Transcranial magnetic stimulation – a potential unobtrusive, non‐invasive and drug‐free treatment for obstructive sleep apnoea

Transcranial magnetic stimulation (TMS) is a technique for stimulating brain neurons by generating a strong magnetic field, which induces an electric current and depolarization of the neurons in the region targeted. The method was approved by the US Food and Drug Administration in 2008 for the treatment of depression, but it has many other uses in the areas of diagnosis, therapy and brain research. The magnetic field (similar in strength to a magnetic resonance imaging scan) is generated by electromagnetic coils held on or near the head, so the method is noninvasive, painless and generally regarded as safe. In the study featured in this issue of Experimental Physiology (Melo-Silva et al. 2013), TMS was used to stimulate the somatotopic representation of the tongue in the motor cortex of sleeping patients with obstructive sleep apnoea (OSA) in order to stimulate contraction of upper airway muscles. Obstructive sleep apnoea is a very common condition, in which the contractions of the upper airway muscles, especially the tongue musculature, fail to stabilize or dilate the airway adequately during inspiration in sleep. As a result, the negative pressure generated in the airway during inspiration causes the airway to collapse and so the patient cannot breathe (apnoea). The apnoeas and their associated effects on the cardiovascular and respiratory systems and the derangement of blood gases lead to long-term adverse, sometimes severe, cardiovascular and neurological consequences. Aside from continuous positive airway pressure, which is not well tolerated by many patients (Weaver & Grunstein, 2008), the treatment options for OSA are limited (Veasey et al. 2006). One obvious strategy would be interventions that improve upper airway muscle activation. The ‘route one’ approach of stimulating the hypoglossal nerve, the motor nerve to the tongue, has been used with some success, but this method requires surgery (Eastwood et al. 2011). The investigation by Melo-Silva et al. (2013) shows that TMS can be used to cause activation of upper airway muscles and improve airflow and inspired volume during episodes of reduced airflow, and this is achieved without waking the patient. There was no effect on activation of chest wall/diaphragm muscles. Presumably, these improvements were brought about by stabilization and dilatation of the airway. The potential for this technique is obvious, but there is still some way to go before it might be possible to use an automated TMS system to prevent airway obstruction throughout sleep in OSA patients. More work is needed in varying stimulus localization, intensity, frequency and timing in relation to the respiratory cycle in order to optimize airflow and tidal volume and to prevent airway collapse. Technological advances may play a significant role, and one future development might be the ability to activate deeper regions using, for example, an H-coil (Zangen et al. 2005) instead of the conventional figure-of-eight coil as used by Melo-Silva et al. (2013). In this way, it might be possible to have greater control of upper airway muscle contraction and airway opening, because the motor and premotor neurons within and near the hypoglossal nucleus could be activated directly instead of indirectly through corticobulbar pathways as in the featured paper. This might lower the threshold intensity for activation and raise the threshold for cortical arousal. Furthermore, it might be possible, with modifications of electromagnetic coil design, to target other regions simultaneously in order to recruit other airway dilators with synergistic effects. Currently, continuous positive airway pressure is the most effective treatment for OSA but, because it requires the wearing of bulky headgear and the application of air under pressure to the airway, there are major difficulties with patient compliance (Weaver & Grunstein, 2008). Other interventions, such as surgery and use of drugs, have been largely either inconsistent or unsuccessful (Veasey et al. 2006). The possibility of an unobtrusive, non-invasive, drug-free method for the treatment of OSA using TMS is exciting and well worth pursuing.