Duloxetine is a reasonable initial option for this patient's painful peripheral neuropathy. The typical presentation for distal symmetric polyneuropathy is a bilateral “stocking-glove” distribution. Damage to the small nerve fibers can result in pain, numbness, burning, and tingling. It can also impair light touch and temperature sensation. Damage to the large nerve fibers leads to abnormal vibration sensation and proprioception. Diminished or loss of ankle reflexes is commonly seen early with diabetic polyneuropathy. Motor weakness can occur as the polyneuropathy progresses. Several classes of drugs are frequently used for symptomatic pain relief, including the tricyclic antidepressants (amitriptyline), other classes of antidepressants (duloxetine, venlafaxine), anticonvulsants (pregabalin, gabapentin, valproate), and capsaicin cream. There are few head-to-head comparison trials for these classes of drugs for distal symmetric polyneuropathy, thus selection must take into consideration the potential risks and benefits associated with each drug for an individual patient. Duloxetine has fewer risks than amitriptyline for this patient given his cardiac history.
Tricyclic antidepressants, such as amitriptyline, should be used cautiously in patients with known cardiac disease due to an association between this class of drugs and arrhythmias, heart block, and sudden death. The patient's history of cardiac disease and a first-degree atrioventricular block may increase his risk of side effects from amitriptyline.
A nerve conduction study is not routinely required for diagnosis or management in patients with diabetes with a typical presentation of symmetric distal polyneuropathy. Atypical clinical features should prompt additional work-up, including electrophysiologic testing.
Vitamin B12 deficiency has been associated with long-term use of metformin and can present with peripheral neuropathy. It is also commonly seen in the setting of megaloblastic anemia. It is unlikely that vitamin B12 deficiency is the cause of this patient's peripheral neuropathy as he has a classic presentation for symmetric distal polyneuropathy, discontinued metformin 2 years ago, and has a normal complete blood count.