For the differential diagnoses of recurrent laryngeal nerve (RLN) paralysis and other causes of vocal fold immobility, e. g., cricoarytenoid joint ankylosis or luxation, a combination of laryngeal electromyography and temporary electrical stimulation of the PCA has been found useful. For this purpose, the PCA -the only abductor muscle- is electrically stimulated via the same electrode (EMG needles of platinum or hooked-wire certified for electrostimulation) used for the EMG examination of this muscle. This can be done with every EMG machine equipped with an internal stimulator usually available for evoked potential examinations.
Stimulation parameters should be set at
- 0.3 to 0.5 ms pulse duration and
- 2 to 3 mA amplitude, starting with lower values (stimulation effect depending on innervation status and electrode position).
- 3 Hz is the optimal frequency parameter for position testing as it causes muscle twitching, and
- 30 Hz is the optimal frequency parameter for testing maximal vocal cord abduction.
A simultaneous endoscopy must be performed in order to detect the effect of the electrical stimulus on vocal cord movement. As known from previous studies, the PCA is best stimulated where the branches of the RLN reach motor end plates. These precise ‘‘hot spots’’ are the optimal contact points to stimulate with biphasic electrical pulses, in order to provoke a subsequent abduction of the vocal folds. They vary in position between individual patients and stimulation sides.
The positioning of the needle for stimulation is similar to the procedure of transcutaneous EMG of the PCA muscle. However, repeated repositioning of the needle may be required to locate the ‘‘hot spot’’.
With the described parameters of stimulation, it is not possible to stimulate the muscle itself but only the small demyelinated branches of the nerve only micrometers before they reach the motor end plates. Thus, only innervated muscles can be detected. Atrophic or denervated muscle fibers would not contract under this electro stimulation.