 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. G93.42 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for G93.42?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. G93.42 is a payment HCC under V28 (Cerebellar Ataxia and Other Degenerative Diseases of Nervous System) but was not retained as a payment HCC under V24. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting G93.42?

This is a rare genetic leukodystrophy; ensure imaging findings confirm the diagnosis before coding Document family history and genetic testing results when available Because G93.42 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

G93.42 ICD-10 Code: Megalencephalic

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupG93.42Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Other disorders of the nervous system (G89-G99)

G93.42

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Megalencephalic leukoencephalopathy with subcortical cysts

A rare inherited brain disorder characterized by abnormal brain enlargement and white matter changes with fluid-filled cysts.

Buddy Insight

Megalencephalic leukoencephalopathy with subcortical cysts is a rare inherited white matter disorder typically presenting in early childhood with macrocephaly and progressive neurological decline.

CMS-HCC V28

Mapped

HCC 200

Cerebellar Ataxia and Other D...

RAF 0.262

CMS-HCC V24

Not risk-adjusted for this model/period.

RAF 0

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Not risk-adjusted for this model/period.

RAF 0

RXHCC

Not risk-adjusted for this model/period.

RAF 0

Code Trumping

Basket needed

Compare selected patient codes to show trumped or trumps impact.

Code Book Path

Official

G93Other disorders of brain

G93.4Other and unspecified encephalopathy

G93.42Megalencephalic leukoencephalopathy with subcortical cysts

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for G93.42 in this effective period.

Excludes 2

Official

alcoholic encephalopathy (G31.2)
encephalopathy in diseases classified elsewhere (G94)
hypertensive encephalopathy (I67.4)
toxic (metabolic) encephalopathy (G92.8)

Related Child Codes

Official

G93.40Encephalopathy, unspecified

G93.41Metabolic encephalopathy

G93.43Leukoencephalopathy with calcifications and cysts

G93.44Adult-onset leukodystrophy with axonal spheroids

G93.45Developmental and epileptic encephalopathy

Includes

Official

ICD-10-CM does not list Includes notes for G93.42 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for G93.42 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for G93.42 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G93.42 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G93.42 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

MRI demonstrating diffuse white matter signal abnormality with characteristic subcortical cysts, particularly in temporal regions

Genetic testing confirming mutations in MLC1 or GLIALCAM genes when available

Documentation of macrocephaly onset, typically in first year of life

Neurological examination documenting progressive motor and cognitive deficits

MEAT Support

HCC Buddy guidance

MRI demonstrating diffuse white matter signal abnormality with characteristic subcortical cysts, particularly in temporal regions

Genetic testing confirming mutations in MLC1 or GLIALCAM genes when available

Documentation of macrocephaly onset, typically in first year of life

Neurological examination documenting progressive motor and cognitive deficits

Audit Caution

HCC Buddy guidance

Using a nonspecific leukodystrophy or white matter disease code when MLC has been specifically diagnosed

Confusing with other childhood-onset leukodystrophies that have distinct codes and pathologies

Failing to document the specific imaging findings (subcortical cysts) that distinguish this from other white matter diseases

Not coding associated manifestations such as seizures, spasticity, or cognitive disability separately

Common Mistakes

HCC Buddy guidance

G93.43 — Leukoencephalopathy with calcifications and cysts: different pathology with calcifications rather than the MLC pattern

G93.44 — Adult-onset leukodystrophy with axonal spheroids: adult onset and different genetic basis (CSF1R)

G90.B — LMNB1-related autosomal dominant leukodystrophy: different gene, adult onset, autosomal dominant

E75.25 — Metachromatic leukodystrophy: lysosomal storage disorder with different enzyme deficiency

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is G93.42 an HCC code?

Yes. G93.42 maps to Cerebellar Ataxia and Other Degenerative Diseases of Nervous System under the CMS-HCC V28 risk adjustment model.

HCC Category Mapping

V28HCC 200, Cerebellar Ataxia and Other Degenerative Diseases of Nervous System

0.262

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 G93.42

For G93.42to 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 G93.42 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

G93.42 is the ICD-10-CM diagnosis code for megalencephalic leukoencephalopathy with subcortical cysts. A rare inherited brain disorder characterized by abnormal brain enlargement and white matter changes with fluid-filled cysts. G93.42 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering other disorders of the nervous system (g89-g99).

Under the CMS-HCC V28 risk adjustment model, G93.42 maps to Cerebellar Ataxia and Other Degenerative Diseases of Nervous System (HCC 200) with a community, non-dual, aged base RAF weight of 0.262. G93.42 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 leukodystrophy; ensure imaging findings confirm the diagnosis before coding. Because G93.42 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 G93.42 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 leukodystrophy; ensure imaging findings confirm the diagnosis before coding
•Document family history and genetic testing results when available

Clinical Significance

Megalencephalic leukoencephalopathy with subcortical cysts is a rare inherited white matter disorder typically presenting in early childhood with macrocephaly and progressive neurological decline. The condition causes progressive loss of motor and cognitive function due to extensive white matter vacuolization and subcortical cyst formation. This is a significant chronic neurological condition requiring lifelong management and support services.

Documentation Requirements

✓MRI demonstrating diffuse white matter signal abnormality with characteristic subcortical cysts, particularly in temporal regions
✓Genetic testing confirming mutations in MLC1 or GLIALCAM genes when available
✓Documentation of macrocephaly onset, typically in first year of life
✓Neurological examination documenting progressive motor and cognitive deficits
✓Current functional status and supportive care plan

Commonly Confused Codes

•G93.43: Leukoencephalopathy with calcifications and cysts: different pathology with calcifications rather than the MLC pattern
•G93.44: Adult-onset leukodystrophy with axonal spheroids: adult onset and different genetic basis (CSF1R)
•G90.B: LMNB1-related autosomal dominant leukodystrophy: different gene, adult onset, autosomal dominant
•E75.25: Metachromatic leukodystrophy: lysosomal storage disorder with different enzyme deficiency