Neurology ICD-10 specificity: stroke, TIA, MS, epilepsy, migraine.

Four diagnosis families carry most of neurology's medical-necessity risk, and every one of them fails the same way: a code that's technically valid but not specific enough for the LCD or payer policy checking against it. This guide is the full crosswalk — every code family verified live against the FY2026 ICD-10-CM code set, organized by the decision logic that actually separates one code from the next, not just a list.

Stroke, TIA, and sequelae: the imaging-confirmation rule

Three code families cover what looks, at the bedside, like a single spectrum of cerebrovascular events — but they answer three different clinical questions, and the decision between them turns on one fact: did imaging confirm an infarction, and is today's visit about the acute event or a lasting deficit from a past one.

The trap runs in both directions. Coding a resolved, non-infarcted event as I63 overstates what happened and can trigger scrutiny on utilization review; coding a genuine completed infarct as G45 because the note never explicitly says "infarction confirmed on imaging" under-codes a real event and can undercut medical necessity for the treatment that followed. The fix is documentation, not memorization: the radiology report or the physician's note has to state explicitly whether imaging showed an infarct, because that single sentence is what the code family hinges on.

I63.x: cerebral infarction by mechanism and artery

I63 codes are built from two axes stacked together: the mechanism (thrombosis, embolism, or unspecified occlusion/stenosis) and the artery affected (precerebral — vertebral, basilar, carotid — or cerebral — middle, anterior, posterior, cerebellar). A final digit adds laterality (right, left, bilateral, or unspecified) wherever the artery is a paired structure.

Example: I63.311 is cerebral infarction due to thrombosis of the right middle cerebral artery — mechanism (thrombosis, the "3" series), artery (middle cerebral), laterality (right). I63.9 (cerebral infarction, unspecified) is valid and billable, but it names neither mechanism nor artery; use it only when the workup genuinely didn't establish more, not as a default when the imaging report is sitting in the chart with the answer already in it. Two codes outside the mechanism-and-artery grid round out the family: I63.6, cerebral infarction due to nonpyogenic cerebral venous thrombosis, and I63.81, infarction due to occlusion or stenosis of a small artery — both frequently under-selected in favor of an unspecified code when the workup actually supports them.

G45.x: transient ischemic attack by syndrome

G45 codes describe a resolved event without imaging-confirmed infarction, and unlike I63, the subcodes are organized by clinical syndrome rather than a mechanism-and-artery grid.

G45.9 is valid and reasonably common where the vascular territory genuinely wasn't localized, but a chart documenting hemispheric symptoms (contralateral weakness, aphasia) supports G45.1, not G45.9, and a chart documenting vertebrobasilar symptoms (vertigo, diplopia, ataxia) supports G45.0. The syndrome-specific codes exist because payers and specialists both use them to track recurrence risk and localization — coding everything as unspecified erases information the chart already has.

I69.3xx: sequelae of cerebral infarction, by deficit

This is the family that gets skipped most often, because it requires recognizing that a visit is about a lasting effect rather than the event itself. It's organized by deficit type, and several subfamilies add a side-and-dominance axis that requires the chart to document the patient's handedness.

The dominance axis is the detail generic guides skip entirely: four of the seven subfamilies split into right-dominant, left-dominant, right-non-dominant, and left-non-dominant, plus unspecified. That distinction matters clinically (a dominant-side deficit carries different functional implications than the same deficit non-dominant) and it requires the chart to state handedness explicitly — a note that documents "left hemiparesis" without recording whether the patient is right- or left-handed can only support the unspecified-side code, not the dominance-specific one, even though the anatomical side is clear.

Chronic migraine: G43.7xx and G43.Exx

Migraine specificity runs on three independent axes stacked together: presence of aura, intractability, and status migrainosus. For chronic migraine specifically — 15 or more headache days per month, at least 8 meeting migraine criteria, sustained 3 months — the two governing categories are G43.7xx (without aura) and G43.Exx (with aura), each splitting the same way.

Beyond chronic migraine, the broader G43 category carries codes that get miscoded into the chronic family when they shouldn't be: G43.4 hemiplegic migraine, G43.5/G43.6 persistent migraine aura without/with cerebral infarction, and G43.A cyclical vomiting in migraine each have their own subcode families with the same intractable/status-migrainosus structure, and none of them are "chronic migraine" even when the patient also happens to meet the 15-day frequency threshold. Code the specific migraine subtype the chart documents, not the frequency pattern alone.

Epilepsy: G40.x by type, syndrome, and severity

G40 is neurology's largest and most granular diagnosis family, and its size is exactly why it gets under-coded — it's easier to reach for unspecified epilepsy than to navigate 90-plus subcodes. Every subcategory below splits further by intractability and status epilepticus; the table shows the category-defining codes, not every combination.

The pattern to watch for on chart review: a note that names a specific syndrome — "consistent with juvenile myoclonic epilepsy" or "Dravet syndrome, genetically confirmed" — but the claim carries G40.909 anyway, usually because the EHR's problem-list mapping was never updated after the diagnosis was refined. That gap doesn't cause a denial by itself, since G40.909 is valid, but it under-documents medical necessity for the AED regimen, EEG frequency, or specialist referral the specific syndrome actually justifies.

Multiple sclerosis: the full FY2026 G35 replacement set

Plain G35 is a category header only as of the FY2026 restructuring — it is not valid for HIPAA transactions and will reject a claim before medical necessity is ever evaluated. The replacement set is built from phenotype (relapsing-remitting, primary progressive, secondary progressive, or unspecified) crossed with disease activity where applicable.

G35.D is the code to reach for when the phenotype genuinely isn't determined yet, not plain G35 — it's the closest equivalent to the old unspecified code and it passes HIPAA validation. The activity designation (active/non-active) on the progressive phenotypes tracks whether the patient has had a relapse or new MRI activity within a defined recent period; a neurologist's note stating "no new relapses or MRI activity in the past 12 months" supports the non-active code, while silence on activity status defaults the coder to the unspecified-activity variant, not an assumption in either direction.

Frequently asked questions

Related resources

• Neurology billing and coding guide
• Botox for chronic migraine billing: J0585 and 64615.
• Neurology modifiers: 25, 59, 26/TC, and telehealth.
• Medical coding overview
• All 25 specialties