Lesson #1: Why Does Neuropathy Hurt?

Before I start, let me make a disclaimer that I am talking about my experience with distal hereditary neuropathy.  The pain that I have experience is different from that experienced by people with autoimmune neuropathy or chemotherapy.  Specifically, neuropathy caused by autoimmune disorders or chemotherapy often causes spontaneous pain.  People that I have talked to with this type of neuropathy often talk about a pins and needles feeling that wakes them up from their sleep.  I have never had this pain.  What I experienced was more akin the pain of an broken bone.  If it was bad, it might ache dully while I was lying in bed, but the real pain started as soon as my feet hit the floor.  Surprisingly, most doctors don't understand this.  As soon as neuropathy enters the picture, the case is closed for them.  I even had one doctor that flatly told me I was wrong when I suggested my pain was not spontaneous.  Unfortunately, I let these experts persuade me that I was doomed to pain and I did pursue relief with as much effort as I should have.

So why does neuropathy cause pain?  This seems like a conundrum.  Doesn't loss of nerve response cause numbness?  It does, but first it lowers your deflector shields (to borrow a term from Star Trek).  Think about what's actually happening when you walk.  As you step forward, you're weight shifts forward on your foot until your heel lifts off the floor and all of your weight is on the ball of your foot.  If you weigh 160 lbs and the contact area of your foot with the floor is about 4" x 2", then the average pressure on your foot is about 20 psi. 

Now let's think about what is going inside the foot.  That 20 psi pressure pushes on the skin on the sole of the foot which in turn applies that pressure to the muscles on the bottom of the foot (plantar muscles).  When activated, these muscles of the foot become somewhat rigid and transmit the force on the bottom to the bones above them.  Since contact area of the muscles with the bones is smaller than between the muscles and the skin (there are gaps between the bones), the pressure on the bones is higher.  Let's suppose that the contact area between the bones and the muscles is a third less.  The pressure would the 3 halves as much, or 30 psi.  This gives us a ball park figure for what kind of stresses the human foot can handle.

So what happens when neuropathy enters the picture?  As the longest nerves die off, the muscles of the forefoot become incompletely activated.  The muscle then becomes soft and stops transmitting force to the bone.  Now, body weight will be transmitted to the bone only where the tissue is trapped between the bone and the floor.  The only place where the tissue can't get be pushed out of the way is right under each  metatarsal phalangeal joint (MPJ) at the base of each  toe.  If we generously assume that all five toes are in contact with floor and each has a contact area about 1/2" in diameter, then your 160 lb body weight is being transmitted to the floor through a total contact area of about 1 sq.in making for a contact pressure of 160 psi. That's five times what we came up with when the muscles were helping distribute the weight.  You start to see why neuropathy hurts.

If you're still not convinced, try this experiment.  Your tongue is all muscle and will illustrate a this point.  Stick your tongue out and tense it as hard as you can.  Keeping your tongue tense, pinch it between your thumb and forefinger.  Be sure to pinch in the middle where your tongue is thick and muscular.  Pinch as hard as you can tolerate.  Now without letting off your pinch, relax your tongue...Ouch!  The moment you relax your tongue all the force goes to the small area of tissue between the tips of your tongue and finger and the pressure skyrockets.

This focusing of pressure is why it is important to use a thick layer of foam to help redistribute the pressure and prevent tissue damage.  On top of any custom orthotics I always add a 1/4" or more of firm polyurethane foam such as McMaster 86375K134  In my next entry, I'll talk about what pedorthists don't know and what to look for in an orthotic.

As I have learned these lessons about neuropathy, I often think of a song by Counting Crows titled Amy Hit the Atmosphere:
Things are getting worse but I feel a lot better
And that's all that really matters to me

Introduction

My name is Tom Herron.  I am 49 years old and I have been diagnosed with hereditary peripheral neuropathy.  The variety I have is strongly distal and slowly progressing.  My first symptoms appeared in 2004 at age 41.  I was into rock climbing at the time and started experiencing pain under my big toe on my right foot when climbing.  Despite rest and anti-inflammation treatments, the problem slowly got worse and I was forced to give up climbing altogether within a year.  At that point, I was diagnosed with arthritis of the first MPJ (joint at the base of the big toe).  Various podiatrists and orthopedic surgeons confirmed this diagnosis, but then would hedge by saying that it wasn't the worst they had seen.  By 2007, the pain had become debilitating in my right foot and was starting in my left foot.  Without any other options, I had the right 1st MPJ fused in late 2007.  This "gold standard" treatment did indeed bring relief from the arthritis inflammation that I had been battling, but the acute pain returned within a few days of resuming use of the foot.

In 2009, a physical therapist that was treating me, asked if I had ever seen a neurologist.  While he was careful to point out that he was not a doctor, he said that he saw "similarities" to neuropathic patients that he had treated in the past.  I did some research and eventually ended up in the office of a physiatrist (pain specialist) who administered an electromyogram (EMG) to measure nerve conduction to my feet.  The test showed virtually no response from the extensor muscles along the top of my foot, and he referred me to a neurologist for further diagnosis.  I quickly learned that peripheral neuropathy comes in two distinct flavors: autoimmune disorders which are often treatable and hereditary disorders which are rarely treatable.  After some (expensive) blood tests, the neurologist was able to rule out the former.  Although he suggested that I be tested for known genetic mutations linked to peripheral neuropathy, I declined since I saw no value in knowing information which did not open the door to treatment.

As the my peripheral nerves have continued to slowly deteriorate, I have repeatedly been at the brink of despair.  At one point, I discussed the option of having my legs amputated below the knee as a way to escape the unending cycle of pain.  Yet each time I was ready to take drastic action, I would make some discovery that would allow me a respite from the pain and allow me to carry on.  Over the next few blog posts, I will describe the tough lessons learned and the techniques I have found for managing the disorder and maintaining an active lifestyle.  Today, nearly 10 years after my first pain symptoms, I am relatively pain free and still ski, mountain bike, and go for walks with my dogs.