LogoCrystal's MCW Coder Hub

G35.C1

11 min read

🧬 ICD-10 CM G35.C1 — Primary Progressive Multiple Sclerosis with Acute Exacerbation

Billable Code Confirmed

ICD-10 CM G35.C1 is a valid, billable 5-character ICD-10-CM code for FY2026. All required characters are present: G35 (category) + .C (primary progressive phenotype) + 1 (with acute exacerbation).

Non-Billable Parent Codes — Never Submit These

  • ❌ G35 — 3-character header — missing phenotype and exacerbation status
  • ❌ G35.C — 4-character header — missing exacerbation status

Always submit G35.C1 (all 5 characters) when PPMS is documented alongside an acute inflammatory flare or relapse.

Clinical Context: "Primary Progressive" vs. "Acute Exacerbation"

ICD-10 CM G35.C1 identifies Primary Progressive Multiple Sclerosis (PPMS) that is actively flaring. PPMS is characterized by a continuous, gradual accumulation of neurologic disability from symptom onset. However, a subset of patients with PPMS can still experience superimposed acute exacerbations (relapses)—sudden, distinct episodes of new or rapidly worsening neurological symptoms driven by acute focal inflammation. The 1 indicates the presence of this acute event, differentiating it from the steady baseline decline.

Code Classification

ICD-10-CM Diagnosis Code — Fields for wRVU, assistant payable, and global period are not applicable. For associated inpatient profee and facility procedure coding, see the CPT Procedural Crosswalk and ICD-10-PCS Crosswalk sections below.

🔍 Code Description

ICD-10 CM G35.C1 classifies Primary progressive multiple sclerosis with acute exacerbation. It denotes an immune-mediated demyelinating disease of the central nervous system marked by a steady functional decline from onset, which is currently complicated by an acute, rapid worsening of neurological function.

In patients with PPMS, the autoimmune pathology is heavily driven by chronic neurodegeneration and smoldering inflammation, particularly in the spinal cord. However, acute inflammatory events—characterized by the influx of active immune cells (T-cells, B-cells, macrophages) across the blood-brain barrier causing focal demyelinating plaques—can still occur.

The qualifier “with acute exacerbation” (also known as a relapse, flare, or attack) signifies that the patient is currently experiencing new focal neurologic deficits, or a severe and rapid deterioration of prior deficits, typically lasting at least 24 hours. These exacerbations often prompt urgent medical evaluation and aggressive acute treatments, such as high-dose corticosteroids or plasmapheresis, to accelerate recovery and minimize residual damage.

🌳 Code Tree / Hierarchy

G35 Multiple Sclerosis ❌ Non-billable  
│  
├── G35.A Multiple sclerosis, unspecified ✅ Billable  
├── G35.B Relapsing-remitting multiple sclerosis (RRMS) ❌ Non-billable 
│ │
│ ├── G35.B0 RRMS without acute exacerbation ✅ Billable
│ └── G35.B1 RRMS with acute exacerbation ✅ Billable
│  
├── G35.C Primary progressive multiple sclerosis (PPMS) ❌ Non-billable  
│ │  
│ ├── G35.C0 PPMS without acute exacerbation ✅ Billable  
│ └── G35.C1 PPMS WITH ACUTE EXACERBATION ◀ THIS CODE ✅ Billable  
│  
├── G35.D Secondary progressive multiple sclerosis (SPMS) ❌ Non-billable  
│ │  
│ ├── G35.D0 SPMS without acute exacerbation ✅ Billable
│ └── G35.D1 SPMS with acute exacerbation ✅ Billable
│ 
└── G35.E Clinically isolated syndrome (CIS) ✅ Billable

Specificity is Key for Proper Care Management

Documenting the exacerbation status is critical. A patient coded with G35.C0 (without exacerbation) is typically managed with maintenance Disease-Modifying Therapies (DMTs) and physical therapy. A patient coded with G35.C1 is experiencing a crisis requiring acute pharmacological intervention. Capturing G35.C1 justifies the medical necessity for urgent MRIs, hospital admissions, and high-dose IV steroid infusions.

