Differentiating Inclusion Body Myositis From Amyotrophic Lateral Sclerosis Based on the Features of Dysphagia: Insights From a Patient With Rapidly Progressive Dysphagia

Dear Editor,

Inclusion body myositis (IBM) is a significant neuromuscular disorder that typically affects individuals aged 45 years and older. It presents insidiously, involving the finger flexors and the quadriceps, with the presence of rimmed vacuoles in muscle pathology.¹ Despite distinct clinical and histopathological features, IBM is often misdiagnosed, particularly as amyotrophic lateral sclerosis (ALS). To address this diagnostic challenge, we report a patient who was initially misdiagnosed with ALS owing to the presence of rapidly progressing dysphagia.

A 76-year-old male presented at our hospital for further management following a previous diagnosis of ALS at another hospital. That ALS diagnosis was based on dysphagia persisting for 1 year, which had progressed rapidly to cause a 10-kg weight loss and had necessitated the insertion of Levin tube 6 months previously. Electromyography (EMG) at the initial hospital revealed active denervation in the muscles of the upper and lower limbs and also the tongue. Upon referral to our hospital, the patient exhibited severe dysphagia but mild and stable limb weakness that only affected tasks such as opening bottle caps. A neurological examination showed no dysarthria, tongue atrophy, or fasciculation, with symmetrical motor weakness in finger flexors and the quadriceps (Medical Research Council grade 3), but normal strength for shoulder abduction and hip flexion. Deep tendon reflexes were hypoactive. Serum creatine kinase was elevated at 242 U/L (normal, <217 U/L). Pronounced weakness of the finger flexors and knee extensors is a typical clinical manifestation of IBM, and the absence of dysarthria further led us to question the diagnosis of ALS. We therefore decided to repeat EMG, which showed positive sharp waves and fibrillation potentials as well as myopathic motor unit potentials (MUPs) in biceps brachii and tibialis anterior. Muscle computed tomography (CT) revealed slight selective atrophy in quadriceps femoris (Fig. 1A), which is also compatible with IBM.² This prompted us to perform a muscle biopsy, which produced findings indicative of IBM, including endomysial inflammation invading nonnecrotic muscle fibers and rimmed vacuoles (Fig. 1B and C). The diagnosis of IBM was confirmed based on the common presentation characteristics and the findings of mandatory investigations as specified in the criteria proposed by the European Neuromuscular Center in 2024.² Since the CT findings were considered definitive, we did not perform ultrasound as specified in the criteria.² Myositis-specific antibody testing was not conducted. A videofluoroscopic swallowing study (VFSS) revealed impaired epiglottis folding, decreased laryngeal elevation, and scores on the penetration-aspiration scale of 3 for bolus liquid and 1 for small amounts of liquid, puree, and semisolid/solid food (Fig. 1D), along with oropharyngeal dysphagia and a cricopharyngeal bar (CPB) (Fig. 1E and F). A CPB refers to a ridge-like thickening of the cricopharyngeal muscle seen in a VFSS that causes a bottleneck appearance in the esophagus.³ Tapering of oral steroids (50 mg) did not significantly improve the dysphagia. The patient experienced two episodes of aspiration pneumonia, necessitating ongoing swallowing rehabilitation, but maintained full daily functionality.

Fig. 1. Muscle CT, myopathological findings, and results from the VFSS. A: Muscle CT revealed selective degeneration of the bilateral quadriceps (arrows) with relative preservation of the posterior thighs. B: Endomysial inflammatory infiltration was evidenced, with some surrounding nonnecrotic fibers (arrow). H&E, scale bar=50 µm. C: Several fibers contain rimmed vacuoles. Modified Gomori-trichrome, scale bar=50 µm. D: Findings of the VFSS including scores on the PAS. E and F: The VFSS revealed that barium had penetrated into the larynx during liquid swallowing (arrow, E), and severe esophageal obstruction represented by a cricopharyngeal bar (arrow, F). CT, computed tomography; H&E, hematoxylin and eosin; PAS, penetration-aspiration scale; VFSS, videofluoroscopic swallowing study.

The presentation of isolated dysphagia is atypical in IBM, although bulbar involvement is frequent during the courses.⁴ While dysphagia gradually worsens over time periods of several years,⁴ our patient experienced rapid deterioration within 1 year, leading to a potential misdiagnosis of ALS. In IBM, dysphagia is characterized by obstruction-related dysphagia and a CPB observed in a VFSS.⁵ The cricopharyngeal muscle is part of the upper esophageal sphincter (UES), and it plays a crucial role in both phenomena through fibrosis and hypertrophy, impairing UES relaxation.⁶ A CPB can be observed in both elderly healthy individuals and in patients with other neuromuscular diseases, and it is reportedly observed in 30%–50% of patients with IBM. Notwithstanding this limitation, the specificity of a CPB in IBM is notably high, being estimated at 96%.³ On the other hand, swallowing in individuals with ALS is primarily compromised by the oral phase showing a longer oral transit time and bolus holding within the oral cavity in a VFSS,⁷ with features of CPB not displayed. This might be because ALS-related dysphagia is attributable to cranial nerve palsy.⁷

Key clinical features distinguishing IBM from ALS include prominent weakness in finger flexors and the quadriceps,⁸ and a higher likelihood of detecting myopathic MUPs in flexor digitorum profundus,⁹ though neurogenic MUPs are not uncommon in IBM. Preservation of the thenar and hypothenar muscles also helps in differentiating IBM from the myotomal pattern of weakness in ALS.¹⁰ Our patient exhibited most of these features, and the pathology findings of the muscle biopsy confirmed the diagnosis. The absence of dysarthria and tongue atrophy/fasciculation is more suggestive of a condition other than ALS. However, the initially developed dysphagia resulted in these features being overlooked during the first diagnostic process. Therefore, in cases manifesting as early dysphagia, discernment based on the distinct features of dysphagia is imperative.

Through our case we further aimed to determine the characteristics of dysphagia in IBM for use as other distinguishing features. Despite its rarity and the current lack of effective treatments, an accurate diagnosis is essential for appropriate prognosis counseling and to avoid unsuitable treatment. This case underscores the importance of considering IBM in patients with isolated dysphagia and highlights the need for a deeper understanding of the pathomechanisms of dysphagia in IBM.

Availability of Data and Material

Data sharing not applicable to this article as no datasets were generated or analyzed during the study.