Isolated sulfite oxidase deficiency (ISOD) is a rare autosomal-recessive metabolic disorder caused by biallelic pathogenic variants in SUOX. Sulfite oxidase cannot function normally, leading to accumulation of toxic sulfite metabolites and severe neurological injury.
Typical severe presentation
Classic ISOD usually begins within the first hours or days of life with intractable seizures, feeding difficulty and rapidly progressive encephalopathy.
Abnormal tone, opisthotonus, spasticity and progressive microcephaly can follow.
Why it can resemble birth asphyxia
Severe neonatal ISOD can produce MRI and clinical features similar to hypoxic-ischaemic encephalopathy.
The diagnosis should be considered when the neurological deterioration is unexplained by the birth history.
Seizure types
Multifocal myoclonic, tonic-clonic and other neonatal seizures are reported.
Seizures are often resistant to conventional antiseizure medicines.
Late-onset disease
Milder disease can present later in infancy or childhood with developmental regression, dystonia, choreoathetosis, ataxia or metabolic stroke-like episodes.
This means absence of a catastrophic neonatal presentation does not completely exclude the disorder.
Ectopia lentis
Lens subluxation or dislocation can develop after the newborn period and is an important diagnostic clue.
Biochemical diagnosis
Elevated urinary S-sulfocysteine and other sulfite-related markers support the diagnosis.
Unlike molybdenum cofactor deficiency, serum uric acid is not typically reduced through loss of xanthine dehydrogenase activity.
Genetics
The disorder is caused by biallelic SUOX variants.
Molecular confirmation distinguishes it from MoCD, which has overlapping clinical features.
Treatment
There is currently no established enzyme- or cofactor-replacement therapy for isolated SUOX deficiency.
Treatment is supportive and includes seizure management, nutrition, respiratory care, management of spasticity and rehabilitation.
Dietary approaches
Specialist low-cysteine or sulfur-amino-acid approaches have been considered in selected patients.
Evidence is limited and these diets require expert metabolic supervision.
Why fosdenopterin does not treat ISOD
Fosdenopterin bypasses the early biosynthetic defect in MoCD type A.
In isolated sulfite oxidase deficiency, the sulfite oxidase enzyme itself is defective, so replacing an upstream molybdenum-cofactor intermediate does not correct the problem.
Prognosis
Classic neonatal disease is usually devastating, with severe neurodevelopmental disability and high early mortality.
Late-onset phenotypes can have a more variable course.
Inheritance
ISOD is autosomal recessive.
Identifying the familial SUOX variants permits carrier, prenatal and preimplantation testing where available.
Worldwide access
The major challenge is recognising a very rare metabolic condition among the many causes of neonatal encephalopathy.
Urinary metabolic testing can provide an important clue even before sequencing results return.
The central point
ISOD and MoCD can look very similar clinically, but they are not the same disorder and do not have the same treatment options.
Related reading
Molybdenum Cofactor Deficiency and Epilepsy →
Metabolic Epilepsies →
Sources
Medical note: severe neonatal seizures with unexplained encephalopathy require urgent metabolic investigation.
Evidence checked: September 2026.