Dancing muscles: the value of real-time ultrasound evaluation of muscle in myositis and mimics

Rheumatology key message

• Myopathies present with increased echogenicity on ultrasound, while fasciculations may point to a neurogenic cause

Dear Editor, Inclusion body myositis (IBM) is the most common myopathy in adults >50, presenting with slowly progressive weakness that can lead to significant disability. Despite increasing recognition of IBM as a clinical entity, the differential diagnosis is broad especially when cardinal features of finger flexor and knee extensor weakness are not fully apparent [1]. In atypical or early presentations, ancillary testing is often needed. As a non-invasive tool, imaging with muscle ultrasound may help to confirm or suggest alternative diagnoses.

A 67-year-old woman presented to Rheumatology for evaluation of hand weakness with a presumed diagnosis of IBM. Her symptoms had progressed over 10 months from dropping items to inability to close her hands secondary to weakness. An MRI of the brain was normal and an MRI of the spinal cord showed multilevel disc disease. Laboratory investigations including muscle enzymes were normal. On physical examination, she had preserved muscle strength in the lower extremities with 5/5 strength by manual muscle testing, but 2/5 weakness at the arm abductors and 4/5 at the finger flexors. There was reduced muscle tone in both arms with wasting around the shoulder girdle, forearms and intrinsic hand muscles. Reflexes were hypoactive in the arms and brisk at both knees. Given the hand weakness, a bedside ultrasound of the forearm was done that demonstrated normal muscle echogenicity of the distal finger flexors (flexor digitorum profundus), not compatible with IBM. However, abnormal fasciculations of multiple muscle groups were detected by ultrasound, which were not visible on physical exam (Fig. 1 and Supplementary Video S1, available at Rheumatology online). An EMG was then completed and showed evidence of acute-on-chronic denervation in cervical and thoracic myotomes asymmetrically, concerning for motor neuron disease. This led to a subsequent referral to Neurology who diagnosed the patient with flail arm variant of amyotrophic lateral sclerosis (ALS). The patient has been started on riluzole.

Fig. 1.

Ultrasound of the forearm showing preserved architecture and normal echogenicity of flexor digitorum profundus (FDP)

Given the chronicity of the disease in IBM, the most common abnormality seen on muscle imaging (MRI) is fat infiltration in affected muscles such as the quadriceps [2]. Recently, muscle ultrasound has also been used in the evaluation of IBM to confirm the presence of muscle disease [3, 4]. Normal muscle tends to be of low echogenicity. Affected muscles in IBM like the flexor digitorum profundus typically show a markedly increased echogenicity correlating with the replacement of normal muscle by fat and fibrosis [5]. As muscles are selectively involved in IBM, a contrasting echogenicity between the flexor digitorum profundus and the adjacent flexor carpi ulnaris has also been described in IBM but not in similar diseases like polymyositis or ALS [4]. The absence of this finding in this patient despite evident hand and finger weakness was helpful in excluding a clinical diagnosis of IBM.

ALS can be a mimicking disease of IBM with shared features of asymmetric weakness, muscle wasting and dysphagia [6]. The diagnosis of ALS requires clinical evidence of progressive upper and lower motor neuron degeneration [7]. Variants such as flail arm/leg and progressive muscular atrophy can manifest with predominantly lower motor neuron features, which can further complicate the distinction from IBM. Nerve conduction studies and needle electromyography (EMG) play a key role in localizing the lesion to muscle vs motor neuron and determining the extent of lower motor neuron involvement. Muscle ultrasound is useful as an adjunct, point-of-care technique to aid in diagnosis of ALS through the enhanced detection of fasciculations [8], which may otherwise be missed on clinical exam. Although increased muscle echogenicity can also be seen with denervation-induced atrophy in ALS, this is often not present early on, nor will it show the regional distribution expected in IBM. In this case, the visualization of muscle fasciculations in this patient led to an appropriate referral and accurate diagnosis. As rheumatologists are increasing their utilization of ultrasound for clinical purposes, an awareness of its value and role in the real-time evaluation of muscle disease, myositis and suspected myositis mimics is needed.