# Ultrasound-Guided Botulinum Toxin Injection for Spasticity

## Introduction

**Botulinum toxin (BoNT)** injection is a first-line focal treatment for spasticity in conditions such as stroke, traumatic brain injury, cerebral palsy, multiple sclerosis, and spinal cord injury. **Ultrasound guidance** improves injection accuracy by enabling direct visualization of target muscles, confirming needle placement within the muscle belly, and avoiding adjacent neurovascular structures. This approach has been shown to improve clinical outcomes compared to landmark-based or EMG-guided techniques for many muscle targets.

## Mechanism of Action

### Botulinum Toxin Pharmacology

BoNT cleaves **SNARE proteins** at the neuromuscular junction, preventing acetylcholine release. **OnabotulinumtoxinA (Botox)** and **abobotulinumtoxinA (Dysport)**: Serotype A; most commonly used. **IncobotulinumtoxinA (Xeomin)**: Free of complexing proteins. **RimabotulinumtoxinB (Myobloc)**: Serotype B; alternative when resistance to type A develops.

Onset of effect: **3-7 days**; peak effect at **2-6 weeks**. Duration: **3-4 months** (clinical re-injection typically every 12-16 weeks). Dose-response relationship; excessive doses risk systemic weakness.

### Spasticity Pathophysiology

**Velocity-dependent** increase in tonic stretch reflexes due to upper motor neuron lesion. Involves hyperexcitability of the stretch reflex arc. Contributes to contracture, pain, impaired function, and hygiene difficulties. BoNT addresses the neural component; does not reverse fixed contracture.

## Advantages of Ultrasound Guidance

Direct visualization of target muscle and surrounding anatomy. Confirmation of needle tip within the muscle belly (not fascia, tendon, or fat). Identification of intramuscular motor points (where nerve enters muscle). Avoidance of neurovascular bundles.

Real-time monitoring of injectate spread. No electrical stimulation required (though can be combined). Improved accuracy for **deep muscles** (e.g., tibialis posterior, flexor digitorum profundus).

![Ultrasound-guided botulinum toxin injection into the flexor digitorum superficialis with visible needle tip](images/us-guided-botox-forearm.png)

## Patient Assessment

### Clinical Evaluation

**Modified Ashworth Scale (MAS)**: Grades spasticity from 0 (no increase in tone) to 4 (rigid). **Tardieu Scale**: Measures velocity-dependent catch angle; more specific for spasticity. **Goal setting**: Functional goals (e.g., improved hand hygiene, gait, dressing) guide muscle selection. Distinguish spasticity from contracture, dystonia, and rigidity. Assess for **spastic pattern** analysis: flexor versus extensor predominance, posturing patterns.

### Treatment Goals

Reduce muscle overactivity to improve active function. Improve passive range of motion for hygiene and care. Reduce pain associated with spastic posturing. Facilitate orthotic fit and use. Prevent progressive contracture development. Complement ongoing rehabilitation therapy.

## Upper Limb Injection Targets

### Common Spastic Patterns

**Shoulder adduction/internal rotation**: Pectoralis major, subscapularis, teres major, latissimus dorsi. **Elbow flexion**: Biceps brachii, brachialis, brachioradialis. **Forearm pronation**: Pronator teres, pronator quadratus. **Wrist flexion**: Flexor carpi radialis, flexor carpi ulnaris.

**Finger flexion**: Flexor digitorum superficialis, flexor digitorum profundus. **Thumb-in-palm**: Flexor pollicis longus, adductor pollicis, first dorsal interosseous.

### Dosing Guidelines (OnabotulinumtoxinA)

| Muscle Target | OnabotulinumtoxinA Dose | Injection Sites |
|--------------|------------------------|-----------------|
| Biceps brachii | 100-200 units | 2-4 sites |
| Brachialis | 50-100 units | 1-2 sites |
| Flexor carpi radialis | 50-100 units | 1-2 sites |
| Flexor carpi ulnaris | 50-100 units | 1-2 sites |
| Flexor digitorum superficialis | 50-100 units | 2-4 sites (divided) |
| Flexor digitorum profundus | 50-100 units | 2-4 sites (divided) |
| Gastrocnemius (medial + lateral) | 150-300 units | 4-6 sites |
| Soleus | 100-200 units | 2-4 sites |
| Tibialis posterior | 100-150 units | 1-2 sites |
| Adductor longus | 100-150 units | 2-3 sites |
| Hamstrings (per muscle) | 100-200 units | 2-4 sites |

Total upper limb dose per session: typically **200-400 units**.

