Crystal's Coder Hub🧬 ICD-10 CM G90.2 — Horner’s Syndrome
Billable Code Confirmed
ICD-10-CM G90.2 is a valid, billable 4-character ICD-10-CM code for FY2026. The first character (G) designates the Diseases of the Nervous System chapter; characters 2-3 (90) identify the G90 category (Disorders of autonomic nervous system); the fourth character (.2) specifies Horner’s syndrome as a distinct clinical entity within that category. No additional characters or extensions are required — G90.2 is complete and terminal as written.
Non-Billable Parent Code — Never Submit This
- ❌
G90— 3-character header — lacks specificity; does not identify which autonomic disorder is presentAlways submit G90.2 (all 4 characters) when Horner’s syndrome or Bernard-Horner syndrome is documented.
Clinical Context: Horner's Syndrome is a Localizing Sign, Not a Standalone Etiology
ICD-10-CM G90.2 captures the oculosympathetic triad (ptosis, miosis, anhidrosis) resulting from interruption of the sympathetic pathway at any level — central (first-order), preganglionic (second-order), or postganglionic (third-order). Because Horner’s syndrome is almost always a manifestation of another condition (malignancy, vascular dissection, demyelinating disease, or surgical injury), the underlying etiology should be identified and coded separately. G90.2 alone does not explain the root cause and may trigger payer scrutiny if no underlying condition is documented.
Code Classification
ICD-10-CM Diagnosis Code — wRVU, assistant payable status, and global period fields are not applicable to diagnosis codes. For procedure coding associated with evaluation of Horner’s syndrome, refer to the CPT Procedural Crosswalk and ICD-10-PCS Crosswalk sections below.
🔍 Code Description
ICD-10 CM G90.2 classifies Horner’s syndrome, a clinical triad of ipsilateral partial ptosis (drooping of the upper eyelid), miosis (pupillary constriction), and anhidrosis (loss of sweating) caused by disruption of the oculosympathetic (sympathetic) nerve pathway at any point along its three-neuron arc from the hypothalamus to the orbit.[¹][²]
The sympathetic pathway originates in the hypothalamus (first-order neuron), descends through the brainstem and cervical spinal cord, exits at C8-T2 to synapse in the superior cervical ganglion (second-order/preganglionic neuron), and then travels along the carotid artery to innervate the dilator pupillae muscle and Müller’s muscle of the eyelid (third-order/postganglionic neuron).[²][³] Lesions at any level — including G35.D multiple sclerosis plaques, I67.0 carotid dissection, C34.10 lung apex tumors (Pancoast), or surgical trauma to the cervical chain — can produce this syndrome.[³]
🌳 Code Tree / Hierarchy
G90 Disorders of autonomic nervous system ❌ Non-billable
│
├── G90.0 Idiopathic peripheral autonomic neuropathy ❌ Non-billable
│ ├── G90.01 Carotid sinus syncope ✅ Billable
│ └── G90.09 Other idiopathic peripheral autonomic neuropathy ✅ Billable
│
├── G90.1 Familial dysautonomia (Riley-Day) ✅ Billable
│
├── G90.2 Horner's syndrome ◀ THIS CODE ✅ Billable
│
├── G90.3 Multi-system degeneration of the autonomic nervous system ✅ Billable
│
├── G90.4 Autonomic dysreflexia ✅ Billable
│
├── G90.5 Complex regional pain syndrome I (CRPS I) ❌ Non-billable
│ ├── G90.51 CRPS I of upper limb ❌ Non-billable
│ │ ├── G90.511 CRPS I, right upper limb ✅ Billable
│ │ ├── G90.512 CRPS I, left upper limb ✅ Billable
│ │ └── G90.519 CRPS I, unspecified upper limb ✅ Billable
│ └── G90.52 CRPS I of lower limb ❌ Non-billable
│ ├── G90.521 CRPS I, right lower limb ✅ Billable
│ ├── G90.522 CRPS I, left lower limb ✅ Billable
│ └── G90.529 CRPS I, unspecified lower limb ✅ Billable
│
├── G90.8 Other disorders of autonomic nervous system ✅ Billable
├── G90.9 Disorder of autonomic nervous system, unspecified ✅ Billable
├── G90.A Postural orthostatic tachycardia syndrome (POTS) ✅ Billable
└── G90.B LMNB1-related autosomal dominant leukodystrophy ✅ Billable
Localizing Order Matters for CDI
Horner’s syndrome coding is complete at G90.2, but payers and physicians both benefit from documentation specifying whether the lesion is central (first-order), preganglionic (second-order), or postganglionic (third-order) — this localization drives the workup (MRI brain/spine vs. CT chest vs. neck imaging) and justifies additional diagnosis codes for the underlying etiology.
