Muscle afferent block (MAB) therapy

MAB therapy, which involves the local injection of diluted lidocaine and ethanol, aims to reduce the effectiveness of muscle spindle afferents without causing unfavorable weakness. Its effect has been shown to be mediated by the blockade of either muscle afferents or gamma motor efferents. In a previous study, the T-reflex of the hand muscles was attenuated whilst their power was preserved after the intramuscular injection of lidocaine, and the muscle spindle afferents or gamma motor efferents that tonically control the sensitivity of the spindles were postulated to be blocked by MAB. In another study, the mean post-MAB response of the jaw elevator muscles (70%), which was evaluated on a self-rating scale, was significantly higher than that of the depressor muscles (38%) (17,18), and it was suggested that by the different numbers of muscle spindles supplying these muscles were responsible for these results. Therefore, MAB therapy is indicated for jaw elevator muscles (the masseter, temporalis and medial pterygoid muscles), but not for jaw depressor muscles (the lateral pterygoid and digastric muscles), which contain fewer muscle spindles.

·EMG recordings

We use surface electrodes to take EMG recordings from superficially located muscles such as the masseter and temporal muscles, whereas we use needle electrodes for deeper muscles like the lateral pterygoid and medial pterygoid muscles or genioglossus muscles. We choose the target muscles for injection based on the patient's symptoms and the results of EMG recordings from the masseter, temporalis, lateral pterygoid (the inferior head), medial pterygoid, genioglossus, trapezius, and sternocleidomastoid muscles, etc.

·Injection

The injection volume (2 to 10 ml) of 0.5% lidocaine delivered to the target muscle is determined according to the size of the target muscle and its contraction force: 3-5 ml for the lateral pterygoid, medial pterygoid, and digastric muscles; 5-10 ml for the masseter, temporal, genioglossus muscles; and 10 ml for the trapezius and sternocleidomastoid muscles.

During the contraction of the target muscle, we slowly inject 80% of the total lidocaine dosage into one site within the bulkiest portion of the muscle using a hollow EMG needle and a standard EMG instrument for guidance. During the injection, we specifically check the subject for any pain, numbness, or weakness. After confirming the absence of these symptoms, we then slowly inject ethanol. The remaining 20% of the lidocaine is added to the ethanol at the same needle site by reversing the connector. We only inject lidocaine in the first treatment session. In the subsequent sessions, a one-tenth volume of 99.5% ethanol is added to the lidocaine at the same needle site using a triple connector.

·Follow-up

The treatment begins to have a noticeable effect soon after the injection, but the effect only lasts for a short period. After repeated injections, the effects of the injections gradually last longer. Eventually, the effects of such injections can last for 6 months or more.