- A disorder affecting the peripheral nerves. These nerves connect the brain and spinal cord to muscles and sensory cells that detect touch, pain, and temperature. In people with this disorder, the peripheral nerves are unusually sensitive to pressure.
- A deletion in the PMP22 gene found on chromosome 17 is the most common cause. This gene encodes an integral membrane protein that is a major component of myelin in the peripheral nervous system. A deletion in PMP22 disrupts this process and results in the symptoms experienced by affected individuals.
- HNPP is inherited in an autosomal dominant manner, which means that each child of a person with HNPP has a 50% chance of inheriting the genetic mutation (however around 15-20% of cases are first-generation sufferers who develop HNPP as a fetus)
- Pressure or trauma to a single nerve results in episodes or periods of numbness and weakness, similar to an arm or leg going to sleep. Unlike a limb going to sleep for a few seconds, each episode of numbness can last from several minutes to several days or months. The most common sites are the wrists, in conjunction with carpal tunnel syndrome, the elbows, and the knees.
- HNPP is a progressive hereditary disorder
- The incidence is currently quoted as 2-3 people per 100,000 population but it may be as high as 1:2500 due to under-diagnosis
- HNPP is characterized by recurrent sensory and motor neuropathy with full recovery over days and months occurring in around half of episodes. Poor recovery correlates with a history of prolonged compression of the nerve.
- Risk factors for pressure palsies-prolonged sitting with legs crossed, occupations requiring repetitive movement of the wrist, prolonged leaning in elbows, and rapid weight loss.
- It may be misdiagnosed as Charcot-Marie Tooth diseaseor Bell’s palsy.
- There is no treatment other than symptomatic measures and avoiding positions that can create pressure such as leaning on elbows, crossing legs or ankles, or sitting for too long without changing position.
SYMPTOMS AND SIGNS
(Not all symptoms and signs may be present)
- Weakness—unable to move an entire limb
- Unable to use certain muscles of the arm (can’t reach, wash hair, eat) or hand (difficulty gripping things or grasping small objects) or face
- Episodes of numbness, weakness, or tingling that do not go away
- Leg/ankle/foot swelling
- Muscle cramps
- Diminished or absent ankle reflexes