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General HNPP Information
HNPP is the acronym for Hereditary Neuropathy with Liability to Pressure Palsy
What is a pressure palsy?
Pressure palsies are the main hallmark of HNPP. They are often the main presenting symptom which will lead to medical intervention and eventual diagnosis. Pressure (or physical trauma) on a peripheral nerve will result in unusual sensation, tingling, then a numb sensation, weakness and eventual loss of function of the muscles supplied by that nerve.
Most people can experience some symptoms of a pressure palsy after prolonged pressure/trauma to a nerve. Most of these will resolve very quickly within a minute or two. However, people with HNPP experience these palsies after only a brief period of pressure. The amount of time for them to resolve the numbness will be prolonged, ranging from minutes to several weeks or months, depending on the type of injury, and any previous nerve injury.
What Nerves Are Affected
HNPP is a sensory motor neuropathy, with muscles, sensory perception for those muscles as well as skin sensations of the overlying tissue, such as touch and vibration affected. Peripheral Nerves - those outside of the spinal column, mostly those at a distance (distal) from the spine.
Common first symptoms will involve the hands and feet, usually only the nerves covered by Myelin (sensory and motor). Myelin helps to protect nerves and speeds up the nerve impulse.
Pressure Palsy - Focal Demyelination in HNPP Pressure, such as mechanical trauma, can damage the myelin and nerve impulses are slowed and even sometimes blocked at the injury site. Mechanical trauma can result from either pressure, stretch, traction, or from repetitive movement. After injury, nerve repair and myelin regrowth is faulty/inhibited. Myelin becomes uncompacted and becomes even more prone to injury.
HNPP is a demyelinating neuropathy.
What Causes This To Happen?
HNPP is a genetic disorder. The main gene involved is one that makes Peripheral Myelin Protein (PMP-22) which is located on chromosome 17. Most people have two copies of this chromosome. With HNPP, there will be only one copy of this gene, as one of the two chromosome 17's is missing that gene. This results in under-expression of the peripheral myelin protein, the same region that is duplicated in CMT Type 1A, in which PMP-22 over-expression is responsible for the neuropathy.
How HNPP Is Inherited
PMP-22 is located on the short arm of chromosome 17. There are two copies of each chromosome, one inherited from the father and one inherited from the mother. In unaffected people, each chromosome 17 has one copy of the PMP-22 gene. In HNPP patients, one of the affected parents will have one of their chromosome 17s with PMP-22 missing (a deletion). Each of their children will have a 50% chance of inheriting this deleted PMP-22 gene on chromosome 17. Both males and females are equally affected.
Is There A Cure?
At present there is no cure. Genetic research (gene therapy and stem cell research) may lead to a cure at some time in the future. Other novel approaches are being researched to find ways of arresting the demyelination. Some possible targets for potential treatment for CMT Type1A may also be appropriate for HNPP, but there is also the possibility that a negative effect might occur.
Common Problems
The age of onset of problems can vary from birth to old age. Most commonly, problems will surface during the second decade of life. Some people affected by HNPP are only very mildly affected. Progression can be variable, but most people will experience problems beyond those of aging. Some will experience a moderate decrease in mobility, as well as some difficulties with daily living.
Foot Drop - the foot muscle weakness/palsy causes the foot to drag. Ankle sprains, and difficulty walking can occur. Weakness of grip and loss of hand function causes difficulty with fine hand control, eg, writing, as well as loss of overall strength.
A list of common problems (from www.HNPP.org by M. Horton)
crossing legs at the knee leaning on elbows sitting with legs crossed, or sitting tailor style
sitting slightly askew in a chair
sitting on something, such as a chair, step, or stool, with legs out, so that there is pressure on the back of the thigh
sitting in one position too long without readjusting
kneeling
gardening
carrying anything by its handle (purse, suitcase, camera case). Loaded plastic grocery bags are among the worst
using scissors
knitting
working with hand power tools
holding the telephone in one position too long
tieing shoes too tight, or tight shoe straps
high heels can make toes numb (even 1 inch heels)
painting too long (holding brush or roller) or painting above head
walking too long (more than an hour)
advanced HNPP:
lifting weights
using a mouse at the computer
typing
any activity on hands and knees
As the illness progresses, a more generalized leg and arm weakness develops. These include neck and shoulder problems; secondary muscle sprains and joint problems; back pain, through muscle imbalance possibly causing the involvement of the nerves to the muscles of the trunk. Some of these problems never resolve.
Tiredness and fatigue can occur causing sleeping difficulties.
Pain is frequently reported anecdotally. Many physicians are adamant that pressure palsies are painless. This may be so for a single pressure palsy occurring in isolation. However, after repeated and multiple incidences, pain can become a feature, possibly though the inclusion of secondary pain due to muscle imbalances, and further physical injury.
Treatment and Management
As HNPP is usually very slowly progressive, and is a demyelinating condition, most treatment is aimed at managing the longer term problems.
These include:
Pain Management - Medication, and sometimes counselling.
Occupational Therapy - work and home adaptations, e.g. Kitchen devices, and also pressure relieving devices.
Orthotic devices - to help with mobility difficulty caused by foot problems which can be helped with AFOs. These can include wrist and arm splints/support.
Physical Therapy - To help maintain mobility. A therapist with a particular interest in neuromuscular conditions would be ideal.
Surgery - Surgery is sometimes offered for nerve entrapment release. It is not generally thought to be effective as any benefits tend to be short-lived. However, individual differences require detailed assessment. Open surgery has been suggested rather than key-hole. Positioning during long periods of anaesthesia is quite important, to minimise the risk of position induced palsies.
