Pain and regional anaesthesia12 / 13

Postherpetic neuralgia in an 81-year-old81-year-old woman, 55 kg, with burning, shooting pain over the right T6 dermatome

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Know this first

PHN: pain still present 120 days or more after the rash began; older age is a major risk.
Gabapentin is cleared by the kidneys, so its dose follows creatinine clearance.
Tricyclic side effects: orthostatic hypotension, sedation, cardiotoxicity, confusion.
Prevention: vaccination over 60 and early antivirals for acute shingles.

The patient

81-year-old woman, 55 kg, with burning, shooting pain over the right T6 dermatome, 5 months after shingles. Allodynia: she can't bear clothes on the area; poor sleep, low mood. CKD with creatinine clearance 35 mL/min; two falls this year; first-degree heart block on ECG. Drugs: paracetamol, amlodipine. Vitals: BP 138/76, HR 66, SpO2 97% on air, RR 14.

The examiner asks

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Consider
What are your considerations?
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- An 81-year-old with thoracic PHN, kidney disease, falls and conduction disease. Patient
- At 81, sedation, delirium and unsteadiness make every sedating drug a risk for her.
- Pain at least 120 days after the rash is PHN; her age is the main risk factor. Disease
Prepare
How do you assess her?
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- Burning, shooting pain and allodynia in one dermatome: her pain is purely neuropathic. Disease
- I score her pain, sleep, mood and function; low mood may need treating in its own right. Patient
- I review her falls and her kidney function before choosing any drug.
Plan
How do you treat her?
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- Topical first: a lidocaine plaster over the area, or a capsaicin 8% patch. (Disease)
- Gabapentin is cleared by her kidneys, so I start low and adjust to her clearance.
- The usual target is 1800–2400 mg a day; hers will be far lower, titrated slowly.
- No amitriptyline: heart block, falls, confusion and postural hypotension all argue against it.
- Opioids only later, in a well-chosen patient with clear goals.
- Nerve blocks give variable results, so I hold them in reserve.
Crisis
Two weeks after her gabapentin was increased she is found on the floor, drowsy and confused. BP 104/60, HR 58, SpO2 95%, RR 12, glucose 6 mmol/L. What do you do?
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- Gabapentin toxicity from poor renal clearance until proven otherwise. Crisis
- ABC; I look for a head injury or hip fracture from the fall.
- Stop the gabapentin; check her renal function and any other sedatives.
- Restart later at a lower renal dose, or rely on topical treatment. After surgery
- Falls review and physiotherapy, with her family involved.

Fatal mistakes

A full gabapentin dose with creatinine clearance 35 mL/min.
Amitriptyline in a woman with heart block and falls.
Treating the fall without stopping the drug that caused it.

Checked against: Faust · Barash 9e · Miller 10e · Stoelting 8e

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