This patient most likely has chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). His rapidly progressive symmetric distal and proximal weakness that plateaued 3 months after onset is consistent with this diagnosis. The diffuse areflexia and sensory and motor neuropathy noted on physical examination and the demyelinating pattern (conduction blocks and slowing of conduction velocities) detected on nerve conduction studies all support the diagnosis of CIDP. CIDP can be idiopathic or associated with a range of systemic conditions, including diabetes mellitus, paraproteinemia, and HIV infection. Recognition of CIDP is essential because the disorder is responsive to glucocorticoids and other forms of immunomodulatory therapy.

Patients with Charcot-Marie-Tooth disease type 1 also have uniform demyelination on nerve conduction studies. However, the clinical course is much slower than in this patient, and symptoms are more prominent in the distal extremities.

Diabetic amyotrophy, also known as proximal lumbosacral radiculoneuropathy, presents with subacute pain and weakness in the proximal lower extremities. The simultaneous involvement of the upper extremities, diffuse areflexia, and diffuse motor nerve abnormalities on nerve conduction studies in this patient are inconsistent with this diagnosis.

The clinical features of CIDP are very similar to those of Guillain-Barré syndrome, but the latter condition has a faster progression and reaches its nadir within 4 weeks, whereas CIDP progression continues beyond 8 weeks from onset.