5. All That glitters is not gold

Abstract Introduction Inflammatory muscle disease is a rare but well-recognised manifestation of systemic vasculitis. It can present as focal myositis or symmetrical proximal muscle involvement seen in polymyositis and dermatomyositis. Investigative findings in inflammatory myositis include raised creatine kinase, abnormal electromyography and muscle biopsy. MR scans can identify muscle abnormality, oedema, fibrosis and the extent of muscle involvement, and are routinely requested to evaluate patients with suspected or definite myositis. They also guide us to the appropriate muscles to biopsy. MR scans are not diagnostic of myositis and treating clinicians must be aware of its mimics. Our case demonstrates one such mimic. Case description A 59-year-old male patient under care of the renal physicians for ANCA-associated vasculitis complained of bilateral thigh pain. He had completed IV cyclophosphamide therapy (CYCLOPs protocol) for vasculitis affecting lungs, kidneys, and ENT. He was taking 30mg prednisolone and 100mg azathioprine daily. He denied muscle weakness, paraesthesia, or sensory symptoms. There was no muscle wasting, fasciculation, or sensory loss. Deep tendon reflexes were normal. 8-months prior, he was investigated for right hip pain and paraesthesia with MR spine and right hip, and neurophysiological studies. Investigations confirmed lumbar disc prolapse with neurophysiology in keeping with right L4 denervation changes. He had been treated with physiotherapy and daily pregabalin, duloxetine, and co-codamol. Renal physicians requested MR thigh, which was reported as muscle oedema over the long length of the right vastus medialis and lateralis, suggesting myositis. Rheumatology referral for myositis related to vasculitis was made. He was PR3+ve-3IU/ml (improved), ESR-2, CRP-4.6mg/L, CK-94U/L. EGFR remained stable at 66ml/min. Clinical features were not supportive of myositis and systemic vasculitis was in remission. The decision was made to discuss scans at the rheumatology- radiology meeting before considering a change in immunosuppression or muscle biopsy. Radiologists felt that findings of localised muscle group involvement suggested a neurogenic cause rather than primary muscle inflammation. They had reviewed MR lumbar spine done previously which had identified degenerative changes with right foraminal stenosis at L3-4 and L4-5 disc space. Electrophysiology showed chronic neurogenic changes of moderate severity in predominately L4 innervated muscles – vastus medialis and adductor magnus of the right lower limb. Findings were supportive of chronic right L4 radiculopathy and not a vasculitic neuropathy or myositis. Hence immunosuppression was left unchanged. He responded well to physiotherapy and did not require spinal surgical intervention. Discussion Our patient developed bilateral thigh pain in the setting of vasculitis. MR imaging was suggestive of myositis. Differential diagnoses included myositis related to vasculitis, primary focal myositis followed by vasculitic neuropathy, or denervation changes due to nerve entrapment. MRI is an excellent imaging modality for skeletal muscle due to its soft tissue resolution. It helps in narrowing differentials in skeletal muscle pathology, clinicians are able to identify individually affected muscle or muscle groups, give information about active muscle inflammation and/or damage. They can assist clinicians in targeting muscle for biopsy. Pattern and extent of muscle involvement can provide valuable clue to peripheral nerve or root lesion. Though MR suggested myositis, clinical assessment was not typical of myositis. Our patient had no systemic features of active vasculitis, which are concurrently present in 80% of vasculitic neuropathies. Neurophysiology in vasculitic neuropathy typically shows evolving multifocal axonal neuropathy with reduced compound muscle action potential amplitudes. His electrophysiology was of chronic denervation in the L4 region and explained the patient’s clinical symptoms and MR findings. For skeletal muscle pathology, three key patterns of MRI signal intensity include mass lesions, fatty infiltration and muscular oedema. In this patient, recognising unilateral muscle oedema localised to the vastus medialis and lateralis (innervated by L4) was key to diagnosis. Other muscles innervated by L4 nerve root are iliopsoas, tibialis anterior and posterior. Neurophysiology did demonstrate changes in iliopsoas muscle. In this patient, the muscle atrophy common to lower motor neurone lesions was not present. This is possible when partial denervation or reinnervation occurs, changes noted on neurophysiological studies in this patient. Our case shows MR evidence of myositis may not always equate to inflammatory myositis. In this patient, awareness of the radiological mimics of myositis avoided a muscle biopsy or further immunosuppression. Key learning points Myositis is an uncommon but recognised manifestation of systemic vasculitis. Chronic denervation can cause muscle abnormalities that appears similar to myositis on MR scan of the affected muscle. As such Clinicians need to be aware of this similarity and in patients with localised or focal myositis. Knowledge of the muscles innervations may help to establish a neurogenic aetiology where muscles are solely affected. Muscle atrophy can be notably absent in denervated muscle if the nerve is partially denervated or regenerating. Conflict of interest The authors declare no conflicts of interest.