✅ Includes
The following clinical terms and scenarios map to G90.2 when documented:
- Bernard(-Horner) syndrome
- Cervical sympathetic dystrophy or paralysis
- Oculosympathetic paresis
- Partial ptosis + miosis + ipsilateral anhidrosis (classic clinical triad)
- Horner syndrome due to Pancoast (superior sulcus) tumor — code the tumor additionally
- Horner syndrome following neck dissection or cervical spine surgery — code the surgical complication additionally
- Horner syndrome associated with carotid dissection — code dissection additionally
❌ Excludes
Excludes 1 — Cannot Be Coded Simultaneously with G90.2
| Code | Description | Note |
|---|---|---|
| G31.2 | Degeneration of nervous system due to alcohol | Autonomic dysfunction in this context is classified entirely under G31.2; G90.x codes are excluded when alcoholic autonomic dysfunction is the cause |
Excludes 1 Violation Risk
A coder may be tempted to assign both G90.2 and G31.2 when a patient with alcohol-related neurodegeneration also has pupillary findings. This is an Excludes 1 violation — if the autonomic dysfunction (including any Horner features) is attributable to alcohol, use G31.2 only and do not layer G90.2 on top of it.
Excludes 2 — May Be Coded in Addition if Separately Present
| Code | Description | Note |
|---|---|---|
| No Excludes 2 noted for G90.2 | — | The underlying etiology (tumor, dissection, demyelinating disease) should be coded separately per coding guidelines as an additional code |
📋 Clinical Overview
Anatomic Level of Lesion — Three-Neuron Arc Localization
The clinical significance of Horner’s syndrome depends entirely on localizing which of the three sympathetic neurons is interrupted, as each level points to a different and potentially life-threatening etiology.
| Feature | First-Order (Central) | Second-Order (Preganglionic) | Third-Order (Postganglionic) |
|---|---|---|---|
| Pathway | Hypothalamus → ciliospinal center (C8-T2) | C8-T2 → superior cervical ganglion | Superior cervical ganglion → orbit |
| Common Etiologies | Brainstem stroke, G35.D, syringomyelia, trauma | Pancoast tumor, cervical rib, neck dissection | Carotid dissection, cluster headache, cavernous sinus lesion |
| Anhidrosis Location | Entire ipsilateral face, arm, and trunk | Ipsilateral face and neck | Face only (or absent) |
| Pharmacologic Testing | Cocaine/apraclonidine confirm; hydroxyamphetamine normal | Cocaine/apraclonidine confirm; hydroxyamphetamine normal | Cocaine/apraclonidine confirm; hydroxyamphetamine fails to dilate |
| Urgent Imaging | MRI brain/cervical spine | CT chest (r/o Pancoast); MRI cervical spine | CT angiography neck (r/o carotid dissection) |
CDI Query Trigger — Underlying Etiology Must Be Documented
Whenever Horner’s syndrome is documented in the medical record, the physician must be queried for the underlying cause if not explicitly stated. Coding G90.2 alone without an etiology code understates severity, misses potential HCC capture on the causative condition, and may leave a Pancoast tumor or carotid dissection uncoded — both of which carry significant DRG and risk-adjustment implications.
