AAV Can Cause Muscle Weakness, With Diagnosis Requiring Biopsy, Case Study Reports

Muscle weakness may be an under-recognized symptom of antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) with diagnosis likely requiring a biopsy of the muscle itself, a case report has found.

The report, titled “Severe proximal muscle weakness with normal CK as a presenting feature of ANCA-associated vasculitis,” was published in BMJ Case Reports.

The most common symptoms of AAV include fever, weight loss, headache, and joint pain. Muscle weakness is generally not considered a symptom of AAV, but previous reports have described a few people who were diagnosed with AAV after experiencing muscle weakness.

The patient in the new case report, an 82-year-old male, went to the hospital with a fever, which developed after a few months of general tiredness and weakness. The patient also described “a fall with his legs giving way underneath him.”

The patient was initially prescribed antibiotics for a presumed chest infection, which led to an improvement at first. However, a few weeks later, he returned to the hospital after another similar fall with symptoms of fatigue, weight loss, and muscle pain.

“On examination, the patient had profound proximal muscle weakness,” the authors wrote. Proximal muscles are those in the limbs that are closer to the torso.

Laboratory tests were conducted, which showed that a number of inflammatory indices, such as white blood cell count and C-reactive protein levels, were abnormally high.

The patient was mildly positive for myeloperoxidase (MPO) antibodies, one of the antibody types associated with AAV. However, because the antibody concentration was relatively low, the increased MPO levels were initially interpreted as a non-significant result, and attributed to the infection.

PET (positron emission tomography) scans did not show any obvious signs of muscle damage. Additionally, levels of creatine kinase (CK), a protein that is elevated in the blood following muscle damage, were within normal limits every time they were tested. This led to some confusion and diagnostic delays.

“The clinicians were reluctant to consider a muscle biopsy given that he had a repeatedly normal CK [level],” the authors wrote.

Nonetheless, a muscle biopsy was eventually performed. It revealed inflammatory infiltrates which — in combination with his positive MPO antibody result — led to a diagnosis of AAV.

The patient was started on a regimen of high-dose steroids (prednisolone) and responded well with reduced fever and muscle weakness. Low-dose prednisolone and Rituxan (rituximab) were then prescribed to maintain his remission.

The patient was able to leave the hospital without requiring rehabilitation and was “independent in all activities of daily living” after a few months of treatment.

This case adds to the small body of literature that shows muscle weakness as a symptom of AAV.

“We propose AAV be considered in the differential diagnosis of proximal muscle weakness after excluding the common causes,” the authors wrote.

Additionally, the report highlights how certain diagnostic tools can be misleading, requiring clinicians to perform the right tests to make a definitive diagnosis.

“When a patient presents with non-specific symptoms with muscle weakness, a normal CK or normal imaging cannot rule out myositis or vasculitis,” the authors wrote.

“We propose a muscle biopsy should be considered in any patient with unexplained muscle pain or proximal myopathy [muscle disease] even if the CK levels are normal,” they said.