# Chronic Migraine: Pathophysiology and Management

## Introduction

Chronic migraine is defined by the International Classification of Headache Disorders (ICHD-3) as headache occurring on 15 or more days per month for more than three months, with migraine features present on at least 8 of those days. It affects approximately 1-2% of the global population and stands as one of the most disabling neurological conditions encountered in pain medicine. Understanding the underlying pathophysiology is essential for selecting appropriate acute, preventive, and interventional therapies.

## Trigeminovascular System

The trigeminovascular system (TVS) is the primary anatomical substrate of migraine pain. Trigeminal ganglion neurons innervate the meningeal vasculature, particularly the dura mater and large cerebral arteries. These first-order neurons project peripherally to perivascular nerve endings and centrally to the trigeminocervical complex (TCC) in the caudal trigeminal nucleus at the C1-C2 level. Second-order neurons ascend from the TCC to the ventral posteromedial (VPM) thalamus and subsequently project to the somatosensory cortex, insula, and anterior cingulate cortex.

When the TVS is activated, perivascular nerve endings release vasoactive neuropeptides -- calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide (PACAP). These neuropeptides drive vasodilation, neurogenic inflammation, and sensitization of the trigeminal pathway. Peripheral and central sensitization of the TVS underlies the progression from episodic to chronic migraine.

<image>Detailed anatomical illustration of the trigeminovascular system showing trigeminal ganglion neurons innervating dural blood vessels, with labeled projections to the trigeminocervical complex at the C1-C2 level, ascending pathways to the VPM thalamus, and cortical targets including somatosensory cortex and insula. Neuropeptide release (CGRP, substance P) at perivascular nerve endings is highlighted.</image>

## Cortical Spreading Depression

Cortical spreading depression (CSD) is a slowly propagating wave of neuronal and glial depolarization followed by prolonged suppression of cortical activity. It moves across the cortex at approximately 3-5 mm/min and is considered the electrophysiological correlate of migraine aura. CSD activates trigeminovascular afferents by stimulating meningeal nociceptors and opening pannexin-1 channels, which release inflammatory mediators into the subarachnoid space. This leads to upregulation of matrix metalloproteinase-9 (MMP-9) and disruption of the blood-brain barrier. In chronic migraine, evidence suggests that subclinical CSD events may occur even without a perceptible aura, contributing to ongoing trigeminovascular activation.

## CGRP Pathway

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide that is central to migraine pathogenesis. It is released from trigeminal sensory nerve endings during migraine attacks and binds to the CLR/RAMP1 receptor complex on vascular smooth muscle and dural mast cells, promoting vasodilation, neurogenic inflammation, and mast cell degranulation. Elevated CGRP levels in jugular venous blood during migraine attacks normalize with effective triptan therapy, providing direct evidence of its involvement. In chronic migraine, interictal CGRP levels remain elevated even between attacks, reflecting persistent trigeminovascular activation that distinguishes the chronic form from its episodic counterpart.

<image>Molecular diagram of the CGRP signaling pathway at the trigeminovascular junction, showing CGRP release from trigeminal nerve terminals, binding to the CLR/RAMP1 receptor complex on meningeal blood vessel smooth muscle cells, downstream cAMP signaling cascade, and resulting vasodilation and neurogenic inflammation with mast cell degranulation.</image>

## Acute Pharmacotherapy

Triptans (5-HT1B/1D agonists) remain first-line therapy for moderate-to-severe attacks. Options include sumatriptan, rizatriptan, and eletriptan, though they are contraindicated in patients with uncontrolled hypertension and cardiovascular disease. Gepants (CGRP receptor antagonists) such as ubrogepant and rimegepant provide effective acute relief without vasoconstrictive risk, making them suitable for patients with cardiovascular contraindications to triptans. Ditans (5-HT1F agonists), specifically lasmiditan, also lack vasoconstrictive activity but carry sedation and driving restrictions. NSAIDs like ketorolac and naproxen sodium are useful as monotherapy for mild-moderate attacks or as adjuncts. Antiemetics such as metoclopramide and prochlorperazine provide independent analgesic benefit in acute migraine beyond their antiemetic effects.

