From MedlinePlus Genetics
The severe, intermediate, and mild forms of Zellweger spectrum disorder were once thought to be distinct disorders. The severe form was known as Zellweger syndrome, the intermediate form was neonatal adrenoleukodystrophy (NALD), and the mild form was infantile Refsum disease. These conditions were renamed as a single condition when they were found to be part of the same condition spectrum. 

People with intermediate or mild Zellweger spectrum disorder have more variable features that progress more slowly than those with the severe form. Affected children usually do not develop signs and symptoms of the disease until late infancy or early childhood. Children with these intermediate and mild forms often have hypotonia, vision problems, hearing loss, liver dysfunction, developmental delay, and some degree of intellectual disability. Most people with the intermediate form survive into childhood, and those with the mild form may reach adulthood. In rare cases, individuals at the mildest end of the condition spectrum have developmental delay in childhood and hearing loss or vision problems beginning in adulthood and do not develop the other features of this disorder.

Children with severe Zellweger spectrum disorder also develop life-threatening problems in other organs and tissues, such as the liver, heart, and kidneys, and their liver or spleen may be enlarged. They may have skeletal abnormalities, including a large space between the bones of the skull (fontanelles) and characteristic bone spots known as chondrodysplasia punctata that can be seen on x-ray. Affected individuals can have eye abnormalities, including clouding of the lenses of the eyes (cataracts) or involuntary, side-to-side movements of the eyes (nystagmus). Severe Zellweger spectrum disorder involves distinctive facial features, including a flattened face, broad nasal bridge, high forehead, and widely spaced eyes (hypertelorism). Children with severe Zellweger spectrum disorder typically do not survive beyond the first year of life.

Individuals with severe Zellweger spectrum disorder usually have signs and symptoms at birth, which worsen over time. These infants experience weak muscle tone (hypotonia), feeding problems, hearing and vision loss, and seizures. These problems are caused by reduced myelin, which is the covering that protects nerves and promotes the efficient transmission of nerve impulses. The part of the brain and spinal cord that contains myelin is called white matter. Reduced myelin (demyelination) leads to loss of white matter (leukodystrophy). 

Zellweger spectrum disorder is a condition that affects many parts of the body. Cases of Zellweger spectrum disorder are often categorizes as severe, intermediate, or mild.  https://medlineplus.gov/genetics/condition/zellweger-spectrum-disorder