Isolated Sulfite Oxidase Deficiency

What else is it called?

  • ISOD

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What causes it?

This disorder is caused by a problem in the SUOX gene. This gene produces the instructions for the sulphite oxidase enzyme which helps breakdown amino acids (the building blocks of proteins) that contain sulphur when they are no longer needed. Sulphur oxidase is specifically involved in the final step of the process where sulphites (sulphur-containing molecules) are converted to sulphates by adding oxygen.

If you have a problem in the SUOX gene, you will not produce enough of the sulphite oxidase enzyme. Therefore, you will be unable to convert sulphites to sulphates in the amino acid breakdown and this causes abnormal accumulation of sulphites in the body. Research suggests that the nervous system is especially sensitive to this accumulation. Excessive levels of sulphur compounds around the brain can also cause brain damage.

There are two forms of ISOD:

  • Classic ISOD: onset occurs around 72 hours into life
  • Late-onset ISOD: onset usually occurs between 6 and 18 months old after experiencing illness involving a fever.

How common is it?

Because of its rareness, the general prevalence of ISOD cannot be estimated, but there have been at least 50 cases mentioned within medical literature.

ISOD is known to affect the sexes equally and do not affect ay ethnic groups specifically.

What are the signs and symptoms?

The signs and symptoms of ISOD differ slightly depending on whether it is the classic or late-onset form. The symptoms of classic ISOD usually develop within 72 hours of birth:

  • Brain dysfunction (encephalopathy)
  • Difficulty feeding
  • Seizures
  • Muscle stiffness
  • Muscle spasms
  • Limited motor skills
  • Head growing at a slower rate than the body

In cases of classic ISOD, the life expectancy is only a few months with children surviving past infancy usually developing displacement of the lenses of the eyes.

In late-onset ISOD, the symptoms are:

  • Developmental delay
  • Developmental regression
  • Muscle tensing
  • Uncontrolled movements of the limbs
  • Difficulty of coordination

Symptoms of late onset ISOD can either get worse over time or they can be episodic. The life expectancy for late-onset is unknown due to its rarity.

How is it diagnosed?

There are various methods for diagnosing ISOD; a sulphite test strip alongside a fresh urine sample can help diagnose ISOD. Molecular genetic testing can be undertaken to test for variants in the SUOX enzyme. Skin cultures (taking a swab using a cotton bud) can also be taken and tested for the absence of sulphite oxidase activity.

MRI and CT scans can be undertaken and are the least invasive methods of diagnosis. The scans can show changes to white matter on the brain and changes in density to specific areas of the brain.

Can it be treated?

There is currently no treatment for the underlying metabolic disorder, so the best form of treatment is based on managing the symptoms of this condition. This includes:

  • Antiepileptic drugs for seizures
  • Medications to reduce spasticity
  • Gastronomy tube to aid difficulty swallowing.

Physiotherapy may be undertaken, especially for the chest in order to prevent respiratory complications. Periodic assessments by a multidisciplinary team will also be performed to monitor nutritional and neurological status alongside other related complications.

Do my family need to be tested?

ISOD is an inherited autosomal recessive disorder meaning that the child will have to have inherited a faulty gene from each parent to develop ISOD. If a faulty gene is inherited from only one parent, the child will be a carrier, and unaffected by ISOD. It is wise to test you family as it can identify carriers within and any undiagnosed cases of ISOD in infants which can help improve treatment. Identifying carriers can also assist in family planning

The parents of the children with ISOD do not need to be diagnosed with ISOD, they’re known as carriers. A carrier is a person that carries one non-working form of the gene, but they do not have the disorder because the other gene in the pair is working correctly. When both parents are carriers of the disorder, the risk of the child developing Hydroxykynureninuria is as follows:  

  • There is a 25% chance that the child is unaffected and not a carrier 
  • There is a 50% chance that the child is unaffected and is a carrier 
  • There is a 25% chance that the child is affected by ISOD

Relevant Organisations


References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing [Resource Library No: AAP002].

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