Manifestations & Symptom Burden
The classic triad and associated findings map to the following codes when separately documented:
- Ptosis (partial/neurogenic): H02.401 - Unspecified ptosis of right eyelid / H02.402 - left eyelid
- miosis: May be captured as part of the syndrome; document as a clinical finding in support
- Anhidrosis: L74.515 - Primary focal hyperhidrosis (anhidrosis captured clinically rather than a specific ICD code in most scenarios)
- Enophthalmos (apparent): H05.401 - Unspecified enophthalmos
- Dilation lag on pupil exam: Documented in support of sympathetic denervation
Coding Manifestations
💰 HCC Risk Adjustment (CMS-HCC v28)
| Field | Detail |
|---|---|
| CMS-HCC Model Version | v28 (2024-2025 Implementation) |
| HCC Assignment | ❌ Not HCC-Mapped |
| HCC Category | N/A |
| RAF Coefficient | N/A |
G90.2 does not map to an HCC under CMS-HCC v28 and does not independently contribute to a risk adjustment factor (RAF) score.[⁴]
Focus Capture on the Underlying Etiology
While G90.2 itself carries no HCC weight, its causative diagnoses frequently do. For example, lung malignancy (HCC 9/10), CNS demyelinating disease including MS (HCC 77), and cerebrovascular disease (HCC 100-103) all carry meaningful RAF coefficients. Accurate, complete coding of the underlying condition — not just the Horner’s triad — is where the risk-adjustment value lives.
🏥 MS-DRG Assignment
MDC 01 — Diseases and Disorders of the Nervous System
| DRG | Title | Est. Relative Weight* |
|---|---|---|
| DRG 091 | Other Disorders of Nervous System with MCC | ~1.45 - 1.65 |
| DRG 092 | Other Disorders of Nervous System with CC | ~0.90 - 1.05 |
| DRG 093 | Other Disorders of Nervous System without CC/MCC | ~0.65 - 0.75 |
Approximate. Verify against IPPS FY2026 Final Rule tables.
Sequencing and Complications
G90.2 most often sequences as a secondary diagnosis when the patient is admitted for the underlying etiology (e.g., Pancoast tumor, carotid dissection, or MS exacerbation). When sequenced secondarily, G90.2 may function as a CC, shifting DRG assignment upward from DRG 093 to DRG 092. Always evaluate whether secondary diagnoses such as dysphagia, aspiration, pain, or neurological deficits are documented to maximize appropriate DRG weight. If Horner’s syndrome is genuinely the reason for admission (e.g., new-onset unexplained Horner prompting workup), it sequences as principal and groups to DRG 091-093 depending on comorbidities.[⁵]
🔗 Related ICD-10-CM Codes
Autonomic Nervous System Disorders (G90 Family)
| Code | Description |
|---|---|
| G90.2 | Horner’s syndrome ← This Code |
| G90.3 | Multi-system degeneration of the autonomic nervous system |
| G90.4 | Autonomic dysreflexia |
| G90.511 | CRPS I, right upper limb |
| G90.9 | Disorder of autonomic nervous system, unspecified |
Commonly Associated Underlying Etiologies
| Code | Description |
|---|---|
| C34.12 | Malignant neoplasm, upper lobe, left bronchus/lung (Pancoast) |
| I77.71 | Dissection of carotid artery |
| G35.D | Multiple sclerosis — see G35.D note |
| S14.109A | Unspecified injury of cervical spinal cord, unspecified level, initial encounter |
| G95.89 | Other specified diseases of spinal cord (syringomyelia context) |
🛠️ Commonly Associated CPT Codes (Neurology / Ophthalmology)
Outpatient, Profee, and Inpatient Consult Setting Context
CPT codes associated with G90.2 span neurology, ophthalmology, and interventional radiology depending on the evaluation stage. Pharmacologic pupil testing and imaging interpretation are the most common profee services billed alongside this diagnosis. In inpatient settings, E/M consultation and neuroimaging interpretation predominate.
