In adults using topical anesthesia, both botulinum toxin type A (BTXA) and bupivacaine (BUP) are readily injected using electromyography (EMG). Medial rectus injection can be done without EMG in adults. In children under anesthesia medial rectus injection can be done reliably without EMG.
31 Injection Technique
31.1 Injection Technique with EMG
31.1.1 Botulinum Toxin Type A
A drop of vasoconstrictor followed at 1-minute intervals by four drops of proparacaine provides adequate anesthesia for most cases. If scar tissue is too painful to push through, then 100 to 200 µL of lidocaine injected at the restriction site will allow electrode penetration.
The skin in an area close to the eye being injected is wiped with alcohol so that the ground (we use a disposable electrocardiogram [ECG] electrode) sticks well and makes good electrical contact. The 1.0-mL syringe is overfilled with botulinum toxin type A (BTXA), the electrode needle is attached, and the exact volume to be injected is left in the syringe. This allows the syringe–needle connection to be tested for leakage. If doing multiple muscles, a separate syringe is used for each, as it is often impossible to see the syringe markings to accurately inject multiple volumes. The electrode is attached to the amplifier, which is turned on rather high, so that touching the needle shaft should make a noise. Hum and static pickup will quiet when the needle tip touches the eye with the ground attached to the patient. Avoid fluorescent lights and other electrical sources or turn them off. Occasionally you need to move to another site.
With the patient looking away from the field of action, insert the electrode needle about 12 mm posterior to the limbus. If there was earlier recession surgery, insert the needle further posterior. We advance the needle on the orbital side of the extraocular muscle (EOM) until past the equator, then turn back to the muscle while advancing the needle to get optimum electromyographic (EMG) amplitude (Video 31.1, Video 31.2, Video 31.3).
What if you don’t hear anything? Check connections, check the ground, and turn up amplitude of gain on the amplifier. Carefully move the needle right and left, and advance it further until the muscle is found. If injecting Botox (Allergan), start over—you don’t want to inject in an unknown place, causing a big vertical deviation or other mishap. The inferior rectus is sometimes hard to access in thyroid-associated eye disease (TED) with large hypotropia. Then move to injecting through the lower lid, remembering to angle medially along the direction of the inferior rectus.Intravenous (IV) ketamine anesthesia in a dose of about 1.0 mg/kg allows preservation of the EMG signal and is useful in children. Anesthesiologists often administer ketamine in larger intramuscular (IM) doses. This works, but it prolongs recovery and induces hallucinations. For children under general anesthesia without EMG guidance, the EOM is grasped with large forceps and injected by insulin syringe. This is easily performed on the medial rectus without incision or surgery. Previously operated EOMs and vertical rectus muscles are more difficult to inject accurately if EMG is not used (Fig. 31‑1, Fig. 31‑2).
Injection under direct view is precise but invasive. It is the obvious choice if BTXA injection is combined with strabismus surgery (Video 31.4).