## Lower Limb Injection Targets

### Common Spastic Patterns

**Equinovarus foot**: Gastrocnemius, soleus, tibialis posterior, flexor digitorum longus. **Stiff knee/knee extension**: Rectus femoris, vastus intermedius. **Hip adduction (scissoring)**: Adductor longus, adductor magnus, gracilis. **Knee flexion**: Hamstrings (semitendinosus, semimembranosus, biceps femoris). **Striatal toe**: Extensor hallucis longus.

### Dosing Guidelines (OnabotulinumtoxinA)

Gastrocnemius: **150-300 units** (medial and lateral heads). Soleus: **100-200 units**. Tibialis posterior: **100-150 units** (deep; US guidance essential). Adductor longus: **100-150 units**.

Hamstrings: **100-200 units** per muscle. Total lower limb dose per session: typically **300-600 units**. **Maximum total body dose**: Generally 400-600 units per session (some guidelines permit up to 800 units).

![Ultrasound anatomy of the posterior lower leg showing gastrocnemius, soleus, and tibialis posterior for injection targeting](images/us-anatomy-posterior-leg-spasticity.png)

## Procedure Technique

### Setup

Patient positioned to expose target muscles. High-frequency linear probe (10-15 MHz) for most targets. Curvilinear probe for deep or large muscles (e.g., iliopsoas). 25-27 gauge needle for superficial muscles; 22 gauge for deep muscles.

### Injection Approach

Identify target muscle in short axis; confirm with long-axis scanning. Insert needle in-plane or out-of-plane depending on target depth and access. Advance to the **mid-muscle belly** (away from myotendinous junction). Aspirate to confirm no vascular entry.

Inject in divided doses across multiple sites within the muscle. Observe injectate spread within the muscle on ultrasound.

## Post-Injection Management

Begin or continue **stretching and therapy** within 24-48 hours. Serial casting or orthotic fabrication 1-2 weeks post-injection (when BoNT takes effect). Functional training targeting treatment goals. Reassess at **4-6 weeks** for peak effect evaluation. Plan re-injection at **12-16 weeks** as needed. Adjust doses based on clinical response and goal attainment.

## Complications

**Excessive weakness** in target or adjacent muscles (dose-related). Injection site pain, bruising, or hematoma. **Systemic spread**: Rare; generalized weakness, dysphagia, respiratory compromise at very high doses. Antibody formation with repeated high-dose treatments (more common with type A complexed formulations). Flu-like symptoms (transient).

## Key Clinical Pearls

1. Ultrasound guidance is particularly advantageous for deep muscles such as tibialis posterior, flexor digitorum profundus, and subscapularis, where landmark-based techniques have poor accuracy. 2. Treatment goals should be established before injection, distinguishing between functional improvement (active use) and passive goals (hygiene, positioning, pain relief), as muscle selection and dosing differ accordingly. 3. Botulinum toxin addresses the neural component of spasticity but cannot reverse fixed contracture; clinical examination should differentiate spasticity from contracture before injection. 4. Post-injection rehabilitation including stretching, serial casting, and functional training is essential to maximize the therapeutic window provided by botulinum toxin.

## References

1. Simpson DM, et al. "Practice Guideline Update: Botulinum Neurotoxin for the Treatment of Blepharospasm, Cervical Dystonia, Adult Spasticity, and Headache: AAN Guideline." *Neurology*. 2016;86(19):1818-1826.
2. Henzel MK, et al. "Comparison of Surface and Ultrasound Localization to Identify Forearm Flexor Muscles for Botulinum Toxin Injections." *PM R*. 2010;2(7):642-646.
3. Picelli A, et al. "Accuracy of Botulinum Toxin Type A Injection into the Gastrocnemius Muscle of Adults with Spastic Equinus: Manual Needle Placement and Electrical Stimulation Guidance Compared Using Ultrasonography." *J Rehabil Med*. 2012;44(5):450-452.
4. Wissel J, et al. "European Consensus Table on the Use of Botulinum Toxin Type A in Adult Spasticity." *J Rehabil Med*. 2009;41(1):13-25.