Conventional needle EMG contributes to the determination of the characteristics and severity of a lesion to the cranial nerve VII, and gives clues regarding pathogenesis and outcome. Needle EMG examination is performed at rest, and with voluntary contraction or after stimulation. EMG is recorded by a concentric needle electrode in at least one muscle innervated by the temporofacial branch (orbicularis oculi or frontalis), and in another innervated by the cervicofacial branch (orbicularis oris or depressor anguli oris). When tolerated, fasting an infant for up to 4 h before the test may facilitate the recording of bursts of activity arising from the facial muscles while the baby is crying or sucking a pacifier. In older children, voluntary contraction can be recorded after asking the child to imitate the examiner by closing the eyes, smiling, and whistling.

The maximum amplitude of a motor unit potential (MUP) is measured from the highest negative peak to the lowest positive peak of the largest EMG burst. Traces are classified as: (1) normal interference pattern; (2) neurogenic (i.e., single or reduced, high-amplitude MUPs units) or; (3) myogenic (i.e., low-amplitude where the maximum amplitude decreased by at least 30%) (Fig. 15.1). The normal maximum MUP amplitude of the interference pattern is 850 μV (range: 400 μV–1.2 mV) with normal MUP duration from 1 to 2.8 ms. The proportion of polyphasic potentials present in a normal muscle may exceed 30% (Table 15.3) [6].

This technique explores both facial and trigeminal nerve functions, as well as the pathways between their respective cranial nuclei within the brainstem. The blink reflex results from the contraction of the orbicularis oculi muscle provoked by stimulation of the supraorbital branch of the trigeminal nerve [7]. A single electric shock is applied to the supraorbital foramen to elicit BRs recorded from the orbicularis oculi muscle on both sides. Stimulation of the ipsilateral supraorbital trigeminal nerve provokes a direct response with two components: R1, which is immediate and brief, and R2, which is delayed and longer lasting. Stimulation of the contralateral nerve V provokes a crossed R2 response.

The maturation from birth of the electrically elicited BRs has been studied [8]. The R1 response is always present and its latency achieves normal adult values (range: 9–13 ms) at 44 weeks postmenstrual age. The ipsilateral R2 response can be evoked in most newborns at term birth (latency range: 34–43 ms), but the contralateral R2 response is not always obtained before the age of 8 months. The R1 component corresponds to an oligosynaptic reflex arc involving at least two and no more than three synapses in the pons between the main sensory nucleus of cranial nerve V and the motor nucleus of the ipsilateral cranial nerve VII. The R2 component follows polysynaptic medullary pathways, which are more caudal and closer to the bulbar formations. The spinal trigeminal nucleus has projections to the adjacent paramedian reticular formation and the motor nuclei of both seventh nerves. Injury in the trigeminal pathway leads to delayed or absent ipsilateral R1 and R2 responses, and contralateral R2 response, while a lesion in the facial pathway shows delayed ipsilateral R1 and R2, but normal contralateral R2. Lesions at the pons show ipsilateral abnormal R1 response, while contralateral stimulus has normal R1. Lateral medullary lesions show abnormal R2 response on the affected side, stimulating either the non-affected or the affected side [9].

Methods for studying trigeminal afferents have several limitations. The specific clinical assessment of the ophthalmic, maxillary, and mandibular nerves is not reliable in the young child. Supraorbital sensory nerve action potential has been reported only in normal adults so far [10]. Somatosensory evoked potentials are misleading because electrical stimuli to the orofacial area generate muscle potentials and trigeminal reflexes that contaminate the scalp signal [11]. Trigeminal motor function is clinically assessed by palpating the masseter-temporalis muscles at rest and on voluntary contraction after asking the child to bite down. Transcranial motor evoked potentials are not routinely available in an ambulatory setting, and normal values during growth are not available. Surface electrodes placed over masseter and temporalis muscles enable the analysis of the timing and the envelope of EMG bursts, taking into account that bursts are contaminated with signals generated by facial muscles. Conventional needle EMG of the masseter and temporalis muscles remains the way to study trigeminal motor function and to detect neurogenic abnormalities.

With the aim to explore bulbar dysfunction and evaluate congenital dysphagia, EMG techniques provide information on brainstem structures involved in oral sensorimotor functions. This includes EMG of muscles of the tongue and soft palate and EMG during bottle-feeding, which investigates the electrophysiology of sucking and swallowing.