A 2-year-old girl from Tamil Nadu is brought to the pediatric clinic with a 6-month history of progressive gait disturbance, loss of developmental milestones, and seizures. Her parents report that she was normal until age 18 months. On neurological examination, she has hyperreflexia, spasticity, and optic disc pallor. MRI brain shows diffuse white matter involvement. Urine shows elevated sulfatide. Arylsulfatase A activity is markedly reduced in leukocytes. What is the most likely diagnosis?

Explanation

## Diagnosis: Metachromatic Leukodystrophy (Late Infantile Form) ### Clinical Presentation This 2-year-old girl presents with the classic features of **metachromatic leukodystrophy (MLD), late infantile form**: - **Age of onset:** 18 months–4 years (late infantile) - **Progressive neurological decline:** gait disturbance, loss of milestones, seizures - **Spasticity and hyperreflexia** (upper motor neuron signs) - **Optic disc pallor** (CNS white matter involvement) - **Diffuse white matter disease on MRI** (hallmark finding) - **Elevated urinary sulfatide** (diagnostic marker) - **Markedly reduced arylsulfatase A activity** (enzyme deficiency) ### Pathophysiology **Key Point:** MLD results from deficiency of **arylsulfatase A (ASA)**, leading to accumulation of **sulfatide** (cerebroside sulfate) in myelin-forming cells and white matter. 1. ASA deficiency → sulfatide accumulation in oligodendrocytes and Schwann cells 2. Myelin destruction → progressive white matter disease 3. CNS and PNS demyelination → progressive neurological decline, seizures, spasticity 4. Urinary excretion → elevated sulfatide ### Classification of MLD by Age of Onset | Form | Age of Onset | Progression | Key Features | | --- | --- | --- | --- | | **Congenital** | 0–6 months | Rapid (death by 2 years) | Severe hypotonia, seizures from infancy | | **Late infantile** | 18 months–4 years | Rapid (death by 4–8 years) | Gait disturbance, loss of milestones, spasticity, seizures | | **Juvenile** | 4–16 years | Slower | Behavioral changes, cognitive decline, ataxia | | **Adult** | >16 years | Slow | Psychiatric symptoms, cognitive decline, peripheral neuropathy | ### Diagnostic Confirmation **High-Yield:** The **triad of progressive white matter disease + elevated urinary sulfatide + reduced ASA activity** is diagnostic of MLD. **Mnemonic — MLD diagnosis: "SULFATE"** - **S**ulfatide elevated in urine - **U**pper motor neuron signs (spasticity, hyperreflexia) - **L**ate infantile onset (18 months–4 years) - **F**unctional decline (milestones lost) - **A**rylsulfatase A deficiency - **T**ransition to dementia/seizures - **E**arly death without treatment ### Neuroimaging Findings **Clinical Pearl:** MRI shows **diffuse, symmetric white matter involvement** with a characteristic **periventricular and subcortical pattern**. The white matter appears hyperintense on T~2~/FLAIR sequences. ### Laboratory Diagnosis | Test | Finding | | --- | --- | | **Urine sulfatide** | Markedly elevated (>100 μmol/L) | | **Arylsulfatase A activity** | <10% of normal in leukocytes or fibroblasts | | **Nerve conduction studies** | Demyelinating pattern (slowed conduction velocity) | | **MRI brain** | Diffuse white matter disease, periventricular involvement | ### Management **Clinical Pearl:** Hematopoietic stem cell transplantation (HSCT) is the only disease-modifying treatment; early intervention (before significant neurological decline) improves outcomes. Gene therapy is emerging. ## Why This Is the Best Answer The **combination of late infantile onset (18 months) + progressive white matter disease on MRI + elevated urinary sulfatide + markedly reduced ASA activity** is pathognomonic for metachromatic leukodystrophy. The clinical phenotype (spasticity, seizures, optic pallor) and biochemical findings are diagnostic.