Zellweger Spectrum Disorder
What else is it called?
- Cerebrohepatorenal syndrome
- PBD, ZSS
- Peroxisome biogenesis disorders, Zellweger syndrome spectrum
- Zellweger spectrum
- Zellweger syndrome spectrum
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What causes it?
Zellweger Spectrum disorder [ZSD] belongs to a group of disorders called Peroxisomal Disorders, all of which are categorised by a PEX gene defect. There can be up to 12 different gene mutations that can occur with this condition. Previously, ZSD was listed as three different disorders; Zellweger Syndrome [ZS] which is the most severe form, Neonatal Adrenoleukodystrophy [NALD] which has a level of intermediate severity and Infantile Refsum Disease [IRD] which is the least severe of the three disorders mentioned. However, they are now all listed under the ZSD spectrum.
The genes that are impacted with this condition are all involved in producing peroxins which assist in the production of the membrane which separates the membrane of your body’s cell from the peroxisomes, they also help to transport enzymes to break substances down within the cells. Peroxisomes help to break down fats in cells and can also be useful for an effective nervous system and for effective digestion. With a gene mutation for ZSD, the body cannot break down these fats which can lead to an accumulation of metabolites in the body.
How common is it?
It is estimated that ZSD impacts 1/50,000 new born babies. Unfortunately, children who have this condition do not tend to make it into adulthood. There is no information on whether this condition is more prevalent in males or in females or occurs more frequently in specific ethnicities.
What are the signs and symptoms?
For ZSD, symptoms, the severity can be individualised, different people can present different symptoms at different time points. The symptom severity generally depends on where on the spectrum your condition is. The common signs and symptoms that you might see for this condition are:
- Weak muscle tone [hypotonia]
- Visual impairment
- Low set of ears
- Failure to thrive
- Small or large head circumference
- Broad nasal bridge
- Adrenal insufficiency
- Protruding tongue
- Hearing loss
- Kidney abnormalities
- Developmental delay
It is not uncommon for children with this condition to present other symptoms such as, renal stones and for them to develop a low mineral bone density which can impact bone strength. In extremely rare cases, some individuals will not develop symptoms until adulthood, these symptoms are most likely to be less severe, these could be problems associated with hearing loss and vision.
How is it diagnosed?
Diagnosis is based upon a clinical evaluation, a detailed patient history, a family history of the condition and through the expression of symptoms relevant to ZSD. An ultrasound may be used to investigate the kidneys for potential cysts that may have developed. Blood and urine analysis may be used to diagnose this condition, this test would be performed to check for levels of very long chain fatty acids [VLCFA] which may provide another clue to diagnosing this condition.
Individuals who are at the severe end of the spectrum for this condition are diagnosed early into the newborn period. Genetic testing is available for this condition, this will be recommended by your health care professional if they believe you may have this condition. This testing will help to confirm if you have or don’t have ZSD.
Can it be treated?
Currently there is no available treatment for ZSD, treatment is usually aimed at managing symptoms as they present. Previous research has looked into using cortisol therapy to treat this condition, this suggested method is due to a lack of cortisol being produced by your adrenal glands which commonly accompanies this condition. You may also be prescribed vitamin K to help with the blood clotting deficiencies and anti-seizure medication to help prevent seizures. In the past, children have also benefitted from treatment with DHA [docosahexaenoic acid].
It may be likely that you are referral to a team of specialists depending on what symptoms or how many symptoms you are expressing such as an audiologist for problems associated with hearing or an ophthalmologist for problems with your vision. Infants who have trouble with feeding may require insertion of a feeding tube, however, this will be discussed with you by your paediatrician and a multi-disciplinary team.
Do my family need to be tested?
ZSD can only be passed on to a child if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. A person who has a copy of the faulty gene is known as a carrier.
If both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.
If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.
Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inherited ZSD.They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.