Continuing Education Activity

Metachromatic leukodystrophy is a rare lysosomal storage disease caused due to deficient activity of arylsulfatase A. It follows an autosomal recessive pattern of inheritance. It is a serious condition and causes death within 5-6 years in early-onset form. Prompt diagnosis and treatment hel in achieving a better quality of life. This activity describes the etiology, evaluation, and management of metachromatic leukodystrophy and highlights the role of the interprofessional team in evaluating and treating the patients with this condition.

Objectives:

• Describe the etiology of metachromatic leukodystrophy.
• Describe the evaluation of metachromatic leukodystrophy.
• Outline the management options available for metachromatic leukodystrophy.

Introduction

Lysosomal storage diseases (LSDs) are a group of hereditary disorders that disrupt lysosomal function, specifically, enzymes involved in cell metabolism, signaling, substrate processing, innate immunity, apoptosis, and other complex cell recycling processes. This process is extremely complex. The accumulation of undigested or partially processed molecules become toxic for the host cell. The onset tends to predominate in early infancy or childhood, with some disease onset in adulthood. LSDs tend to have a progressive neurodegenerative course and can cause multi-organ failure and, ultimately, death.[1]

Leukodystrophies are inherited disorders that predominantly affect the central nervous system (CNS) white matter tracts, and it's cellular components. These may include glial cells, myelin sheath, and axons. Genetic leukodystrophies tend to combine features of leukodystrophies with development issues caused by inborn errors of metabolism, disorders of DNA transcription, translation, production of critical CNS proteins including myelin, and neuronal cytoskeletal dysfunction.

Metachromatic leukodystrophy is a demyelinating, autosomal recessive genetic leukodystrophy and LSD, caused by an inborn error of metabolism in the arylsulfatase A lysosomal enzyme. This leads to the accumulation of sulfatides, which result in the dysfunction and destruction of the CNS/PNS myelin sheaths. It also accumulates in other organs, including the kidneys, testes, and gallbladder. It can be classified based on the age of onset and clinical features of the disease. All forms of the disease involve a progressive deterioration of neurodevelopment and neurocognitive function.[2]

Etiology

Metachromatic leukodystrophy is caused by deficient activity of arylsulfatase A. In almost all cases, mutations are in the arylsulfatase A gene (ARSA gene), on chromosome 22q13.3-qter. Two alleles, A and I have contributed to approximately 50 percent of cases and are responsible for different clinical expression of the disease.[3] In some cases, it is due to the deficiency of sphingolipid activator protein SAP-B (saposin B), which is responsible for the degradation of sulfatides by ARSA. This form is caused by mutations in the prosaposin gene (PSAP gene).[4][5]

Epidemiology

The prevalence of metachromatic leukodystrophy ranges from 1/40,000 to 1/100,000 in the northern European and North American populations.[6] Incidence is estimated to be 1/40,000 births in the United States of America. There is no sexual and racial predilection. The disease is categorized based on the age of onset.

• Late Infantile form: 6 months to 4 years of age.
• Juvenile form:
  • Early Juvenile: 4 to 6 years of age.
  • Late Juvenile: 6 to 16 years of age.
• Adult form: Beyond 16 years of age.

Pathophysiology

Metachromatic leukodystrophy is a lysosomal storage disease characterized by the inability to degrade sulfated glycolipids, mainly the galactosyl-3-sulfate ceramides. It is caused by deficient activity of lysosomal enzyme arylsulfatase A, most commonly due to mutations in the arylsulfatase A (ARSA gene). During the process, the sulfated glycolipids are degraded into galactocerebroside by the enzyme Arylsulfatase A.

Histopathology

Metachromatic granules may be seen in the tissue specimen. In the nervous system, the loss of myelinated oligodendrocytes is seen.

