Two Labs Specialize in Muscle and Nerve Puzzles
Date: June 1, 2009
The patient had progressive muscle weakness serious enough to require hospitalization. His symptoms pointed to an inflammatory myopathy. But which one? An autoimmune form of the disease? Inclusion-body myositis? Or another category of muscle disease?
Solving these mysteries is the business of Johns Hopkins Hospital’s Neuromuscular Pathology Lab, which uses muscle and nerve biopsies to diagnose a wide variety of diseases. The comprehensive staining, which the lab provides for physicians at both Hopkins and other medical centers, is not offered at community hospitals, says lab co-director Andrea Corse.
For the biopsy, three cylindrical sections (1.5 x 0.5 cm) of muscle are removed from the patient under local anesthesia. The lab applies histochemical stains to highlight abnormalities. “We look for inflammatory cells, myofiber necrosis and architectural changes,” says Corse. The results may reveal systemic diseases such as vasculitis or muscle disease such as myositis, toxic myopathy, muscular dystrophy or rarer metabolic diseases. (The lab also conducts biopsies on the sural or other sensory nerves.)
In the case of the man who was hospitalized, his biopsy indicated colchicine myopathy. He’d been taking colchicine for gout. After his diagnosis, he stopped the medication, and his myopathy improved.
Another Hopkins lab, the Cutaneous Nerve Laboratory, offers a different set of specialty diagnostic services. It uses skin biopsies to diagnose neuropathies that leave their signature in the small nerve fibers that interlace the epithelium. One of the first labs to specialize in this field, it handles several thousand biopsies a year from patients around the world and is one of only a handful of labs in the country to offer such services.
When a patient has numbness, pain or tingling in the feet, a physician might perform a standard nerve conduction test to assess for neuropathy, says lab co-director Michael Polydefkis. While those tests can identify neuropathies that affect the large myelinated nerve fibers, they can’t detect small nerve fiber neuropathies. Another test, a biopsy of the sural nerve at the ankle, can be used to diagnose small fiber neuropathy. But it is invasive and removes a relatively large segment of nerve.
Instead, the Cutaneous Nerve Lab uses a skin punch biopsy technique, which involves removing a 3-millimeter in diameter segment of skin from the patient’s ankle, knee and thigh, and examining the density and morphology of the samples’ epidermal nerve fibers. “Nerves might appear thick, have blebs or swelling, or their density could be depleted, and these changes could mark nerve degeneration,” says Polydefkis. The test, he adds, can also reveal the progression of the disease, which generally begins in the feet and moves up.
A variety of different conditions, from vitamin deficiencies and inflammatory disorders to diabetes, can lead to small fiber neuropathy. “If we can catch somebody at the earliest stages of neuropathy,” says Polydefkis, “we have a better chance of arresting the process.”
Information: Neuromuscular Lab, 410-614-4278\
Cutaneous Nerve Lab, 410-614-6399
Michael Polydefkis and Andrea Corse