OnabotulinumtoxinA (Botox) for Migraine
OnabotulinumtoxinA is FDA-approved for the preventive treatment of chronic migraine (≥15 headache days/month). It works through a unique mechanism — inhibiting release of CGRP, substance P, and glutamate from trigeminal sensory afferents — making it complementary to, rather than overlapping with, CGRP-targeted therapies. It is not effective for episodic migraine.
Bottom Line
- Approved for chronic migraine only (≥15 headache days/month) — PREEMPT trials showed no benefit in episodic migraine
- Protocol: 155 units minimum across 31 fixed injection sites in head and neck every 12 weeks
- Assess after 2-3 cycles (6-9 months) before declaring failure — benefit is cumulative
- Reduces headache days by ~8-9/month (vs ~6-7 placebo) in PREEMPT; effect size grows with subsequent cycles
- Can be combined with CGRP mAbs for refractory CM — different mechanisms, additive benefit
- Well-tolerated: Main side effects are injection site pain, neck weakness/stiffness, and rare ptosis
Mechanism
OnabotulinumtoxinA cleaves SNAP-25, a protein essential for vesicle fusion at nerve terminals. In migraine, this inhibits the release of pain-signaling neuropeptides from trigeminal C-fiber afferents:
- CGRP — blocks release at the peripheral terminal (complementary to mAbs which block extracellular CGRP)
- Substance P and glutamate — reduces neurogenic inflammation and peripheral sensitization
- TRPV1 receptor trafficking — prevents insertion of pain receptors into the cell membrane
The effect is localized to sensory neurons near injection sites. It does not cross the blood-brain barrier. Onset takes 1-2 weeks as existing SNAP-25 is gradually depleted; nerve terminals regenerate over 10-12 weeks, necessitating repeat injections.
PREEMPT Trials
PREEMPT I (Aurora 2010)
1,384 adults with chronic migraine randomized to onabotulinumtoxinA 155-195 units or placebo, injected every 12 weeks for 2 cycles (24 weeks).
- Primary endpoint (headache episodes): Not statistically significant in PREEMPT I (-5.2 vs -5.3, p=0.344)
- Headache days (key secondary): -7.8 vs -6.4 (p=0.006) — significant
- Multiple secondary endpoints favored onabotulinumtoxinA
PREEMPT II (Diener 2010)
705 adults with chronic migraine, same protocol as PREEMPT I.
- Primary endpoint (headache days): -9.0 vs -6.7 (p<0.001) — significant
- ≥50% reduction in headache days: 28% vs 17%
Pooled PREEMPT Analysis (Dodick 2010)
| Outcome | OnabotulinumtoxinA | Placebo | Difference |
|---|---|---|---|
| Headache days/month reduction | -8.4 | -6.6 | -1.8 (p<0.001) |
| Migraine days/month reduction | -8.2 | -6.2 | -2.0 (p<0.001) |
| ≥50% responder rate (headache days) | 27.1% | 17.5% | p<0.001 |
| Acute medication days/month | -7.3 | -5.5 | -1.8 (p<0.001) |
Open-Label Extension (56 weeks)
- Patients completing the double-blind phase entered open-label treatment with onabotulinumtoxinA for up to 3 additional cycles
- Cumulative improvement: Headache day reductions continued to increase through cycles 3-5
- By cycle 5, mean reduction was ~11 headache days/month from baseline
- Supports the recommendation to assess after at least 2-3 treatment cycles
Injection Protocol
Fixed-Site Paradigm (PREEMPT Protocol)
The approved protocol uses 31 fixed injection sites across 7 head/neck muscle groups, with a minimum total of 155 units:
| Muscle Group | Sites | Units per Site | Total Units |
|---|---|---|---|
| Corrugator | 2 (1 per side) | 5 U | 10 U |
| Procerus | 1 | 5 U | 5 U |
| Frontalis | 4 (2 per side) | 5 U | 20 U |
| Temporalis | 8 (4 per side) | 5 U | 40 U |
| Occipitalis | 6 (3 per side) | 5 U | 30 U |
| Cervical paraspinal | 4 (2 per side) | 5 U | 20 U |
| Trapezius | 6 (3 per side) | 5 U | 30 U |
| Total | 155 U (31 sites) | ||
Follow-the-Pain (Additional Units)
- Up to 40 additional units (total 195 U) can be distributed across temporalis, occipitalis, or trapezius based on the patient’s predominant pain location
- Follow-the-pain dosing is commonly used in practice and was permitted in PREEMPT
- The additional sites should target areas where the patient reports maximal tenderness or pain radiation
Injection Pearls
- Use a 30-gauge, 0.5-inch needle for most sites; 1-inch needle for trapezius and cervical paraspinal muscles
- Frontalis: Stay ≥2 cm above the brow to avoid ptosis; inject superficially into the muscle belly
- Corrugator: Medial brow; directed laterally to avoid eyelid weakness
- Temporalis: Fan injections across the muscle belly following the temporal line
- Occipitalis: Avoid injecting too medially (near midline) to prevent neck extensor weakness
- Trapezius: Inject into upper fibers; stay above the scapular spine
- Reconstitution: 100 U vial with 2 mL preservative-free normal saline yields 5 U per 0.1 mL
When to Assess and When to Stop
- Minimum trial: 2 treatment cycles (6 months). Many patients respond only after the second or third cycle.
