Numbness in Charcot-Marie-Tooth Disorders

Each of the Charcot-Marie-Tooth (CMT) disorders is characterized by varying degrees of degeneration of peripheral nerve axons. In CMT type I, the primary pathology is degeneration of the myelin sheath but later there is associated axonal degeneration. In CMT type II the disorder directly affects the axon, resulting in primary axonal degeneration. Peripheral nerve axons are relatively simple structures whose function is to transmit electrical impulses to and from the brain; motor nerves transmit impulses from the brain to muscles while sensory nerves transmit impulses from the skin and other body tissues back to the brain.

When nerves degenerate, those parts farthest from the brain almost always are the first to be affected. Thus, symptoms usually begin in the feet and spread up the legs to about the level of the knees and then begin to involve the hands and spread up the arms in a similar fashion. It is very unusual for the nerves to the body or the face and scalp to be involved to any great extent. Deviation from this pattern is very rare in CMT disorders other than hereditary neuropathy with liability to pressure palsies (HNPP), a condition I will discuss separately.

Involvement of motor nerves predominates in all CMT disorders, resulting in weakness and muscle atrophy (wasting). Involvement of sensory nerves is ubiquitous and yet sensory symptoms are not as prominent. However, some degree of loss of sensation can always be found on careful examination in CMT patients. It is only rarely disabling but it can be very annoying. Other symptoms that result from sensory nerve involvement include paresthesias (tingling), pain and loss of balance. The latter particularly occurs in the dark or when the eyes are closed.

It is not clear why sensory symptoms are so minor in CMT patients. It may be that since the sensory loss evolves so slowly, over years or decades, in some patients the brain learns to ignore the symptom. The brain also has the capacity to amplify signals received from the peripheral nerves so that if any signal is received, no matter how small, a sensation may be felt and any sensory loss will be inapparent to the patient but can be found on careful examination. In many patients the numbness is worse in the hands than in the feet. This is probably because the hands are normally more sensitive and we feel any sensory loss more acutely. Even in those patients who complain more of their hands, neurological examination will show a greater degree of sensory loss in the feet. Nor is it clear why sensory symptoms are so variable with some patients having very annoying symptoms while others are completely unaware of sensory involvement.

Sensory loss appears to be more severe when the axons degenerate and therefore may occur earlier and be more prominent in CMT type II. In CMT type I the earliest abnormality is slowing in the speed of conduction and yet the signal may still eventually reach the brain and be amplified so sensation is relatively normal. Later, as the associated axonal degeneration occurs, the sensory loss may become more obvious.
HNPP differs from other CMT disorders in that it is characterized both by acute episodes of demyelination and by a more slowly progressive degeneration of peripheral nerve axons. HNPP patients frequently complain of episodic numbness and tingling, lasting from minutes to several days. These episodes are usually precipitated by minor trauma to the nerve such as compression or stretching. In more severe episodes, the sensory symptoms are associated with weakness but sensory symptoms usually predominate. These symptoms may occur in any part of the body, head or limbs. By contrast, the slowly evolving degeneration of the nerves that also occurs in HNPP patients is more like the other CMT disorders in that it begins in the feet and mainly produces symptoms of weakness although sensory loss can always be found on examination. This lends support to the idea that sensory symptoms are, at least in part, dependent on the pace at which they develop.

Linda here - Not everyone experiences numbness. Some people can have numbness and movement loss together, some just loss of movement and no numbness.
We asked Dr. Parry to write about numb hands and feet because so many of our readers have asked about it. Believe me, you are not alone. We'll print here some of the comments, some via e-mail, some by post, we've had from people experiencing numbness in their extremities.
Numbness is something new
Cindy Jacobsen wrote on our cmtilist e-mail: Does anyone else out there have numbness in their hands? I think this must be something new with me. I've had my hands in wat