LOPD can be misdiagnosed as inflammatory muscle disease: Study

Report highlights need for muscle imaging tests to reduce delay in treatment

Patricia Inácio, PhD avatar

by Patricia Inácio, PhD |

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A man with late-onset Pompe disease (LOPD) was initially misdiagnosed with an inflammatory muscle disease called polymyositis, according to a case study highlighting the need to incorporate imaging tests in clinical practice to reduce the delay in diagnosis and treatment.

After medication failed to improve his muscle strength, the patient underwent an ultrasound that showed muscle involvement suggestive of Pompe and a genetic test confirmed the LOPD diagnosis.

“Patients with apparent polymyositis, which persists despite treatment, require reconsideration of the diagnosis, with particular attention to treatable genetic causes,” researchers wrote in the report “Pompe disease misdiagnosed as polymyositis,” which was published in the journal Practical Neurology.

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Pompe disease leads to progressive muscle weakness

Pompe disease is caused by low to no levels of working acid alpha-glucosidase (GAA) — an enzyme that that breaks down a complex sugar molecule called glycogen — due to mutations in the GAA gene.

Deficiency in GAA leads to the toxic buildup of glycogen in several tissues, most often muscles, resulting in progressive muscle weakness. Symptoms can appear in the first year of life (infantile-onset Pompe disease) or later in life (late-onset).

In the report, researchers from Brazil described a case of LOPD initially misdiagnosed as polymyositis.

The 46-year-old man had an eight-year clinical history of muscle weakness affecting both upper and lower limb muscles. No one in his family had history of neuromuscular disease.

Physical examination revealed larger-than-normal body parts, including nose and hands, suggestive of a condition known as acromegaly which occurs when the pituitary gland produces too much growth hormone.

Blood work showed elevated levels of IGF-1, a hormone that manages the effects of growth hormone, and a brain MRI scan revealed a small benign mass (adenoma) in the pituitary gland.

After a diagnosis of acromegaly, he underwent a so-called transsphenoidal surgery to partially remove the adenoma. Subsequent treatment included octreotide to lower the levels of growth hormone and low-dose prednisolone, a corticosteroid.

However, his muscle weakness continued to worsen over the next three years, with blood tests showing mildly elevated levels of creatine kinase, suggestive of chronic muscle damage.

An electromyography, which is a diagnostic procedure to detect alterations in the electrical activity in muscles, and a muscle biopsy overall showed evidence of inflammatory damage.

The patient was diagnosed with polymyositis linked to acromegaly. His dose of prednisolone was increased but with little effect.

His muscle weakness continued to worsen and was accompanied by fatigue, difficulties while standing, and back pain, and he was referred to a neurologist. Examination confirmed weakness in the muscles of the arms, legs and abdomen, as well as a waddling gait.

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Ultrasound revealed distinct pattern of muscles affected

Ultrasound, a noninvasive imaging test, revealed a distinct pattern of muscle involvement consistent with Pompe disease. Alterations included the tongue, abdominal muscles, and the vastus intermedius, one of the quadriceps muscles in the thigh. In contrast, the deltoid muscles, which cover the shoulder and are commonly involved in inflammatory myopathy, showed no significant changes.

The patient underwent genetic testing which confirmed two mutations in the GAA gene, one in each gene copy, and further tests found a low activity of the GAA enzyme.

He was finally diagnosed with Pompe disease and referred for enzyme replacement therapy.

Overall, this case highlights “the need to incorporate muscle imaging to our daily practice. This knowledge may reduce the delay in diagnosing potentially treatable conditions,” the scientists concluded.