Clinical characteristics: Zellweger spectrum disorder (ZSD) is a phenotypic continuum ranging from severe to mild. While individual phenotypes (e.g., Zellweger syndrome [ZS], neonatal adrenoleukodystrophy [NALD], and infantile Refsum disease [IRD]) were described in the past before the biochemical and molecular bases of this spectrum were fully determined, the term "ZSD" is now used to refer to all individuals with a defect in one of the ZSD-PEX genes regardless of phenotype.

Individuals with ZSD usually come to clinical attention in the newborn period or later in childhood. Affected newborns are hypotonic and feed poorly. They have distinctive facies, congenital malformations (neuronal migration defects associated with neonatal-onset seizures, renal cysts, and bony stippling [chondrodysplasia punctata] of the patella[e] and the long bones), and liver disease that can be severe. Infants with severe ZSD are significantly impaired and typically die during the first year of life, usually having made no developmental progress.

Individuals with intermediate/milder ZSD do not have congenital malformations, but rather progressive peroxisome dysfunction variably manifest as sensory loss (secondary to retinal dystrophy and sensorineural hearing loss), neurologic involvement (ataxia, polyneuropathy, and leukodystrophy), liver dysfunction, adrenal insufficiency, and renal oxalate stones. While hypotonia and developmental delays are typical, intellect can be normal. Some have osteopenia; almost all have ameleogenesis imperfecta in the secondary teeth.

Diagnosis/testing: The diagnosis of ZSD is established in a proband with the suggestive clinical and biochemical findings above by identification of biallelic pathogenic variants in one of the 13 known ZSD-PEX genes. One PEX6 variant, p.Arg860Trp, has been associated with ZSD in the heterozygous state due to allelic expression imbalance dependent on allelic background.

Management: Treatment of manifestations: The focus is on symptomatic therapy and may include gastrostomy to provide adequate calories, hearing aids, cataract removal, glasses to correct refractive errors, supplementation of fat-soluble vitamins, and cholic acid supplementation; varices can be treated with sclerosing therapies; anti-seizure medication, early intervention services for developmental delay and intellectual disability; adrenal replacement therapy; vitamin D supplementation and consideration of bisphosphonates for osteopenia; treatment as per dentist for ameliogenesis imperfecta. Supportive treatment for renal oxalate stones has included hydration, lithotripsy, and surgical intervention. Annual influenza and respiratory syncytial virus vaccines should be provided.

Surveillance: Growth and nutrition should be assessed at each visit. Annual audiology and ophthalmologic evaluations; annual monitoring of liver function and coagulation factors, and ultrasound and/or fibroscan to evaluate liver architecture; monitor for changes in seizure activity; head MRI to evaluate for white matter changes that may explain changes in cognitive and/or motor ability; monitor developmental progress and educational needs; ACTH and cortisol levels by age one year and annually thereafter. Dental examinations every six months. Annual urine oxalate-to-creatinine ratio with consideration of renal imaging when performing liver imaging. Assessment of family needs at each visit.

Genetic counseling: ZSD is typically inherited in an autosomal recessive manner (one PEX6 variant, p.Arg860Trp, has been associated with ZSD in the heterozygous state). At conception, each sib of an individual with biallelic ZSD-causing pathogenic variants has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives is possible if the pathogenic variants have been identified in an affected family member. Prenatal testing for a pregnancy at increased risk is possible by DNA testing if both ZSD-related pathogenic variants have been identified in an affected family member, or by biochemical testing if the biochemical defects have been confirmed in cultured fibroblasts from an affected family member.