Allodynia

It’s a kind of pain associated with a damaged nervous system – particularly, but not exclusively, the peripheral nerves. It’s characterized by increased sensitivity to common sensations that would not normally feel like pain – like a light touch to the skin or perhaps picking up a cold or warm glass. It’s the sort of thing that folks with long-standing diabetes might experience because of diabetic nerve damage. Folks who have had Shingles can develop persistent allodynia in the nerve distribution affected by the Shingles lesions.

My personal experience with allodynia is related to the thalamic stroke I had some months ago. From the moment of its onset until today, the stroke has caused a “pins and needles” sensation over the right side of my body – especially my face and hand. At first, I described these annoying sensations as paresthesias, but allodynia is probably a better description. When I am at rest, these unpleasant sensations disappear completely. So, in peaceful sleep, am not aware of them. Some individuals who have severe allodynia can hardly stand to feel the bedsheets on their bodies; I am much luckier than they.

If I manipulate an object, type on the keyboard, or use my right hand in any way, the unpleasant sensations return. What I am experiencing is common in thalamic stroke, I’ve read. Therapy to desensitize the affected parts has had some success, and I do that for myself with the “artificial cat” that Susan made for me (a soft cloth bag-type glove filled with pinto beans that I can microwave and use for heat therapy. I also do things deliberately to trigger the allodynia in an effort to desensitize myself – especially the right hand and right lip. Writing in the blog is one of those things. The remainder of my right side is affected minimally.

And, that brings me to the homunculi. There are two – the sensory homunculus and the motor homunculus. These are mappings of the brain areas that process sensations and control of the voluntary muscles respectively. They are located on either side of a dividing channel that spans from ear-to-ear, as it were. The homunculi look like a distorted person draped over the surface of the brain. The thumb, hand, lips and tongue are huge; the thorax and torso are scrawny; the leg is bigger but still overshadowed by the hand; and the genitals get more brain surface than their size alone would merit. In other words, the parts of the body with the most sensory input are the mouth, hands and genitals. You can see the sensory homunculus here.

And, before you ask, I have not experienced allodynia down there. 🙂