✅ Includes

The following clinical terms and scenarios map to G35.C1 when the patient is in an active flare:

  • Primary progressive MS (PPMS) with acute exacerbation
  • Primary progressive MS (PPMS) in relapse
  • Active flare of primary progressive multiple sclerosis
  • PPMS with superimposed attack

❌ Excludes

Excludes 1 — Cannot Be Coded Simultaneously with G35.C1

CodeDescriptionNote
G36.0Neuromyelitis optica [Devic]Mutually exclusive — NMO involves antibodies targeting aquaporin-4 (AQP4); pathophysiology is distinct from MS.
G36.9Acute disseminated demyelinationADEM is typically a monophasic post-infectious process, unlike chronic MS.
G37.0Schilder’s diseaseA rare progressive demyelinating disorder distinct from MS.
G37.81MOGADMyelin oligodendrocyte glycoprotein antibody-associated disease is a distinct demyelinating syndrome.

Excludes 1 Violation Risk

You cannot assign MS codes concurrently with other specified central demyelinating diseases like NMO or MOGAD. If a patient’s diagnosis is revised from an MS flare to an NMO flare based on positive AQP4 autoantibody testing, use the NMO code exclusively.

Excludes 2 — May Be Coded in Addition if Separately Present

CodeDescriptionNote
G37.-Other demyelinating diseases of central nervous systemMay be coded additionally if a completely distinct demyelinating condition is confirmed alongside MS.

📋 Clinical Overview

True Exacerbation vs. Pseudoexacerbation

A critical clinical and documentation distinction during an MS flare is separating a true exacerbation from a pseudoexacerbation.

FeatureTrue Exacerbation (Relapse/Flare)Pseudoexacerbation
PathophysiologyNew area of active central nervous system inflammation and demyelination.Temporary unmasking or worsening of old symptoms due to physiological stress.
Common TriggersAutoimmune activity; sometimes preceded by viral illness.Heat (Uhthoff’s phenomenon), UTI, pneumonia, systemic infection, fever, extreme stress.
DurationLasts >24 hours, often weeks to months without treatment.Typically resolves completely once the underlying trigger (e.g., fever, infection) is treated or subsides.
MRI FindingsOften shows new gadolinium-enhancing lesions (active active plaque).Usually no new enhancing lesions.
Code AssignmentG35.C1 (This Code)Usually G35.C0 plus the code for the trigger (e.g., N39.0 for UTI).

CDI Query Trigger — "Pseudoexacerbation"

If the physician documents a “flare” but notes it is entirely secondary to a severe UTI or fever, and no steroid treatment is initiated for the MS, a CDI query is warranted to determine if this is a true exacerbation or a pseudoexacerbation. If it is a pseudoexacerbation, G35.C0 is the more accurate code.

Clinical Presentation of an Exacerbation

Symptoms of a true acute exacerbation depend on the location of the new demyelinating plaque in the CNS, but frequently present rapidly (over hours to days) and may include:

  • Optic neuritis: Unilateral eye pain and acute vision loss.
  • Acute transverse myelitis symptoms: Sudden severe weakness or paralysis in the legs.
  • Sensory changes: Ascending numbness or severe paresthesia.
  • Brainstem/Cerebellar signs: Acute severe vertigo, ataxia, or diplopia (double vision).

Coding Manifestations

Always code the specific acute neurological deficits driving the clinical encounter. This supports the medical necessity for high-level E/M or admission. Examples include:

  • H46.9 — Unspecified optic neuritis
  • G82.20 — Paraplegia, unspecified
  • R27.0 — Ataxia, unspecified
  • H53.2 — Diplopia

💰 HCC Risk Adjustment (CMS-HCC v28)

FieldDetail
CMS-HCC Model Versionv28 (2024-2025 Implementation)
HCC Assignment✅ Mapped
HCC CategoryHCC 77 — Multiple Sclerosis
RAF Coefficient~0.45 - 0.65 (varies by demographic/status)

G35.C1 maps directly to an HCC and contributes significantly to the RAF score. Capturing the acute exacerbation status also provides a clear narrative in the patient’s claims history regarding sudden spikes in resource utilization (inpatient stays, biologic therapies, intensive rehab).