## Preventive Pharmacotherapy

| Agent | Class | Dose Range | Route/Frequency | Key Considerations |
|-------|-------|-----------|-----------------|-------------------|
| Topiramate | Anticonvulsant | 50-100 mg/day | Oral daily | Cognitive side effects; weight loss; teratogenic |
| Propranolol | Beta-blocker | 80-240 mg/day | Oral daily | Avoid in asthma; fatigue; contraindicated in bradycardia |
| Amitriptyline | TCA | 25-75 mg/day | Oral nightly | Sedation benefit for insomnia; anticholinergic effects |
| Valproic acid | Anticonvulsant | 500-1500 mg/day | Oral daily | Teratogenic; weight gain; hepatotoxicity monitoring |
| Erenumab | Anti-CGRP receptor mAb | 70-140 mg/month | SC monthly | Constipation; hypertension (rare) |
| Fremanezumab | Anti-CGRP ligand mAb | 225 mg monthly or 675 mg quarterly | SC | Injection site reactions |
| Galcanezumab | Anti-CGRP ligand mAb | 240 mg loading, then 120 mg monthly | SC | Well-tolerated |
| Eptinezumab | Anti-CGRP ligand mAb | 100-300 mg quarterly | IV | Fastest onset among mAbs |
| OnabotulinumtoxinA | Neurotoxin | 155-195 units q12 weeks | IM (PREEMPT protocol) | Minimum 3 cycles to assess; FDA-approved for CM |
| Atogepant | Oral CGRP receptor antagonist | 60 mg/day | Oral daily | Approved for episodic and chronic migraine |

First-line oral preventive agents include topiramate (50-100 mg/day), propranolol (80-240 mg/day), amitriptyline (25-75 mg/day), and valproic acid (500-1500 mg/day). CGRP monoclonal antibodies -- erenumab (targeting the CGRP receptor), fremanezumab, galcanezumab, and eptinezumab (targeting the CGRP ligand) -- are administered monthly or quarterly via subcutaneous or intravenous injection and have demonstrated efficacy in patients who have failed multiple preventive classes. OnabotulinumtoxinA (Botox) is FDA-approved specifically for chronic migraine, administered as 155-195 units across 31-39 injection sites using the PREEMPT protocol; it works by inhibiting CGRP release from trigeminal afferents. Atogepant, an oral CGRP receptor antagonist, has been approved for both episodic and chronic migraine prevention.

## Criteria for Interventional Treatment

Interventional approaches should be considered when patients have failed at least two classes of preventive medications at adequate doses and duration (minimum 8 weeks each), experience intolerable side effects from pharmacotherapy, have contraindications to standard preventive medications, or demonstrate significant disability (MIDAS score greater than 20 or HIT-6 score greater than 60).

Interventional options include greater occipital nerve blocks with local anesthetic and corticosteroid, which serve both diagnostic and therapeutic purposes as a bridge therapy. Sphenopalatine ganglion (SPG) blocks can be performed via transnasal approach. Peripheral nerve stimulation of the occipital nerves is reserved for medically refractory chronic migraine. Trigger point injections address associated cervical myofascial pain that often accompanies chronic migraine.

<image>Clinical illustration demonstrating the PREEMPT injection protocol for onabotulinumtoxinA in chronic migraine, showing a posterior and lateral view of the head and neck with labeled injection sites across the frontalis, corrugator, procerus, temporalis, occipitalis, cervical paraspinal, and trapezius muscles, with dosing annotations at each site.</image>

## Clinical Pearls

Chronic migraine frequently coexists with medication overuse headache, and analgesic overuse should always be assessed before escalating preventive therapy. The transition from episodic to chronic migraine is driven by central sensitization, and early, aggressive preventive therapy may prevent this chronification. CGRP monoclonal antibodies have a favorable side effect profile compared to traditional preventives and should be considered earlier in treatment algorithms for patients with significant comorbidities. OnabotulinumtoxinA requires a minimum of three treatment cycles (9 months) before its efficacy can be fairly judged. Comorbid conditions -- depression, anxiety, sleep disorders, and obesity -- should be addressed as part of a comprehensive management plan, as they independently contribute to migraine chronification and treatment resistance.

## References

1. Dodick DW. Migraine. Lancet. 2018;391(10127):1315-1330.
2. Silberstein SD, Holland S, Freitag F, et al. Evidence-based guideline update: pharmacologic treatment for episodic migraine prevention in adults. Neurology. 2012;78(17):1337-1345.
3. Ashina M, Katsarava Z, Do TP, et al. Migraine: epidemiology and systems of care. Lancet. 2021;397(10283):1485-1495.
4. Aurora SK, Dodick DW, Turkel CC, et al. OnabotulinumtoxinA for treatment of chronic migraine: results from the double-blind, randomized, placebo-controlled phase of the PREEMPT 2 trial. Cephalalgia. 2010;30(7):804-814.