History and Physical

Leukodystrophies are generally suspected in pediatric patients with difficulties in meeting appropriate development milestones when previously was able to do so. Peripheral neuropathy can present prior to dysarthria and other CNS manifestations.[7][8] A decline in gross and fine motor skills at any age should be evaluated for metachromatic leukodystrophy. Clinical manifestations of the diseases can be categorized by the age at which the disease onset.[9]

• Late infantile onset: Onset < 30 months. Typically early milestones are met, followed by a progressive regression of motor skills, ataxia, dysarthria, hypotonia, spasticity, hypotonia, hyporeflexia, extensor plantar posturing, and optic atrophy.[10] Accounts for approximately 50% of cases. Neuropsychiatric testing can uncover these deficits.
• Juvenile onset: Onset between 30 months and puberty, with a heterogeneous presentation. Early milestones are met, followed by psychomotor regression, intellectual decline (e.g., dropped school performance), behavioral difficulties, personality changes, ataxia, upper motor neuron signs, and peripheral neuropathy. In late juvenile-onset, the patient presents similarly, with +/- seizures. Neurocognitive tests demonstrate dementia, memory loss, disinhibition, impulsiveness, decreased motor function, and optic atrophy.[11]
• Adult onset: Adult-onset (can present as early as adolescence) shows neurocognitive and neuropsychiatric difficulties. Patients may have dysesthesias (peripheral neuropathy), psychosis, schizophrenia, or seizures. It is frequently misdiagnosed with bipolar disorder and dementia. Tends to be exclusively neuropsychiatric with minimal or no motor findings. Neurocognitive tests in this group of patients present similar to the juvenile onset group.

Evaluation

Laboratory Studies: 

Arylsulfatase A enzyme activity in leukocytes or cultured skin fibroblasts may be decreased.[12] Values generally range from undetectable to less than 10 percent of the normal values. However, Metachromatic Leukodystrophy must be distinguished from arylsulfatase A pseudo deficiency (present in approximately one percent of the general population). Patients with arylsulfatase A pseudo deficiency has arylsulfatase A levels ranging from 5% to 20% of normal values without clinical or radiographic disease. The followings tests can be used to differentiate them:

• Urine sulfatide levels
• Radiolabeled sulfatide fibroblast loading
• DNA mutation analysis or next-generation sequencing- biallelic ARSA pathogenic variants

Imaging Studies

Brain MRI shows T2-weighted FLAIR symmetric and confluent hyperintensities diffusely on the frontal and parietal periventricular white matter, which are characteristic of the diseases but nonspecific. T1-weighted images tend to be hypointense, given it is a demyelinating disorder. A normal MRI does not exclude metachromatic leukodystrophy.[13]

Ultrasound or CT abdomen may reveal hyperplastic gallbladder polyps, which can predispose for gallbladder carcinoma.[14]

Additional Tests

The following tests can be conducted with could further help in diagnosing the case. 

• Nerve conduction and ENG studies
  • EMG- can show a demyelinating polyneuropathy pattern affecting both compound muscle (CMAPs) and sensory action potentials (SNAPs)
• Neurocognitive, neuropsychological testing, or both

Procedures

• Lumbar Puncture: Elevated cerebrospinal fluid protein concentrations may be helpful to arrive at the diagnosis.
• Peripheral nerve biopsy: usually not performed, but may show metachromatic lipid deposits

Newborn Screening

• It is still under development. It uses mass spectrometry. However, it can not be distinguished from pseudo deficiency.[15]

Treatment / Management

No curative treatment options are currently available for this disease—the focus in the enhancement of the quality of life by focusing on symptom management. Symptomatic supportive care is needed to address to neurocognitive and neuropsychiatric disturbances, seizures, dystonias, spasticity, feeding problems, and constipation.

Symptomatic Treatment

• Seizures - broad spectrum anti-epileptics such as levetiracetam, zonisamide, lacosamide, and valproic acid
• Spasticity - baclofen, cyclobenzaprine, botox toxin A
• Dystonia - botox toxin A
• Dysautonomia (drooling, orthostasis) - anticholinergic therapy, oral midodrine or fludrocortisone, 
• Pain - NSAIDs, gabapentin, pregabalin, serotonin-norepinephrine reuptake inhibitors (SNRIs), etc
• Nutrition and GI issues - percutaneous feeding tubes, famotidine or pantoprazole, bowel regimen (docusate, senna)
• Insomnia/mood issues - SNRI such as mirtazapine, tricyclic anti-depressants (TCAs), serotonin reuptake inhibitors (SSRI), etc
• Arthralgias/myalgias, mobility, functional impairments in activities of daily living - physical, occupational, cognitive, and gait therapy[16]

Genetic Counseling

MLD is inherited in an autosomal recessive manner. At conception, each sib of an affected individual 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 of at-risk family members and prenatal testing for a pregnancy at increased risk are possible if both ARSA pathogenic variants have been identified in an affected family member.

