Myasthenia gravis (MG) is a rare, autoimmune antibody-mediated T cell-dependent neuromuscular junction disorder that often presents with fluctuating and fatigable painless weakness involving specific muscle groups. It has a prevalence rate of approximately 1/5000. The onset of MG is influenced by gender and age in a bimodal fashion. In patients younger than 40, women predominate and in the fifth decade new cases of MG are evenly distributed between men and women. After age 50, new cases of MG are slightly more common in men. The course is variable, and most patients with initial ocular weakness develop bulbar or limb weakness within three years of initial symptom onset. Ocular weakness involving asymmetric ptosis and binocular diplopia is the most typical initial presentation [1
]. Early or isolated oropharyngeal weakness is less common, however approximately 20% of patients with MG will present with dysarthria and dysphagia [2
]. Recurrent dysarthria and dysphagia not uncommonly signal exacerbations in patients with chronic MG. Therapeutic modalities in relapsing disease include steroids and other immunosuppressants, along with primary immunomodulatory agents (e.g., Intravenous Immunoglobulin [IvIg]), plasmapharesis, and cholinergic agents.
In a chronic MG patient with worsening symptoms, the doses of cholinergic agents are often increased. Cholinergic crisis may develop with excessive dosing of acetylcholinesterase inhibitors in patients with more severe MG. In cholinergic crises increased muscarinic activity generates copious oropharyngeal and bronchial secretions that may obstruct the airway or be aspirated and may simulate dysphagia. Signs of cholinergic crisis include weakness indistinguishable from myasthenic weakness, muscle fasciculations, and symptoms of increased muscarinic activity including bradycardia [1
]. In the absence of cholinergic crises, it is often assumed that worsening dysphagia and dysarthria is caused by MG itself and secondary non-neurologic causes are forgotten or overlooked.
The anatomical regulatory mechanisms of dysphagia in neurologic disease are complex (Please see Schaller and colleagues in Reference 3 for an excellent review). Swallowing and gastrointestinal motility depend on coordinated interactions between central and peripheral structures regulating both voluntary and involuntary neuromuscular activity. With regards to central causes (e.g., stroke) exact cortical and brainstem localization is debatable given a dearth of any detailed studies, however a variety of new imaging studies are promising [3
Cricopharyngeal sphincter achalasia is a common disorder producing dysphagia in the elderly with a variable clinical presentation. Although the exact incidence of cricopharyngeal dysfunction is unknown, the literature reports cricopharyngeal achalasia as the primary cause of or as a contributor to dysphagia in 5-25% of patients being evaluated for clinical symptoms of dysphagia [4
]. Cricopharyngeal achalasia may be primary or secondary. Primary cricopharyngeal achalasia implies that the abnormality that leads to the persistent spasm or failure of relaxation of the cricopharyngeus muscle is confined to the muscle, with no underlying neurologic or systemic cause. This primary group can be further subdivided into primary cricopharyngeal achalasia with no underlying cause (i.e., idiopathic) or cricopharyngeal achalasia caused by intrinsic disorders of the cricopharyngeus muscle (e.g., polymyositis, muscular dystrophy, hypothyroidism, or inclusion body myositis [4
Unfortunately medical therapy for cricopharyngeal dysfunction has been largely refractory to medical management, including therapy with muscle relaxants. Botulinum toxin injection into the cricopharyngeus muscle has recently been explored as a possible therapeutic intervention, but experience is limited [4
]. Given this patients' history of neuromuscular disease, these were not considered viable options.
Several surgical approaches may be considered for treatment of cricopharyngeal dysfunction. Although trans-oral and endoscopic approaches have also been advocated with limited experience, the classic approach is the external cricopharyngeal myotomy [4
The presented patient elected to undergo the less invasive esophageal dilation procedure as previously described. Given the overall rarity of MG, it is unknown whether cricopharyngeal myotomy is superior to esophageal dilation in these patients.
In retrospect, this patient's course including dysphagia characteristics, lack of diurnal fatigability and fluctuation and paucity of other MG-associated symptoms made CSA more likely. The history of previous MG however confused the picture, and this case serves as an important lesson. In myasthenic patients with worsening dysphagia, all clinicians need to consider non-neuromuscular causes.