This is quite a difficult topic because there is a long held belief that HNPP only causes painless palsies. I believe this view to be wrong and the result of misinterpretation. However that flies in the face of the medical profession, which is not always a smart thing to do.
Recently there was a report in the medical journals which described a case of HNPP where pain was reported. HNPP Manifesting By Recurrent Neuropathic Pain
Hereditary Neuropathy With Liability to Pressure Palsies Manifesting By Recurrent Neuropathic Pain.
Journal of Clinical Neuromuscular Disease. 8(1):26-30, September 2006.
Barroso, Fabio A MD *+; Leiguarda, Ramon MD +; Nogues, Martin A. MD *+
Hereditary neuropathy with liability to pressure palsies is an autosomal dominant disease that presents with focal neurological deficits in the territory of peripheral nerves. The usual cause is a deletion of 1.5 Mb at chromosome 17p11.2.
It has been clear from several clinical series that other phenotypes, such as isolated sensory deficits, chronic mononeuropathies, motor sensory neuropathies like CMT1A, polyneuropathy like CIDP, and Davidenkow’s syndrome, may also occur in affected families.
In this report, the authors present a new family with hereditary neuropathy with liability to pressure palsies (HNPP) in which one of the members suffered recurrent episodes of pain in the territory of peripheral nerve trunks along with generalized conduction nerve abnormalities and the common deletion of 1.5 Mb at 17p12.1.
On this basis, the authors propose that HNPP should be considered in the differential diagnosis of painful multiple mononeuropathy.
(C) 2006 Lippincott Williams & Wilkins, Inc.
From my own experience and that of my son and many others that I have spoken to, this is a common presentation. However, often it is not reported. I believe this to be due many neurologists not having much experience of HNPP and the first patient they see prompts them to quickly read up about HNPP, whereupon they read the commonly reported ‘old’ studies which talk only of ‘painless palsies’. Even if the patient then talks about pain, this will be dismissed or misinterpreted as being of another cause or just ‘anxiety’ on the part of the patient.
As far as I am aware there is no device or medical technique available for measuring pain, so Doctors have to rely on the patients own description. I find it quite incredible that most will deny that HNPP can cause pain. I have long wondered why this situation has been allowed to develop and I can only think that the scenario described above is common.
Another misconception is that HNPP is milder than CMT1a, and as CMT1a predominantly affects large fibre myelinated sensory-motor nerves, then pain cannot be a part of these diseases as pain is mostly carried by c-type unmyelinated fibres.
Slowing of conduction in motor and sensory nerves was believed to cause weakness and numbness. However, a study by Krajewski et al (2000) suggests that neurologic dysfunction and clinical disability in CMT 1A are caused by loss of or damage to large-diameter motor and sensory axons. Pain and temperature sensations usually are not affected because they are carried by unmyelinated (type C) nerve fibers.
This seems to me to be a rather simplistic explanation. Isn’t it well known that to be able to react quickly enough to a pain stimulus so as to quickly pull our fingers from the flame, that some pain has to be transmitted by myelinated A-delta fibres. If we only relied on c-type fibres with mean conduction speeds of around 0.1-1.0 m/s, then we would happily leave our fingers burning for several seconds before responding to the burn. A-delta fibres are thinly myelinated and whilst not having the ‘racy’ conduction speeds of the large myelinated fibres, their conduction speeds are in the range of between 5-25 metres/second. Now that’s more like it. Indeed the book that specified these conduction speeds, actually refers to A-delta fibres as First pain and c-type as second pain. ‘First’ neuropathic pain from A-delta fibres is likely to be experienced as lancinating, stabbing, or electric like symptoms.
Now it might be possible that with CMT1a A-delta fibres are not involved, although I cannot think why they would be spared. But with HNPP which causes demyelination by focal compression or other focal ‘injury’ then it would seem to me that A-delta fibres would be involved, especially in areas of localised cutaneous nerve involvement.
Another aspect with long-term neuropathic pain is ‘recruitment’. There have been several studies which have shown that injured myelinated sensory nerves (deafferent injury) will cause other neighbouring nerves to fire in a random fashion. These can be c-type fibres and may explain the ‘burning’ sensations expereinced by many with HNPP. There is also the possibility of further axonal loss by repeated episodes of compression induced demyelination adding to this picture of deafferent chronic pain.
Deafferent pain is a term often used in the case of amputation injuries. Afferent peripheral nerves are sensory nerves. Pressure palsies cause conduction block and slowing of nerve impulses, and could be viewed as episodes of deafferent loss, when sensory nerves are involved.
Finally, many nerve conduction studies have shown that many with HNPP will have significant slowing and conduction block of sensory nerves, even if they have few clinical signs of a motor neuropathy. It has even been suggested that HNPP causes far more sensory symptoms than motor symptoms. Surely this can only lead to the conclusion that neuropathic pain in well defined peripheral nerve distributions is in fact very common in HNPP.
(I will add links to this article as time passes… in the meantime it is published in it’s unfinished state)