Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.
An 86-year-old woman is evaluated in her assisted-living facility for pain. Four weeks ago, she developed herpetic lesions on her right posterior thorax in a T7 distribution. She was treated with acyclovir, and the lesions healed; however, she has persistent severe burning pain. The pain is so severe that she is unable to leave her bed to attend meals. Medical history is remarkable for hypertension, mild cognitive impairment, and osteoporosis. She ambulates short distances but uses a walker for longer distances. Medications are amlodipine and as-needed acetaminophen. She cannot tolerate opioid medications because they have caused delirium in the past.
On physical examination, the patient is afebrile, blood pressure is 140/86 mm Hg, pulse rate is 62/min, and respiration rate is 14/min. BMI is 18. Examination of the back reveals allodynia and hyperalgesia in the right posterior T7 dermatome. All zoster skin lesions have resolved. On neurologic examination, she exhibits short-term memory impairment, which her family reports is her baseline. The remainder of the examination is unremarkable.
Which of the following medications is the most appropriate pharmacologic therapy for this patient’s pain?
A: Fentanyl patch
B: Oral gabapentin
C: Oral tramadol
D: Topical lidocaine
MKSAP Answer and Critique
The correct answer is D: Topical lidocaine.
A lidocaine patch is the most appropriate treatment in this patient with neuropathic pain. Topical lidocaine in the form of either a patch or the less expensive cream has been shown in randomized controlled trials to be very effective in treating postherpetic neuralgia and diabetic peripheral neuropathy. It has also been shown to be better tolerated and to have fewer side effects than systemic therapies. Similarly, topical capsaicin is an effective topical therapy for neuropathic pain. In patients with pain amenable to localized therapy, and particularly in those in whom systemic treatment may be problematic, such as this patient with cognitive impairment and an increased risk for falls, a topical agent that avoids central nervous system toxicity is preferable as first-line therapy.
Fentanyl is a potent opioid that is indicated only in patients who are opioid tolerant due to chronic treatment, and this patient has not been taking opioids regularly. Furthermore, fentanyl carries the same risk of delirium as other opioids and would therefore not be an appropriate choice based on her previous episodes of opioid-associated delirium.
Gabapentin is first-line therapy for systemic neuropathic pain conditions that affect large portions of the body that are difficult to treat topically. However, dizziness and drowsiness are major adverse effects of gabapentin and might increase this patient’s risk of falls and worsen her cognitive function. This patient has localized pain, which is better suited to topical therapy; therefore, gabapentin would be considered second-line therapy.
Tramadol binds to opioid receptors in the central nervous system and can cause adverse reactions similar to those of other opioid medications, which this patient does not tolerate. Tramadol also has a wide range of potential drug-drug interactions and a significant side effect profile, making it a poor choice for older patients. It would therefore not be an appropriate choice for treating this patient’s pain, which is amenable to topical treatments.
Key Point
- Topical lidocaine is effective in the treatment of postherpetic neuralgia.
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