DIAGNOSTIC WORK-UP AND DIFFERENTIAL DIAGNOSIS
Although the diagnosis of diabetic amyotrophy is made primarily through detailed history taking and neurologic examination, other studies—electromyography, nerve conduction, imaging and labs, and nerve biopsy—may provide confirmation. Referral to neurology should also be considered.
The differential diagnosis is extensive and includes myopathies, muscular dystrophies, intervertebral disc disease, spinal stenosis, polyradiculopathies due to porphyria, amyloid, heavy metal poisoning, anterior horn cell diseases (eg, poliomyelitis), neoplasms, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré syndrome, monoclonal gammopathy, inflammatory vasculitis, hypothyroidism, vitamin B6 or B12 deficiencies, syphilis, AIDS, Lyme disease, and Charcot-Marie-Tooth disease.2,5-7 Diabetic neuropathic cachexia should also be considered in the differential, as it presents with weight loss and lower limb pain but no weakness.5
Lab evaluation should begin with analysis of fasting plasma glucose, complete blood count, comprehensive metabolic profile, A1C, erythrocyte sedimentation rate (ESR), creatine kinase, vitamin B12, and thyroid-stimulating hormone levels.7 Elevations in ESR and positive rheumatoid factor and antinuclear antibody can occur in patients with diabetic amyotrophy and are suggestive of a coexisting autoimmune disorder.6 Serum creatine kinase and thyroid function studies are normal.4 Additional lab tests, if clinically indicated, include paraneoplastic panel to evaluate for occult malignancy, antimyelin-associated glycoprotein antibodies, antiganglioside antibodies, cryoglobulins, cerebrospinal fluid analysis, porphyrin titers, and testing for heavy metals.7
Electrodiagnostic studies are recommended if the diagnosis of diabetic amyotrophy remains unclear following history taking, physical examination, and preliminary testing. Electromyography and nerve conduction studies typically reveal findings consistent with denervation and axonal damage in proximal muscles of the lower extremities.4 If demyelination is observed, a diagnosis of chronic demyelinating polyneuropathy should be considered.5
Nerve biopsy is considered if the diagnosis remains unclear after laboratory and electrodiagnostic testing or when confirmation of the diagnosis is needed before starting aggressive treatment. The sural and superficial peroneal nerves are preferred for biopsy. In cases of diabetic amyotrophy, sural nerve biopsy reveals significant fiber loss in an asymmetric fashion, resembling focal ischemia.5
MRI or CT scan of the lumbosacral spine is employed to exclude mass lesions and structural disorders such as spinal stenosis and disc disease.4 Cerebrospinal fluid is typically acellular, with a mildly elevated protein level of 60 to 100 mg/dL (but occasionally as high as 400 mg/dL).5
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