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Pelizaeus-Merzbacher disease
Pelizaeus-Merzbacher disease usually begins in infancy or early childhood with abnormal eye movements, low muscle tone and delayed development. It mainly affects boys and men and varies from very severe to mild. Most cases are linked to changes in PLP1.
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Overview
Pelizaeus-Merzbacher disease (PMD) belongs to a group of conditions called leukodystrophies, which affect the white matter of the brain. White matter relies on myelin, a fatty layer that helps nerve signals travel quickly. In PMD, myelin is under-produced or unstable, so signals are transmitted less efficiently and brain function is affected.
The condition is rare. The metadata for this page gives a prevalence of roughly 1 in 200,000 to 1 in 500,000, although exact figures are not well established and may differ between populations. PMD is listed in OMIM under number 312080.
PMD sits on a spectrum with a related condition, spastic paraplegia type 2, which is generally milder and mainly affects leg movement. Clinicians often describe PMD as severe (connatal), classic, or milder, depending on the age at which symptoms appear and how they progress.
Symptoms & clinical features
In the classic form, the first sign is often rapid, wandering eye movements called nystagmus, noticed in the first weeks or months of life. Babies may also have low muscle tone (hypotonia), feeding difficulties and delayed early milestones such as head control and sitting. Over time, muscle stiffness (spasticity), unsteadiness (ataxia) and involuntary movements such as dystonia may develop.
Speech and walking are often delayed or limited, although many affected children make slow developmental progress. Understanding of language and social engagement are often better preserved than movement, so communication aids can be valuable. In the most severe connatal form, symptoms start at birth and can include noisy breathing (stridor), swallowing problems and seizures.
Milder forms may show leg stiffness and walking difficulties with relatively few other features, sometimes appearing later in childhood or adulthood.
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Affected organs
PMD primarily affects the central nervous system, meaning the brain and spinal cord. MRI scans typically show reduced myelin in the white matter. Secondary effects on other systems can follow from neurological impairment, including swallowing and nutrition, breathing, the spine (such as scoliosis, a sideways curve) and the joints.
Risks & severity
Severity varies widely, even between people with the same underlying type of genetic change. Connatal PMD is generally the most serious and can shorten life expectancy, while classic and milder forms often allow survival into adulthood. Outlook depends on the individual, so families are best guided by their specialist team.
Symptoms usually appear in infancy or early childhood. Progress may plateau or decline slowly over time, and complications involving swallowing, breathing and mobility are common concerns. Some female carriers of PLP1 variants may have mild or late-onset neurological features, though many have none.
Genetic causes
PMD is associated with changes in PLP1, which is located on the X chromosome. PLP1 provides instructions for proteolipid protein 1, a major structural component of myelin made by cells called oligodendrocytes. This protein helps myelin form a compact, stable layer around nerve fibres.
The most common cause is an extra copy of PLP1 (a duplication), which leads to too much protein and cellular stress. Other causes include point variants that alter the protein, and deletions or loss-of-function changes that reduce or remove it. Because the mechanism differs, features can vary between these groups; for example, complete absence of the protein is often associated with a milder picture than some altered-protein variants.
A small number of people with a PMD-like picture have changes in other genes, which is why broad testing is often used when PLP1 testing is negative.
- PLP1 proteolipid protein 1The PLP1 gene provides instructions for producing proteolipid protein 1, which is vital for forming the myelin sheath that insulates nerve fibres in the central nervous system.
Inheritance pattern
The metadata for this page lists autosomal recessive inheritance, but PLP1 lies on the X chromosome and PMD is generally described as X-linked recessive. This page follows the X-linked pattern, which is consistent with the gene location. A clinical genetics team can confirm the pattern in an individual family.
Males have one X chromosome, so a single altered PLP1 copy usually causes the condition. Females have two X chromosomes and are more often carriers. A carrier mother has a 1 in 2 chance with each pregnancy of passing on the altered copy: sons who inherit it are typically affected, and daughters who inherit it are carriers. Affected men pass the variant to all of their daughters but none of their sons. Sometimes the change arises for the first time in the affected child.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis typically combines clinical assessment, brain MRI showing reduced myelin, and genetic testing. Testing for PLP1 usually needs to detect duplications and deletions as well as small sequence changes, so more than one technique may be used. If results are negative, wider panels or genome sequencing may be considered.
In the NHS, testing is arranged through the Genomic Medicine Service, usually after referral from a paediatric neurologist or clinical geneticist. Eligibility is set out in the National Genomic Test Directory under R-codes covering leukodystrophies and early-onset neurological conditions, and the referring team confirms which applies. PanelApp gene panels define the genes tested.
Management & lifestyle
There is currently no cure for PMD, and care focuses on supporting function and comfort. This is generally coordinated by a multidisciplinary team, which may include paediatric neurology, physiotherapy, speech and language therapy, dietetics, orthopaedics and respiratory specialists. Support for communication, posture, nutrition and spasticity is tailored to each person by their team. Research into new treatments is ongoing, but plans for any individual are decided with their clinicians.
UK care pathway
In the UK, people with suspected PMD are usually seen by paediatric neurology and referred to a regional NHS clinical genetics service. Genetic testing is delivered through the NHS Genomic Medicine Service, using the National Genomic Test Directory and PanelApp gene panels.
Genetic counsellors can explain results, discuss carrier testing for female relatives, and outline reproductive options such as prenatal testing or preimplantation genetic testing. Families can generally ask their GP or specialist for a referral.
Frequently asked questions
Why are mostly boys affected by Pelizaeus-Merzbacher disease?
PLP1 is on the X chromosome. Boys have only one X chromosome, so a single altered copy typically causes the condition. Girls have a second copy that often compensates, so they are more often carriers.
Can a mother who has no symptoms still be a carrier?
Yes. Many carrier females have no symptoms, though some may have mild features. Carrier testing through NHS clinical genetics can clarify this, and sometimes the change appears for the first time in the affected child.
Is there a cure or specific treatment?
No cure is currently available. Care focuses on supporting movement, communication, feeding and breathing through a multidisciplinary team, and research into new approaches is ongoing.
How do we get a genetic test for suspected PMD?
Testing is usually arranged by a paediatric neurologist or clinical geneticist through the NHS Genomic Medicine Service. Families can ask their GP or specialist about referral, and a genetic counsellor can explain the process and results.
References
- Singh R, Samanta D. Pelizaeus-Merzbacher Disease. 2023. PMID: 32809357
- Thakkar RN, Patel D, Kioutchoukova IP. Leukodystrophy Imaging: Insights for Diagnostic Dilemmas. Medical sciences (Basel, Switzerland). 2024. PMID: 38390857
- TYLER HR. Pelizaeus-Merzbacher disease. A.M.A. archives of neurology and psychiatry. 1958. PMID: 13558772
- Koeppen AH, Robitaille Y. Pelizaeus-Merzbacher disease. Journal of neuropathology and experimental neurology. 2002. PMID: 12230321
- Inoue K. Pelizaeus-Merzbacher Disease: Molecular and Cellular Pathologies and Associated Phenotypes. Advances in experimental medicine and biology. 2019. PMID: 31760646
- Torii T, Miyamoto Y, Yamauchi J. Pelizaeus-Merzbacher disease: cellular pathogenesis and pharmacologic therapy. Pediatrics international : official journal of the Japan Pediatric Society. 2014. PMID: 25040584
- Perlman SJ, Mar S. Leukodystrophies. Advances in experimental medicine and biology. 2012. PMID: 22411242
- Adam MP, Bick S, Mirzaa GM. PLP1-Related Disorders. 1993. PMID: 20301361