Experimental and emerging therapies:

• A combination of gene therapy and hematopoietic stem cell transplantation (HSCT)- Offers the most promise at this time. The benefits may be more apparent in the asymptomatic or early symptomatic juvenile-onset disease. HSCT is not recommended for individuals with symptomatic, late-infantile form. Studies show that patients with juvenile disease may have a 5-year survival of approximately 59%. However, many patients decline post-transplant (based on measures such as imaging, gross motor function, cognitive skills, nerve conduction velocities, and other metrics). Therefore, the neurocognitive function may be stabilized by bone marrow transplantation; however, there is still a progressive loss of the motor function. In late juvenile and adult-onset forms, bone marrow transplantation may slow the disease progression.[17][18]
  • Phase I/II clinical trial gene therapy (using retroviral vector technology) plus autologous hematopoietic stem cell transplantation: A study of asymptomatic late-onset infantile disease, juvenile disease, and early-onset juvenile disease that showed delayed onset of MRI abnormalities and recovery of some gross motor function and cognition.[19][20]

• Gene therapy, enzyme replacement, and small molecule therapy (e.g., substrate reduction and chaperone therapies)

Hence, preliminary evidence suggests that gene therapy and hematopoietic stem cell transplantation combined with gene therapy are promising treatment options.[21] However, the cellular pathogenesis of metachromatic leukodystrophy is complex. Several other LSDs have used disease-specific gene and enzyme replacement therapies with some success. Small-molecule therapies are emergent therapy for some LSDs. Although gene therapy and genome microRNA editing are at advanced preclinical stages, there is a need for phase III/IV clinical trials.

Differential Diagnosis

Metachromatic leukodystrophy must be differentiated from other LSDs with similar presentation and with arylsulfatase A pseudodeficiency. Arylsulfatase A pseudodeficiency can be differentiated using gene mutation analysis or evaluation of radiolabeled sulfatide fibroblast uptake and accumulation. Other differentials that must be kept in mind while diagnosing metachromatic leukodystrophy are:

• Krabbe diseases:[22][23] LSD that is autosomal-recessive, caused by deficient activity of the beta-galactosidase. Clinically it presents with irritability, hypertonia, hyperesthesia, and psychomotor arrest.

• X-linked adrenoleukodystrophy:[24][25] Primarily affects boys, it is a leukodystrophy that presents with adrenal insufficiency, neurocognitive (low IQ) and neurobehavioral issues (ADHD-like behavior), dysarthria, dysgraphia, deficits in vision, hearing. They are found to have elevated very-long-chain fatty acids (VLCFA), abnormal white matter disease on MRI, and a pathogenic variant of the ABCD1 gene on molecular genetic testing.

• Canavan disease:[26][27] Primarily affecting the Ashkenazi Jewish population, it is an infantile autosomal-recessive leukodystrophy characterized by intellectual disability, irritability, macrocephaly, dysphagia, early hypotonia, late spasticity, ataxia, seizures, and optic atrophy. It is progressive and neurodegenerative in nature. Typically they have elevated N-acetyl aspartate (NAA) in urine, abnormal diffuse white matter disease on MRI, and pathogenic variants in ASPA on molecular genetic testing.

• Peroxisomal biogenesis disorders (e.g. Zellweger disease):[28][29] Leukodystrophy caused by a mutation in one out of 13 different PEX genes, PEX1 being the most common, leading to dysfunctional peroxisomes. Clinical symptoms may include intellectual delay, craniofacial dysmorphia, retinopathy (e.g. glaucoma, retinitis pigmentosa), visual blindness, sensorineural deafness, hypotonia, hepatomegaly with coagulopathy, diffuse jaundice, dysphagia, chondrodysplasia punctata, and seizures. MRI can show neocortical dysplasia, diffuse white matter atrophy, and ventriculomegaly with cysts. Elevated transaminases and hyperbilirubinemia are common.

