HNPP (hereditary neuropathy with liability to pressure palsies) is not an autoimmune disease. It is a genetic condition caused by a missing copy of a gene that helps build the protective insulation around your peripheral nerves. Because HNPP and certain autoimmune neuropathies can look similar on the surface, with episodes of numbness, tingling, and weakness, the two are sometimes confused. But their causes, progression, and treatment are fundamentally different.
What Actually Causes HNPP
In autoimmune neuropathies, the immune system mistakenly attacks healthy nerve tissue. In HNPP, the immune system is not involved at all. The problem is structural: a deletion on chromosome 17 removes one working copy of the PMP22 gene, which provides instructions for a protein critical to myelin, the insulating sheath that wraps around nerve fibers. With only one functional copy of PMP22 instead of two, your body produces less of this protein than it needs.
The result is myelin that forms abnormally. Under a microscope, nerves from people with HNPP show distinctive “sausage-shaped” swellings called tomacula. These are spots where myelin folds over on itself excessively and fails to compact properly. The loose, overfolded myelin does a poor job of insulating the nerve, allowing electrical signals to leak. The swellings also physically squeeze the nerve fiber inside, further disrupting signal transmission. This combination of poor insulation and constricted nerve fibers makes HNPP nerves unusually vulnerable to even mild physical pressure.
The vast majority of HNPP cases result from this specific chromosome 17 deletion. A small number are caused by other mutations that knock out function of the PMP22 gene through different mechanisms, but the end result is the same: not enough PMP22 protein, abnormal myelin, and nerves that are fragile at pressure points.
Why HNPP Gets Confused With Autoimmune Conditions
The overlap with autoimmune neuropathies like CIDP (chronic inflammatory demyelinating polyneuropathy) is understandable. Both conditions involve demyelination of peripheral nerves. Both can cause episodic weakness and numbness. And nerve conduction studies in both conditions show slowed signal transmission.
But the patterns differ in important ways. HNPP produces a distinctive signature on nerve conduction testing: sensory nerve conduction is abnormally slow in 93% of affected nerves, and distal motor latencies (the time it takes signals to travel through the ends of motor nerves) are prolonged in 78%. Notably, the slowing in HNPP is worst at the far ends of nerves and at common compression points like the wrist and elbow, whereas autoimmune neuropathies tend to produce more patchy or widespread slowing. HNPP also shows a background polyneuropathy that exists even between symptomatic episodes, something that helps distinguish it from a purely episodic autoimmune process.
The most definitive distinction is genetic testing. A simple blood test can identify the PMP22 deletion and confirm HNPP, removing any ambiguity about whether the immune system is involved.
What HNPP Episodes Feel Like
The hallmark of HNPP is recurrent episodes of numbness, tingling, and muscle weakness triggered by ordinary physical pressure on a nerve. The kinds of pressure that most people’s nerves handle without issue, like leaning on an elbow, crossing your legs, carrying heavy grocery bags, or sitting in one position too long, can set off an episode in someone with HNPP.
Episodes typically affect one limb at a time. Compression of the nerve at the wrist (carpal tunnel area) can make it hard to write, open jars, or fasten buttons. Pressure on the nerve at the knee can cause foot drop, where you lose the ability to lift your foot, making walking, climbing stairs, or driving difficult. Pain in the hands and limbs is also common.
About 50% of episodes resolve fully over a period of days to months. Recovery is generally complete, and when it isn’t, the lingering disability tends to be mild. The strongest predictor of poor recovery is prolonged compression, meaning a nerve that was under pressure for an extended time before the pressure was relieved.
How HNPP Is Managed
Because HNPP is genetic rather than autoimmune, the immunosuppressive drugs used for conditions like CIDP are not standard treatment. There is no pharmacological therapy proven to address the underlying cause. Management centers on protecting your nerves from the pressure that triggers episodes.
In practical terms, this means learning which positions and activities compress vulnerable nerves and finding alternatives. Avoiding prolonged leaning on elbows, not crossing legs for extended periods, using ergonomic supports at work, and being mindful of repetitive pressure during exercise or manual labor can all reduce the frequency of episodes. Occupational adjustments matter: jobs that involve sustained gripping, kneeling, or leaning on hard surfaces pose higher risk.
Interestingly, there are isolated case reports of corticosteroids helping during prolonged or severe episodes. Two adolescents with HNPP who had incomplete recovery showed rapid improvement after receiving steroids. This does not mean HNPP is inflammatory in nature. Steroids have broad effects on nerve swelling and fluid balance that could explain short-term benefit. But steroid use in HNPP remains anecdotal, not routine, and it highlights how important accurate diagnosis is: if HNPP is misdiagnosed as an autoimmune neuropathy, a patient could end up on long-term immunosuppressive therapy they don’t need.
How Common HNPP Is
HNPP is considered rare, with prevalence estimates ranging from 7 to 16 per 100,000 people. The numbers vary dramatically by population: as low as 0.84 per 100,000 in Ireland and as high as 16 per 100,000 in southwestern Finland. A Korean newborn screening study found rates as high as 1 in 1,698, suggesting HNPP may be significantly underdiagnosed in many populations. Many people with mild symptoms likely go years without a diagnosis, attributing their episodes to sleeping in an odd position or “pinching a nerve.”
HNPP follows an autosomal dominant inheritance pattern, meaning each child of an affected parent has a 50% chance of inheriting the PMP22 deletion. If you’ve been diagnosed, genetic counseling can help you understand the implications for family members who may have unexplained episodes of numbness or weakness.