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Why is HNPP here
Similar Genetics
The same mechanism that causes the genetic fault in CMT Type 1a, is also responsible for Hereditary Neuropathy with liability to Pressure Palsies (HNPP). However with HNPP this leads to a deletion of the genetic material rather than a duplication. It is the same region of DNA that is affected, and causes an under-expression of the Peripheral Myelin Protein (PMP22). In other words, in HNPP, there is only one gene in the relevant pair, rather than the "normal" two, or three in CMT Type 1a.
As with CMT1a, HNPP is autosomal dominant. This means there is an equal chance of both males and females inheriting the condition, and a 50% risk for each child. The incidence is around 2-5 in 100,000 of the population – however the rate is thought to be 15-20 in 100,000, ie similar to that of CMT Type1a. Many people are undiagnosed or misdiagnosed.
Similar nerve conduction results
Studies show a "mild background demyelinating neuropathy”. Nerve conduction studies often reveal increased latency, decreased velocity, decreased amplitude. Notable are the focal areas of conduction slowing, reflecting nerve compression/trauma at sites of anatomical vulnerability - such as the elbow, wrist, fibular neck. This shows itself as the characteristic pressure palsy.
Because of the generalised demyelinating neuropathy, HNPP families have occasionally been diagnosed as having CMT, and only since the advent of genetic testing, has the cause been identified as HNPP.
What is the effect of the PMP-22 deletion?
HNPP can be very variable, with some affected family members having few if any symptoms. The age of onset of first symptoms can be from birth, although more commonly during teenage years or as a young adult. Because of the transient nature of the pressure palsy, it may be some years before the person realises that all is not quite right. Some people will have a very delayed onset, and may not have a long lasting palsy, leading to diagnosis much later in life. Once diagnosed they often recognise its steady progression through the years.
