HNPP – A Short Description
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Hereditary Neuropathy with Liability to Pressure Palsy
Hereditary – Can be passed on to children with a 50% chance of inheritance.
Neuropathy – A disease affecting the Nerves.
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. Mostly these will resolve very quickly within a minute or two.
People with HNPP will experience these palsies after only a brief period of pressure. The amount of time for them to resolve will be prolonged. This may take 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. Muscles, Senses for those Muscles and Skin sensations (touch, vibration). (see note 1)
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.
Only the nerves covered by Myelin (sensory and motor).
Myelin helps to protect nerves and speeds up the nerve impulse.
Pressure Palsy. – Focal Demyelination
Pressure (mechanical trauma) damages the myelin and the nerve impulses are slowed and sometimes blocked at the injury site.
Mechanical trauma can be pressure, stretch, traction, and from repetitive movement.
After injury nerve repair and myelin regrowth is inhibited/faulty. It becomes uncompacted and 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).
Normally two copies of this gene are needed. With HNPP there will be only one copy of this gene.
This is called Under-Expression of the peripheral myelin protein.
This is the same region that is duplicate in CMT1A, in which PMP22 over-expression is responsible for the neuropathy.
How Is It 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.
Each chromosome 17 has one copy of the PMP-22 gene.
In HNPP the affected parent will have one of their chromosome 17’s with PMP-22 missing (Deletion).
A child will have a 50% chance of inheriting this PMP-22 deleted 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 CMT1A may also be appropriate for HNPP, there is also the possibility that a negative effect might occur.
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 are only very mildly affected. Progression can be variable, most people will experience some problems beyond those of aging, a smaller number will experience a moderate decrease in mobility, and some difficulties with daily living.
Foot Drop – Foot muscle weakness/palsy, causing the foot to drag. Ankle sprains, and difficulty walking can occur.
Weakness of grip and loss of hand function – Difficulty with fine hand control (e.g. writing), and loss of overall strength.
A list of common problems (from HNPP.org by M. Horton)
- crossing legs at the knee
- leaning on elbows
- sitting with legs crossed, tailor style
- sitting slightly askew in a chair
- sitting on a something (chair, step, stool) with legs out so there is pressure on the back of the thigh
- sitting in one position too long without readjusting
- carrying anything by the handle (purse, suitcase, camera case). Loaded plastic grocery bags are among the worst
- using scissors
- working with hand power tools
- holding the telephone in one position too long
- tying shoes too tight or tight shoe straps
- high heels can make toes numb (even 1 inch)
- painting too long (holding brush or roller) or painting above head
- walking too long (more than an hour – time varies)- advanced HNPP
- lifting weights
- using a mouse at the computer
- any activity on hands and knees
As the illness progresses – more generalised leg and arm weakness. 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.
Tiredness and fatigue can occur with sleeping difficulties.
Pain is frequently reported, anecdotally. Many physicians are adamant that pressure palsies are painless. This maybe so for a single pressure palsy occurring in isolation, but after repeated and multiple incidence, 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 very slowly progressive, and is a demyelinating condition, most treatment is aimed at managing the longer term problems.
Pain Management – Medications, Counselling.
Occupational Therapy – Work and Home Adaptations, e.g. Kitchen aids, pressure relieving aids.
Orthotic devices – To help with mobility caused by foot problems. 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 that effective, any benefits being 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.