Skeletal muscle channelopathies are divided into periodic paralyses and nondystrophic myotonias [1]. Nondystrophic myotonia is characterized by muscle stiffness on voluntary movement owing to delayed skeletal muscle relaxation. Nondystrophic myotonias include myotonia congenita, paramyotonia congenita, and SCM. Because SCM is very rare, there have been no reports describing perioperative anesthetic management of patients with SCM to date. For guidance, we referred to reports on anesthetic management of patients with myotonic dystrophy and other types of skeletal muscle channelopathies.
First, the anesthetic to be used was considered. Previous reports on the anesthetic management of patients with myotonic dystrophy and myotonia congenita [4,5,6,7] suggest that propofol can be used safely for induction and maintenance of general anesthesia in patients with SCM. Volatile agents, including sevoflurane and desflurane, may also be safe for use in patients with SCM because they are not contraindicated in patients with myotonic dystrophy and other myotonic diseases [3, 7, 8]. However, depolarizing muscle relaxants must be avoided because they may cause exaggerated contracture, masseter spasm, and laryngospasm, thus complicating extubation [3, 7]. The use of non-depolarizing muscle relaxants may be acceptable with monitoring of neuromuscular blockade [4], although the use of cholinesterase inhibitors might worsen the symptoms of SCM as it does in other myopathies. In the present case, we avoided muscle relaxants because we anticipated that the patient’s rhinoplasty and potential exacerbation of myotonia would independently increase the difficulty of her airway postoperatively.
An increase of serum potassium was noted during anesthesia, which might result from the potassium-containing solution and its redistribution [9]. It was rapidly decreased after switching it to a potassium-free solution. The mutation at V445M causes some alternations in the gating mechanism of NaV1.4 as impairment of fast inactivation and enhanced slow inactivation result in increased excitability of the muscles [10]. Additionally, pain or hypothermia can cause muscle contraction with excessive sodium influx in patients with SCM, which may result in an elevation of the extracellular potassium level. Thus, repeated intraoperative arterial blood gas analysis is recommended.
We made a concerted effort to prevent our patient from shivering, because shivering can exacerbate symptoms of myotonia [4]. This required particular focus on body temperature control and pain management. We rewarmed the patient and maintained her body temperature using a forced-air warming device throughout the surgery. To control pain, we administered intravenous fentanyl with a continuous intercostal nerve block, bearing in mind that excessive fentanyl during anesthesia should be avoided for patients with difficult airways. Combined use of regional anesthesia should be considered, if possible, in order to reduce opioid dose.
Oral administration of mexiletine might be used if the patient had an acute myotonic episode, because it is generally regarded as the only effective treatment for myotonic episodes [11]. It is a non-selective sodium channel inhibitor and belongs to the Vaughan-Williams class IB anti-arrhythmic group of drugs [12]. Although IB and IC anti-arrhythmic group of drugs have been studied, no other drug has been shown effective for treatment of myotonia. Additionally, the effectiveness of intravenous mexiletine is unknown. Usually myotonia lasts for seconds to minutes and improves with time; however, sometimes it lasts longer and involves respiratory and/or swallowing muscles. Therefore, we need to observe the patient carefully for signs of deterioration postoperatively [13]. Intensive treatment, including constant monitoring, is definitely required for patients with SCM after general anesthesia. The patient should be followed up for several hours after extubation in a postanesthesia care unit [14]. In addition, we need to provide preoperative education to the patient to enable self-recognition of worsening myotonia and notify the medical staff for help.
The present patient had a history of re-intubation following postoperative respiratory failure. For this reason, we decided to exchange the device used for airway maintenance as described above. Supraglottic airway devices allow cautious observation of respiratory conditions without tracheal stimulation [15], which can trigger myotonia in patients with SCM. This procedure should be considered for patients requiring tracheal intubation.
In conclusion, we safely administered total intravenous anesthesia to a patient with SCM, although the current literature does not present contraindications to the use of inhaled anesthetics in these patients. Given the clinical course of the present patient, close monitoring of electrolytes, airway patency, and symptoms is recommended.
