Hereditary neuropathy with pressure palsies (HNPP) is an inherited nerve disorder that typically causes numbness, tingling and muscle weakness.
It affects one or more of the peripheral nerves, which connect the brain and spinal cord to the muscles and to the sensory cells that detect touch, pain and temperature.
HNPP is thought to affect between 1 in 50,000 and 1 in 20,000 people in the UK. Other terms for it are "entrapment neuropathy" and "compression neuropathy".
It is just one of many inherited conditions that damage the peripheral nerves, which are collectively known as Charcot-Marie-Tooth disease.
What are the symptoms of HNPP?
HNPP ranges in severity – some people get no symptoms at all or find it a minor nuisance, while others are so badly affected they can't even move the affected limb.
Most people with HNPP have attacks of numbness, tingling and muscle weakness in:
- the leg or foot – which can make walking, climbing stairs and driving difficult or impossible, and can result in foot drop (inability to lift the front part of the foot because of muscle weakness)
- the elbow, wrist or hands – which can result in "wrist drop" and loss of sensation in the index finger and thumb (symptoms similar to those of carpal tunnel syndrome), making fine movements such as fastening buttons impossible
These symptoms happen because the layer of protein covering and protecting the nerve (the myelin sheath) is faulty. For more information, see What's the cause?
Some episodes last just a few minutes, while others can last months.
Symptoms can be triggered by simple actions, usually:
- squashing the nerve – by leaning on an affected arm, for example
- repetitive movement – you may be able to do an activity one day without problems, but find it increasingly difficult if the same activity is continued many days in a row (you're thought to be damaging more and more fibres inside the nerve each time)
- stretching the vulnerable limb
Progression of the disease
HNPP is not life-threatening. How severe it is and the way it progresses can differ greatly from person to person.
Symptoms usually start in the teenage years or 20s-30s, although they can sometimes develop later in life or in childhood.
Some people start off with mild symptoms, or episodes in one particular area that come and go.
About half of people with HNPP fully recover after experiencing symptoms, never to have another episode, as the nerves have rebuilt their protective covering.
But the nerves don't completely recover for everyone with HNPP. For some people, the nerve only partially heals after being damaged, and they are left with recurrent or permanent nerve symptoms and muscle problems. However, these symptoms are usually mild, and it's unusual to be left with a severe disability.
What's the cause?
We need two PM22 genes, one from each parent, for the normal development of the peripheral nerves. These genes provide instructions for the creation of PM22 protein, which makes up the protective outer layer of each nerve. This outer layer, which is a bit like the plastic coating that insulates a wire, is called myelin.
In HNPP, one copy of the PM22 gene is lost (deleted) so that people have only one copy of this gene. A person with HNPP will have inherited a faulty copy of the PM22 gene from one of their parents.
With only one normal PM22 gene, their myelin is fragile and more susceptible to injury. Just the slightest pressure, stretch or repetitive movement on the nerve causes sections of the myelin to be lost, leading to muscle weakness – or "pressure palsies". Signals cannot be relayed to and from the muscles properly.
How is it diagnosed?
A diagnosis of HNPP is based on symptoms (such as recurrent loss of sensation in one particular area), evidence of a family history and electrical nerve tests. A genetic test will confirm the diagnosis.
How is it managed?
There's currently no cure for HNPP, but some of the symptoms can be treated, and lifestyle changes can make a huge difference.
If you can feel any warning signs (such as tingling), stop what you're doing to try to prevent further damage to the nerve.
It's wise to avoid:
- prolonged sitting with crossed legs
- leaning on elbows, resting the backs of arms on a chair or having someone rest on your arm
- repetitive movements of the wrists or elbows
Make a note of the sorts of activities that trigger symptoms and change the way you do them next time – for example, use a machine or tool to do the work, or get someone to help you.
An occupational therapist can advise on ways you can best carry out day-to-day activities.
Physiotherapy is not usually needed. A damaged nerve needs time to repair itself, so if you have muscle weakness, carry on using the limb just enough to keep the blood flowing to it to help it heal, but go easy and don't push it.
If you're overweight, now may be a good time to lose the weight – if the disease gets worse, you may find it difficult to be active.
Orthoses, splints and protective pads
People with foot drop or wrist drop may benefit from an ankle-foot orthosis or wrist splint, which can be worn at night.
Protective pads for the elbows and knees may prevent the nerves from being aggravated.
A foam pad on top of the mattress may also lessen pressure on the affected nerve.
If you're suffering from nerve pain, some antidepressants and anti-seizure drugs may help. Read more about medications for nerve pain.