• Polyglucosan body disease:[30][31] An autosomal recessive adult-onset disease characterized by progressive neurogenic bladder, gait difficulties (e.g. spasticity and weakness) from mixed upper and lower motor neuron involvement, sensory loss predominantly in the distal lower extremities, and some cognitive difficulties (often executive dysfunction). MRI of the brain and spinal cord, and sural nerve biopsy shows clusters of polyglucosan deposition. The glycogen brancher enzyme (GBE) activity in skin fibroblasts is abnormal, and molecular genetic testing reveals a mutation in GBE1.

• Fucosidosis:[32][33] very rare, caused by FUCA1 mutation, characterized by coarse facies, recurrent opportunistic infections, generalized dystonia, spasticity, dysostosis multiplex, diffuse angiokeratoma, and organomegaly

• Childhood-onset schizophrenia:[34] Childhood-onset schizophrenia is a severe form of psychotic disorder that occurs at age 12 years or younger. It has some neurologic features and has to be differentiated from metachromatic leukodystrophy. Schizophrenia presents with delusion, hallucinations, disorganized speech, grossly disorganized or catatonic behavior, and negative symptoms.

Prognosis

Metachromatic leukodystrophy is a progressive disease. This means that the symptoms tend to get worse over time. People who have this disease lose all muscle and mental functions eventually. Lifespan often depends on the age at which a person is first diagnosed. 

• Late infantile form: The prognosis is worse than later-onset forms of the diseases; progression to death typically occurs within five to six years.
• Juvenile form: Progression is slower in this form of the disease, and the patients may survive until early adulthood.
• Adult form: The disease course in this form of the disease may be static or of insidious progression.

Complications

Therapies focus on the quality of life, and functional activities of daily living can help in areas of mobility, cognition, communication, and oral intake. Safety measures are to avoid falls at home. The most common complications of the disease include:

• Neurocognitive decline (dementia)
• Blindness (e.g., optic atrophy)
• Malnutrition
• Aspiration pneumonia
• Death (5 to 6 years in the late infantile form)

Consultations

Regular consultations with followings specialists are generally needed:

• Pediatric and adult general neurologists
• Epileptologists
• Dietitians and nutritionists
• Gastroenterologists
• Genetic counselor and specialist
• Pediatrician
• Ophthalmologist

Deterrence and Patient Education

Patients often find it difficult to carry out activities of daily living as the condition worsens. Patients and family members should be properly counseled about the progressive nature of the disease and the prognosis. Some of the related complications and co-morbidities are gastroesophageal reflux, constipation, dental caries, impaired vision, among others. Pharmacological management, physical therapies, and family support can help prevent further decline of the patient and improve quality of life.

Metachromatic leukodystrophy is an autosomal recessive condition. Parents require counseling about the inheritance pattern of the condition if the family has a positive family history of the condition.

Enhancing Healthcare Team Outcomes

Metachromatic leukodystrophy is an autosomal recessive lysosomal disorder that results in a buildup of sulfatides that leads to the destruction of the myelin sheath, leading to progressive demyelination of the central and peripheral nervous system. Once the diagnosis is made, an interprofessional approach is vital. 

Medical centers with specialty teams can offer information about the disorder, coordinate care among specialists, help evaluate options, and provide treatment. Primary care physicians, neurology physicians, pathologists, radiologists, physiotherapists, among others, can form a collaborative team for the best possible outcome. A physical therapist, occupational therapist, orthopedist, ophthalmologist, neuropsychologist, and other specialists may be involved are often needed for long term follow up and evaluation. Working with a nutrition specialist (dietitian) can help provide proper nutrition. Eventually, it may become difficult to swallow food or liquid. This may require assistive feeding devices as the condition progresses.

The role of the nurse in education is indispensable. The patient and the family need to know about the course of the disease, lifestyle modifications, and the need to follow up. The physical and occupational therapist should be consulted to assist with ambulation, use of an ambulatory device, and how to perform daily living activities. Couples with a family history of the disease should be offered genetic counseling during pregnancy.