🏥 MS-DRG Assignment

MDC 01 — Diseases and Disorders of the Nervous System

DRGTitleEst. Relative Weight*
DRG 058Multiple Sclerosis & Cerebellar Ataxia with MCC~1.30 - 1.50
DRG 059Multiple Sclerosis & Cerebellar Ataxia with CC~0.90 - 1.10
DRG 060Multiple Sclerosis & Cerebellar Ataxia without CC/MCC~0.65 - 0.80

Approximate. Verify against IPPS FY2026 Final Rule tables.

Sequencing the Flare

When a patient is admitted primarily for the management of the MS exacerbation (e.g., for 5 days of IV Solu-Medrol due to acute paraparesis), G35.C1 is sequenced as the principal diagnosis. It will group directly to MDC 01.

🔗 Related ICD-10-CM Codes

Exacerbation Status Variants

CodeDescription
G35.C1PPMS with acute exacerbation ← This Code
G35.C0PPMS without acute exacerbation

Phenotype Variants

CodeDescription
G35.B1Relapsing-remitting multiple sclerosis with acute exacerbation
G35.D1Secondary progressive multiple sclerosis with acute exacerbation
G35.AMultiple sclerosis, unspecified

🛠️ Commonly Associated CPT Codes (Neurology / PM&R)

Outpatient and Profee Setting Context

The CPT codes below are associated with the diagnostic workup and acute management of a PPMS exacerbation in profee and outpatient facility settings.

CPT CodeDescriptionProfee Coding Notes (Modifier 26)
70553MRI brain without contrast, followed by with contrastContrast is crucial during a flare to identify active (enhancing) lesions. Append -26 for profee interpretation only.
72156MRI cervical spine w/ and w/o contrast
96365Intravenous infusion, for therapy, prophylaxis, or diagnosis; initial, up to 1 hourUsed for administering high-dose IV methylprednisolone (Solu-Medrol).
96366Intravenous infusion, each additional hour
99222Initial hospital inpatient or observation care, moderate complexityTypical E/M for acute admission during a flare.

NCCI Bundling Considerations

  • Infusion services (96365) billed on the same day as an E/M visit require the E/M to be significant, separately identifiable, and above/beyond the standard work of the infusion. If supported by documentation, append Modifier -25 to the E/M code.

🔬 ICD-10-PCS Crosswalk (Inpatient Procedures)

When G35.C1 is an inpatient principal diagnosis for a flare, the following therapeutic ICD-10-PCS sections and root operations are highly relevant for acute management.

PCS SectionBody SystemRoot OperationClinical Application
3 (Administration)E (Physiological Systems)0 (Introduction)High-dose IV Corticosteroid therapy. Example: 3E033VZ (Introduction of Hormone into Peripheral Vein, Percutaneous).
6 (Extracorporeal Therapies)A (Physiological Systems)5 (Pheresis)Therapeutic plasma exchange (plasmapheresis) for severe relapses unresponsive to steroids. Example: 6A550Z3 (Pheresis of Plasma, Single).

💊 Coding Scenarios and Examples

Scenario 1 — Inpatient Admission for Acute MS Flare

Clinical Vignette: A 45-year-old female with a known history of primary progressive MS presents to the ER with sudden onset of severe right eye pain and profound vision loss over 24 hours. She has a history of slow gait decline, but this visual loss is acute. MRI of the brain and orbits with contrast reveals a new, strongly enhancing lesion on the right optic nerve. She is admitted to the neurology service for acute exacerbation of PPMS manifesting as optic neuritis. She is started on 1,000 mg IV methylprednisolone daily for 5 days.