- Optimal assessment: After 3 cycles (9 months). If no meaningful improvement, consider discontinuation.
- Responders: Continue every 12 weeks. AHS suggests reassessing after 12-18 months of good response to determine if continued treatment is needed.
- Breakthrough at end of cycle: Some patients notice worsening in weeks 10-12 (“wearing off”). Shortening the interval to 10 weeks is off-label but commonly done in practice.
Combining With CGRP Therapies
OnabotulinumtoxinA and CGRP mAbs work through complementary mechanisms. Botox blocks CGRP release from the nerve terminal; mAbs block CGRP in the extracellular space. Growing evidence supports combination therapy for refractory CM:
- Blumenfeld et al. (2021): Adding erenumab to Botox reduced monthly migraine days by an additional 4.4 days
- Ailani et al. (2022): 54% of Botox partial responders achieved ≥50% response after adding fremanezumab
- No dose adjustment needed for either agent when combined
- Gepants can also be combined with Botox (different mechanism); do not combine a gepant + mAb
Side Effects
| Side Effect | Frequency | Management |
|---|---|---|
| Injection site pain | Common | Resolves within hours; ice pre/post injection |
| Neck pain / stiffness | ~6% | Usually mild, resolves in 1-2 weeks; may relate to cervical paraspinal injection |
| Neck weakness | Uncommon | Difficulty holding head up; reduce cervical paraspinal dose at next cycle |
| Eyelid ptosis | ~2-4% | From toxin diffusion to levator palpebrae; resolves in 2-4 weeks. Minimize by staying ≥2 cm above brow. |
| Brow ptosis / “heavy” feeling | Uncommon | Frontalis weakness; reduce dose or adjust site positions |
| Headache worsening (first 1-2 days) | ~5% | Transient; treat with usual acute medication |
Contraindications
- Infection at injection site
- Known hypersensitivity to botulinum toxin or excipients
- Neuromuscular disorders (myasthenia gravis, Lambert-Eaton) — risk of systemic weakness
- Pregnancy: FDA Category C; generally avoided, though limited case data has not shown teratogenicity
- Concurrent use of aminoglycosides or other agents that impair neuromuscular transmission — use caution
Why It Doesn’t Work in Episodic Migraine
- Earlier randomized trials of onabotulinumtoxinA in patients with <15 headache days/month (episodic migraine) showed no benefit over placebo
- Hypothesized reason: In chronic migraine, trigeminal afferents are in a state of persistent peripheral sensitization with ongoing CGRP and substance P release. Botox addresses this tonic release. In episodic migraine, sensitization is intermittent, so blocking release between attacks has little effect.
- Do not use onabotulinumtoxinA for episodic migraine, even if close to the 15-day threshold (12-14 days/month) — insurance will deny and evidence does not support it
References
- Aurora SK, et al. OnabotulinumtoxinA for treatment of chronic migraine: results from the double-blind, randomized, placebo-controlled phase of the PREEMPT 1 trial. Cephalalgia. 2010;30(7):793-803.
- Diener HC, 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.
- Dodick DW, et al. OnabotulinumtoxinA for treatment of chronic migraine: pooled results from the double-blind, randomized, placebo-controlled phases of the PREEMPT clinical program. Headache. 2010;50(6):921-936.
- Blumenfeld AM, et al. Real-world evidence for the addition of CGRP monoclonal antibodies to onabotulinumtoxinA treatment for migraine. Headache. 2021;61(8):1246-1256.
- Simpson DM, et al. Practice guideline update: Botulinum neurotoxin for the treatment of blepharospasm, cervical dystonia, adult spasticity, and headache. Report of the Guideline Development Subcommittee of the AAN. Neurology. 2016;86(19):1818-1826.