| CPT Code | Description | Profee Coding Notes (Modifier -26) |
|---|---|---|
| 99205 | New patient E/M, high complexity (outpatient) | Use when new Horner’s syndrome warrants high MDM (new diagnosis with uncertain etiology); no modifier needed for profee E/M |
| 99215 | Established patient E/M, high complexity (outpatient) | Appropriate for follow-up with ongoing workup; no modifier needed |
| 92018 | Ophthalmological exam, new patient, comprehensive | Used when ophthalmology evaluates ptosis and pupil asymmetry; no -26 modifier (professional-only code) |
| 70553 | MRI brain with and without contrast | Append Modifier -26 for interpretation only; pairs with G90.2 when central lesion is suspected |
| 70492 | CT angiography, neck with contrast | Append Modifier -26 for interpretation; used when carotid dissection is the suspected etiology |
| 71250 | CT thorax without contrast | Append Modifier -26; used when Pancoast tumor workup is indicated |
NCCI Bundling Considerations
- 99205/99215 (E/M) billed on the same day as a diagnostic imaging order or pharmacologic drop test: append Modifier -25 to the E/M to demonstrate it is a separately identifiable service from any minor procedure performed at the same encounter.
- Pharmacologic pupil testing (cocaine 4-10% or apraclonidine 1%) does not have a dedicated CPT code; it is typically included in the ophthalmologic exam code. Do not separately bill a drug administration code for diagnostic eye drops in this context without payer-specific guidance.
🔬 ICD-10-PCS Crosswalk (Inpatient Procedures)
When G90.2 is an inpatient diagnosis, these PCS codes are relevant for procedures associated with the underlying etiology driving the Horner’s syndrome.
| PCS Section | Body System | Root Operation | Clinical Application |
|---|---|---|---|
| B (Imaging) | B3 (Upper Arteries) | 3 (Magnetic Resonance Imaging) | MRI of carotid arteries when carotid dissection is suspected; example PCS: B331YZZ |
| B (Imaging) | B8 (Respiratory System) | 4 (Ultrasonography / CT) | CT of chest/thorax for Pancoast tumor evaluation; example PCS: BB24ZZZ |
| 0 (Medical & Surgical) | 00 (Central Nervous System) | N (Release) | Surgical release of cervical sympathetic chain compression; example PCS: 01NY0ZZ |
| 3 (Administration) | 3E0 (Peripheral Nervous System) | 3 (Introduction) | Stellate ganglion block for diagnostic or therapeutic sympathetic interruption; example PCS: 3E0T3GC |
💊 Coding Scenarios and Examples
Scenario 1 — Outpatient Neurology: New-Onset Horner’s Syndrome, Etiology Unknown
Clinical Vignette: A 47-year-old male presents to neurology clinic with a 3-day history of right-sided partial ptosis and noticed his right pupil appears smaller than the left, especially in dim lighting. He denies pain. Vital signs stable. Exam confirms right miosis, 2mm ptosis, and dilation lag at 5 seconds. No anhidrosis elicited on exam today. Pharmacologic testing with apraclonidine 1% confirms sympathetic denervation. Provider documents: “New Horner’s syndrome, right eye, etiology under investigation — ordering MRI brain/cervical spine and CT chest.”
CPT / HCPCS (Profee):
- 99205 — New patient E/M, high complexity (high MDM: new neurologic diagnosis with uncertain etiology requiring extensive workup)
ICD-10-CM Principal/Primary Diagnosis:
- G90.2 — Horner’s syndrome (reason for visit; etiology not yet established)
ICD-10-CM Secondary Diagnoses:
- H02.401 — Unspecified ptosis, right eyelid (separately documented manifestation)
Scenario 2 — Inpatient: Pancoast Tumor with New Horner’s Syndrome
Clinical Vignette: A 61-year-old female with a 6-week history of right shoulder pain and new right-sided Horner’s syndrome is admitted for staging workup. CT chest reveals a 4.2 cm right upper lobe apical mass with involvement of the first rib. Biopsy confirms non-small cell lung carcinoma (adenocarcinoma). Neuro consult documents: “Horner’s syndrome consistent with preganglionic (second-order) sympathetic interruption due to Pancoast tumor.”
