This patient should have a glucose tolerance test. He most likely has small-fiber neuropathy, a condition sometimes associated with impaired glucose tolerance. Although many classic neuropathies associated with diabetes mellitus occur later in the course of the disease, impaired glucose tolerance is being increasingly recognized as an underlying cause of distal peripheral neuropathies, especially those involving the small fibers, which are unmyelinated peripheral nerves that carry sharp pain, temperature, and autonomic nerve fibers. Pure small-fiber neuropathy can present with distal upper and lower extremity pain and paresthesia without sensory or motor deficit. Autonomic deficits also may be present. Clinical examination findings are typically normal, including normal results on sensory, motor, and reflex testing with the possible exception of a mild distal sensory deficit. Results of electromyography (EMG), which assesses the large nerve fibers, can be normal. The presence of glucose intolerance should be confirmed by a glucose tolerance test.

Given the normal results of the clinical examination and EMG, an MRI of the lumbosacral spine to assess for myelopathy and radiculopathy is not warranted.

Although vitamin D deficiency can cause myopathy and central nervous system–related symptoms, it usually does not cause small-fiber neuropathy. Although multiple other conditions may be associated with small-fiber neuropathy, impaired glucose metabolism is one of the most common causes, and evaluating for this possibility should be the initial investigation. Further evaluation of small-fiber neuropathy should consider the possibility of vitamin B₁₂ deficiency, HIV infection, amyloidosis, Sjögren syndrome, paraproteinemia, celiac disease, and sarcoidosis.

Sural nerve biopsy has low sensitivity in detecting small-fiber disorders. The diagnosis is instead made on the basis of autonomic testing, including quantitative sudomotor axon reflex testing and skin biopsy, to assess intraepidermal nerve fiber density.