Diagnosis of McArdle disease is frequently delayed by many years following the first presentation of symptoms to a health professional. The aim of this study was to investigate the importance of misdiagnosis in delaying diagnosis of McArdle disease. The frequency of misdiagnosis, duration of diagnostic delay, categories of misdiagnoses and inappropriate medical interventions were assessed in 50 genetically confirmed patients. The results demonstrated a high frequency of misdiagnosis (90%, n = 45/50) most commonly during childhood years (67%; n = 30/45) compared with teenage years and adulthood (teenage: n = 7/45; adult n = 5/45; not known n = 3/45). The correct diagnosis of McArdle disease was rarely made before adulthood (median age of diagnosis 33 years). Thirty-one patients (62%) reported having received more than one misdiagnosis; the most common were "growing pains" (40%, n = 20) and "laziness/being unfit" (46%, n = 23). A psychiatric/psychological misdiagnosis was significantly more common in females than males (females 6/20; males 1/30; p < 0.01). Of the 45 patients who were misdiagnosed, 21 (47%) received incorrect management. This study shows that most patients with McArdle disease received an incorrect explanation of their symptoms providing evidence that misdiagnosis plays an important part in delaying implementation of appropriate medical advice and management to this group of patients.

Glycogen storage disease type XIII (GSDXIII) is a very rare inherited metabolic myopathy characterized by autosomal‐recessive mutations in the ENO3 gene resulting in muscle β‐enolase deficiency, an enzymatic defect of the distal part of glycolysis. Enzyme kinetic studies of two patients presenting with exertion intolerance and recurrent rhabdomyolysis are reported. Next generation sequencing confirmed patient 1 was homozygous for p.E187K in ENO3, while patient 2 was homozygous for p.C357Y. ENO3 variants pathogenicity was confirmed by functional studies in skeletal muscle. p.E187K caused extremely low total enolase activity. p.C357Y was associated with a higher level of residual activity but kinetic studies showed a lower maximum work rate (V max). This study illustrates that GSDXIII may be caused by either null mutations leading to β‐enolase deficiency or by mutations that alter the enzyme's kinetic profile. This study highlights the importance of carrying out functional studies as part of the diagnostic process following the identification of variants with next generation sequencing.

Patient: Female, 44Final Diagnosis: McArdle diseaseSymptoms: Exercise intolerance • muscle contracture • myalgia • myoglobinuria • recurrent rhabdomyolysisMedication: —Clinical Procedure: —Specialty: NeurologyObjective:Rare diseaseBackground:McArdle disease is a glycogen storage disorder mainly characterized by exercise intolerance. Prolonged muscle contracture is also a feature of this condition and may lead to rhabdomyolysis (RM), which is a serious event characterized by acute skeletal muscle damage.Case Report:A 44-year-old female patient presented with an acute contracture of the posterior neck muscles, causing severe nuchal rigidity. The contracture was induced during a dental extraction as she held her mouth open for a prolonged period, with her neck in a rigid position. She presented with severe pain in her ear and head, as well as fever, vomiting, and confusion. Based on her symptoms, she was initially misdiagnosed with bacterial meningitis and experienced an acute allergic reaction to the systemic penicillin she was subsequently administered. Lumbar puncture results were normal. High serum creatine kinase (CK) levels, recurrent exercise-related muscle symptoms, and a previous history of recurrent myoglobinuria raised the suspicion of an underlying neuromuscular condition. McArdle disease was confirmed by muscle biopsy and a genetic test, which revealed that the patient was homozygous for the R50X mutation in the PYGM gene.Conclusions:This case illustrates that even seemingly innocuous movements, if rapid isotonic or prolonged isometric in nature, can elicit a muscle contracture in McArdle disease patients. Here, we highlight the need for careful management in this patient population even during routine healthcare procedures. The allergic reaction to antibiotics emphasises that misdiagnoses may result in iatrogenic harm.