A Case Report of Congenital Fiber Type Disproportion with an Increased Level of Anti-ACh Receptor Antibodies

Congenital fiber type disproportion (CFTD) is a form of congenital myopathy, which is defined by type 1 myofibers that are 12% smaller than type 2 myofibers, as well as a general predominance of type 1 myofibers. Conversely, myasthenia gravis (MG) is an acquired immune-mediated disease, in which the acetylcholine receptor (AChR) of the neuromuscular junction is blocked by antibodies. Thus, the anti-AChR antibody is nearly specific to MG. Herein, we report on a case of CFTD with increased anti-AChR antibody levels. A 23-month-old boy exhibited muscle hypotonia and weakness. Although he could walk by himself, he easily fell down and could not control his head for a long time. His blood test was positive for the anti-AChR antibody, while a muscle biopsy revealed characteristics of CFTD. We could not explain the relationship between MG and CFTD. However, we considered different diagnoses aside from MG, even when the patient’s blood is positive for the anti-AChR antibody. 1. Introduction Congenital fiber type disproportion (CFTD) is a form of congenital myopathy [1]. CFTD is defined as a type 1 myofiber that is 12% smaller than the type 2 myofiber. Fiber type 1 predominance, where type 1 fibers can occupy more than 55% of all fiber types, has been seen in many cases. CFTD is usually characterized by hypotonia and mild-to-severe generalized muscle weakness at birth or within the first year of life. CFTD is often associated with a high-arched palate, kyphoscoliosis, contracture, and, less commonly, a mild increase in CK levels. Mutations of actin alpha 1 skeletal muscle (ACTA1), and several genes [2–5] have all been associated with CFTD. Fiber type disproportion is a morphological finding common to cases of neurogenic atrophy and many other congenital myopathies, such as nemaline myopathy (NM) and centronuclear myopathy (CNM). CFTD requires diagnosis by exclusion of nemaline and other myopathies. Myasthenia gravis (MG) is an acquired immune-mediated disease, in which the acetylcholine receptor of the neuromuscular junction is blocked by antibodies [6]. The disease is roughly classified into generalized and ocular myasthenia gravis (GMG and OMG, resp.). The symptoms of GMG involve easy fatigability of the skeletal or bulbar muscles, which results in dysphonia, dysphagia, general fatigue, and occasionally respiratory failure. The predominant symptoms of OMG are extraocular muscle weakness, ptosis, and limitations of eye movements. Daily variation in symptoms, with a worsening of muscle weakness in the evening, is a characteristic finding of OMG. The diagnosis of