Principal Diagnosis:

  • G35.C1 — Primary progressive multiple sclerosis with acute exacerbation (Drives the MDC 01 DRG grouping)

Secondary Diagnoses:

  • H46.9 — Unspecified optic neuritis, right eye (Captures the specific acute manifestation)

Inpatient Procedures (PCS):

  • 3E033VZ — Introduction of Hormone into Peripheral Vein, Percutaneous Approach (Captures the IV steroid therapy)

Scenario 2 — Outpatient Infusion Center for Flare Management

Clinical Vignette: A 50-year-old male with PPMS calls his neurologist reporting acute, severe worsening of lower extremity weakness over the last two days, making him entirely unable to bear weight (a significant decline from his baseline of walking with a cane). The neurologist evaluates him in the clinic, confirms an acute relapse, and sends him directly to the outpatient infusion center for 3 days of IV Solu-Medrol.

CPT / HCPCS (Profee/Outpatient):

  • 99215-25 — Office/outpatient visit, high complexity, established patient (Modifier 25 denotes the separate evaluation leading to the decision to treat)
  • 96365 — IV infusion, initial, up to 1 hour
  • J2920 — Injection, methylprednisolone sodium succinate, up to 40 mg (Billed based on total units administered)

ICD-10-CM:

  • G35.C1 — Primary progressive multiple sclerosis with acute exacerbation
  • G82.20 — Paraplegia, unspecified (To capture the acute weakness)

Scenario 3 — CDI Query: True Exacerbation vs. Pseudoexacerbation

Clinical Vignette: A patient with PPMS is admitted for urosepsis. The H&P states: “Patient admitted for severe sepsis secondary to UTI. Also experiencing an MS exacerbation today with marked worsening of her baseline spasticity and fatigue.” The patient is treated with IV Rocephin and fluids. No IV steroids or acute MS-specific therapies are ordered. As the infection clears on day 3, the physician notes, “MS symptoms returning to baseline.”

Action / Outcome: The documentation uses the word “exacerbation,” which would lead a coder to assign G35.C1. However, the clinical picture—symptoms resolving upon treatment of an underlying infection without steroid therapy—strongly points to a pseudoexacerbation (Uhthoff’s phenomenon/infection-related worsening).

CDI Query: The coder queries the provider to clarify if this is a true immunologic acute exacerbation or a transient pseudoexacerbation secondary to sepsis.

Query Response: The physician clarifies it was a pseudoexacerbation driven by the UTI.

Corrected ICD-10-CM Coding:

  • A41.9 — Sepsis, unspecified organism (Principal Diagnosis)
  • N39.0 — Urinary tract infection, site not specified
  • G35.C0 — Primary progressive multiple sclerosis without acute exacerbation (Corrected from C1 to C0 based on query)

⚠️ Coding Pitfalls and Tips

Pitfall or Tip
❌Do not confuse a pseudoexacerbation with a true exacerbation. Worsening of symptoms solely due to heat, fever, or systemic infection (which resolves when the trigger resolves) should be coded as G35.C0, not G35.C1.
❌Do not code G35.A (Unspecified) if an exacerbation is documented. The presence of a flare demands the use of the 1 character (B1, C1, or D1).
✅Sequence properly for admissions. If the patient is admitted specifically to treat the MS relapse (e.g., IV steroids, plasmapheresis), sequence G35.C1 as the principal diagnosis.
✅Code the acute deficits. Optic neuritis, acute paraparesis, or severe ataxia that prompted the encounter should be coded secondarily to paint a full picture of the severity of illness.
✅Look for MRI contrast use. In outpatient diagnostics, if an MS patient presents with new symptoms and the provider orders an MRI with contrast, they are looking for enhancing lesions—a strong clinical indicator of an acute flare.

📚 Sources

  1. CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026. 2. National Multiple Sclerosis Society. Managing Relapses (Exacerbations) in MS.
  2. CMS. 2025-2026 Medicare Advantage Risk Adjustment — CMS-HCC Model v28 ICD-10-CM Mappings. 4. CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43. MDC 01 logic tables.
  3. CMS. ICD-10-PCS Reference Manual FY2026. Section 3 (Administration), Section 6 (Extracorporeal Therapies).
  4. AMA. CPT Professional Edition 2026. Neurology and Medicine subsections.

Graph View

Recent Notes