Transcutaneous Vagus Nerve Stimulation of the vagus nerve (see Figure 1B).9 During the sectioning of the


vagus on the left side, the anaesthetist noted that the heart rate dropped to 40 beats per minute. Following section of the vagus root, the cutaneous area of complete anaesthesia covered the posterior wall of the external auditory canal, the concha and, with only a slight degree of pain, the antihelix and antitragus. The authors concluded that there is no doubt that the major supply to the anaesthetised area is by means of the vagus nerve.9


Isolated Vagus Nerve Palsy with Herpes Zoster A 31-year-old woman was admitted to hospital due to difficulty swallowing fluid, hoarseness and painful vesicles on the right ear. Neurological examination revealed poor elevation of the soft palate on the right side. Herpetic vesicles were present on the right concha and the posterior wall of the external auditory canal. No facial palsy, loss of hearing or mucosal lesions in the mouth or pharynx were present. The authors diagnosed an isolated vagus nerve palsy due to varicella zoster infection (see Figure 1C), highlighting the distribution of the cutaneous receptive field of the ABVN.10


Auricular Syncope


A 13-year-old girl had been receiving drug treatment for presumed absence epilepsy without any anticonvulsive effect. The medical history indicated that recurrent syncopal attacks were precipitated by external auditory canal stimulation. Targeted autonomic function tests confirmed a hyperactive vagal response with bradycardia and light-headedness provoked by tactile stimulation of the left external auditory canal. Abstinence from ear-scratching led to complete alleviation of symptoms without any drug treatment. The authors proposed reflex syncope (see Figure 1D), due to stimulation of the ABVN, as the pathophysiological mechanism.11


Referred Otalgia


Referred otalgia arises from non-otological, remote diseases and occurs in up to 50 % of adult patients who consult a general physician for ear pain.12


important pathology associated with referred otalgia. Twenty-six patients with non-metastatic lung cancer primarily suffered from auricular pain localised ipsilaterally to the lung mass.13


Head and neck malignancy is the most Lung masses


which abut or infiltrate visceral vagus nerve afferents can refer pain to the ear by convergence of visceral fibres from the lung and somatic afferents of the ABVN onto common secondary sensory neurons in the NTS (see Figure 1E).


Ear-cough Reflex


A young boy complained about a chronic dry cough. On examination, an accumulation of epidermal cerumen surrounding a skin ulceration in a narrowed external auditory canal was found. Stimulation of the wall of the ear canal with a cotton bud triggered a marked cough reflex (see Figure 1F). After removal of the accumulated cerumen the cough disappeared.14


The ear-cough


reflex was elicited in 12 patients. It was bilaterally induced in three patients. Lacrimation was additionally observed in one patient (auriculo-lacrimal reflex).15,16,17


Twenty-one out of 500 patients


studied had a clinically positive ear-cough reflex. Gagging and lacrimation were seen in nine and 10 patients, respectively. While vomiting was present in one case (ear-vomiting reflex), severe cardiac inhibition with syncopal attack was seen in three patients (auriculo-cardiac reflex).15


Similar reflex phenomena documenting EUROPEAN NEUROLOGICAL REVIEW


the functional connection between the ABVN and the autonomic nervous system are the gastro-auricular phenomenon, the auriculo-genital reflex and the auriculo-uterine reflex.18


Preferential Excitation of Thick Myelinated Nerve Fibres by Transcutaneous Vagus Nerve Stimulation


The clinical efficacy of VNS requires activation of thick myelinated afferent fibres of the vagus nerve.4,7


The fibres of a sensory


peripheral nerve such as the ABVN mediate touch sensation. Consequently, the stimulus intensity of electrical t-VNS is adjusted to a level above the individual’s detection threshold and clearly below the individual’s pain threshold. The detection threshold is defined as the lowest stimulus intensity that evokes the first perceptible sensation that reliably corresponds to a tingling sensation. The pain threshold is defined as the lowest stimulation intensity that elicits the first pricking or unpleasant sensation. Both psychophysical thresholds are determined by the method of limits, with several runs of electrical stimuli applying ramps of decreasing and increasing intensity. In 18 healthy volunteers (36 ears) the electrical detection threshold with a single-pulse stimulation (200 µs duration) averages out at 0.8 ± 0.3 mA in the cymba conchae.19


as measured in the face or the forearm.20–22


This intensity conforms to published thresholds Touch sensation


is clinically assessed by the mechanical detection threshold via application of von Frey filaments. The mechanical detection


255


Figure 1: Brainstem Mechanisms of Transcutaneous Vagus Nerve Stimulation


NTS E


XXSuperior ganglion DN


NA B C VZV


Lung F Cough Heart D Syncope A Cymba conchae ABVN


Sensory fibres of the auricular branch of the vagus nerve (ABVN) (red) supply the skin of the concha (yellow). The cymba conchae is exclusively supplied by the ABVN. Sensory vagus nerve fibres from different organs project via the superior ganglion to the nucleus of the solitary tract (NTS). NTS neurons (dark blue) project to visceral efferent neurons located in the dorsal nucleus of the vagus nerve (DN) and the nucleus ambiguus (NA). Visceral efferent nerve fibres (green) supply, e.g., the heart and the lung. For the sake of clarity, afferent pathways and efferent pathways of the vagus nerve are separately illustrated on the right and left sides of the figure, respectively. A to F refer to the text. VZV = varicella zoster virus causing herpes zoster; X = vagus nerve.


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