ALLODYNIA AND NEUROPATHIC PAIN

A patient presented with chest pain radiating to the back. She has chronic eczema covering the thorax. The day before she had a full work-up, including CT scan of the chest.  The resident suspects shingles.  “How would you demonstrate that in a patient like this?” you ask the resident. The resident draws vertical lines with Q-tips on her chest and allodynia is detected at upper and lower borders of a band-like distribution.  No vesicles were seen through the eczematous tissue, but gentle rubbing unroofed vesicles in this band-like area but not the remainder of the chest. “Well done!”

 

The clues to the diagnosis in some of the more challenging cases we see in emergency medicine arise from unique characteristics of nerves.  Ravi Morchi in “Diagnosis Deconstructed” (Emergency Medicine News, December 2011) describes a case where a patient presented with anterior thigh pain which traced out to the area of the femoral nerve. He gently pinched the skin off the thigh and it still hurt, indicating allodynia and the absence of deeper pathology in this area. This ultimately led him upstream to the iliopsoas abscess that was irritating the femoral nerve.

 

Nerve-related pain is identified through the following steps:

1) Topography

Trace the boundaries of the painful area.  If they correspond to a dermatome or peripheral nerve then this is suspicious for nerve-mediated pain. Of course, the painful area can spread outside of the area of the specific dermatome, so one should not be rigid in the interpretation of this pattern.(Oaklander, Archives of Neurology, 1999)

2) Allodynia

A painful response to nonpainful stimuli suggests neuropathic pain, in the absence of other disease.  A gentle touch of the finger is sufficient to provoke this.  Any modality can exhibit this pattern.  One author describes punctate hyperalgesia as abnormal pain from pinprick, static hyperalgesia from blunt pressure, heat hyperalgesia from hot stimuli and cold hyperalgesia from cold stimuli.(Haanpaa, Am J Med, 2009)

3) Positive findings

In addition to allodynia, other positive nerve-mediated phenomena include paresthesia and lancinating transient sharp pain

4) Negative findings

Sensory deficits are common in neuropathic pain, and can include hypoesthesia to light touch, temperature, and vibration.

 

Take Home Points:

Map out areas of pain when nerve-related pain is a possibility.

Check for allodynia – the experience of pain in response to non-painful stimuli

Recognize positive and negative phenomena of nerve-related pain.

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