Metachromatic leukodystrophy (MLD) is a rare metabolic disorder characterized by the build-up of sulfatides in cells.  This accumulation of sulfatides especially affects the myelin producing cells of the nervous system, eventually resulting in their degeneration.  As the disease progresses, affected patients suffer from developmental delay, psychomotor regression, spasticity, dysarthria, dysphagia, incontinence, hypotonia and peripheral neuropathy.  Patients may also suffer from seizures and cognitive decline.  Depending on the age of onset, MLD can be further categorized into three types: late-infantile, juvenile and adult-onset disorder.  Late-infantile disorder is the most common type of MLD and it occurs within the second year of life.  Juvenile MLD begins between the ages of 4 and 14 while adult MLD presents after the age of 16.  The severity of the disorder depends on the age of onset, with an earlier onset resulting in a more severe disease progression. 

While the common form of MLD (caused by Arylsulfatase A deficiency) has been shown to affect about 1 in every 40,000 individuals worldwide, Metachromatic Leukodystrophy due to Saposin B Deficiency has only been reported in a handful of cases so far.  Diagnosis of the disorder can be done through urine sulfatide tests, nerve conduction studies, brain imaging tests, and genetic analysis of the PSAP gene.  There is presently no cure for MLD.  However, in certain infantile-onset cases, bone marrow transplantation can help delay the progression of the disorder.  Physical and occupational therapy can also help patients manage their symptoms.  Despite this, the prognosis for MLD remains poor, with an expected life span of about 6-14 years following the onset of symptoms. 

Metachromatic Leukodystrophy due to Saposin B Deficiency follows an autosomal recessive pattern of inheritance.  It is caused by mutations in the PSAP gene.  This gene encodes a sphingolipid activator protein, called saposin B, that is necessary for the lysosomal degradation of sulfatide.  So far about seven mutations in the PSAP gene have been found to cause this disorder.  These are mostly homozygous single nucleotide substitutions.  The mutation C241S has been found in several Arab metachromatic leukodystrophy patients.