Principal Diagnosis:
- C34.11 — Malignant neoplasm of upper lobe, right bronchus or lung (reason for admission; Pancoast tumor)
Secondary Diagnoses:
- G90.2 — Horner’s syndrome (manifestation of tumor’s involvement of sympathetic chain; functions as CC)
- M54.12 — Radiculopathy, cervical region (shoulder/arm pain documented)
MS-DRG Assignment: With G90.2 functioning as a CC and the principal diagnosis being a thoracic malignancy, this case likely groups to MDC 04 (Respiratory) or MDC 17 (Hematologic/Neoplastic depending on sequencing) — verify in grouper. G90.2 as CC supports a higher-weighted DRG within the applicable MDC.[⁵]
Scenario 3 — CDI Query: Horner’s Noted on Exam, No Etiology Documented
Clinical Vignette: A 38-year-old female is admitted with sudden onset right-sided neck pain and headache. Attending documents “right Horner’s syndrome” in the physical exam findings but attributes the admission to “cervicogenic headache” in the discharge summary. No imaging results are referenced in the assessment and plan. MRI neck ordered during admission shows right internal carotid artery irregularity, but the radiology report is not integrated into the physician’s documentation.
Action / Outcome: The coder identifies a conflict — Horner’s syndrome is documented but no etiology is linked, and imaging suggests carotid pathology. A CDI query is warranted to clarify whether the Horner’s syndrome and carotid irregularity represent a carotid dissection and whether the provider agrees that carotid dissection is the underlying etiology driving the admission.
Query Response: Provider updates documentation to confirm: “Right internal carotid artery dissection confirmed on MRI angiography, with secondary Horner’s syndrome due to disruption of the postganglionic sympathetic fibers traveling with the carotid. Carotid dissection is the principal diagnosis.”
Corrected ICD-10-CM Coding:
- I77.71 — Dissection of carotid artery (now correctly sequenced as principal diagnosis)
- G90.2 — Horner’s syndrome (secondary; manifestation of carotid dissection)
⚠️ Coding Pitfalls and Tips
| Pitfall or Tip | |
|---|---|
| ❌ | Coding G90.2 Without the Underlying Etiology. Horner’s syndrome is almost never idiopathic in adults — failing to code and sequence the causative condition (tumor, dissection, MS, surgical trauma) understates severity and misses critical HCC and DRG opportunities. |
| ❌ | Sequencing G90.2 as Principal When an Etiology Is Known. If the patient is admitted due to a Pancoast tumor or carotid dissection that is causing Horner’s syndrome, the etiology sequences as principal and G90.2 sequences secondarily per UHDDS and ICD-10-CM Official Guidelines. |
| ❌ | Violating the G31.2 Excludes 1 Note. Never assign G90.2 alongside G31.2. If autonomic dysfunction is due to alcohol, G31.2 is the only appropriate code for that component. |
| ✅ | Use G90.2 as a CC Trigger. When G90.2 sequences secondarily, confirm whether it elevates the DRG from a without CC/MCC to a with CC tier — this can meaningfully impact reimbursement without requiring any additional documentation if Horner’s is already documented. |
| ✅ | Query for Localization Level. First-, second-, and third-order Horner’s syndromes point to very different etiologies. Querying the provider for localization often surfaces a codeable underlying cause (lung mass, carotid dissection, or CNS lesion) that changes both the DRG and risk-adjustment profile. |
| ✅ | Code Manifestations Separately. If the physician documents ptosis, miosis, or anhidrosis as distinct clinical problems — especially in the context of ophthalmology consultation — code them separately under the appropriate H-code series to fully represent the patient’s condition complexity. |
📚 Sources
- CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026. Centers for Disease Control and Prevention.
- Walton, K.A., & Buono, L.M. (2003). Horner syndrome. Current Opinion in Ophthalmology, 14(6), 357-363. (Source for three-neuron arc anatomy and clinical triad description.)
- Kanagalingam, S., & Miller, N.R. (2015). Horner syndrome: Clinical perspectives. Eye and Brain, 7, 35-46. (Source for etiology localization, pharmacologic testing, and imaging guidance.)
- CMS. 2025-2026 Medicare Advantage Risk Adjustment — CMS-HCC Model v28 ICD-10-CM Mappings. Centers for Medicare & Medicaid Services.
- CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43. MDC 01 logic tables.