Leigh syndrome (also known as subacute necrotizing encephalomyelopathy) is a rare, devastating neurodegenerative disorder characterized by bilateral, symmetric lesions in the brainstem, basal ganglia, and cerebellum. The disease typically presents in infancy or early childhood with progressive neurological deterioration, including loss of motor skills, respiratory failure, and metabolic crises. Leigh syndrome represents the most common inherited mitochondrial disorder and serves as a paradigm for understanding the relationship between mitochondrial dysfunction and neurodegeneration.
Leigh syndrome was first described by the British neurologist Denis Leigh in 1951, who reported cases of infants with progressive encephalopathy, lactic acidosis, and characteristic neuropathological findings of necrotizing lesions in specific brain regions. The disease results from defects in mitochondrial energy metabolism, leading to impaired ATP production and progressive neuronal death.
The hallmark neuropathological finding is bilateral, symmetric necrotizing lesions with spongiform changes, neuronal loss, and capillary proliferation in the brainstem, basal ganglia, and cerebellum. These lesions are thought to result from episodes of severe metabolic decompensation, leading to energy failure and cell death in vulnerable brain regions.
¶ Genetics and Molecular Biology
Leigh syndrome can result from pathogenic variants in over 100 different genes, with inheritance patterns including:
- Autosomal recessive: Most common (e.g., SURF1, PDHA1, NDUFS1)
- Maternal inheritance: MT-ATP6, MT-ND genes (mitochondrial DNA)
- X-linked: PDHA1 (most common X-linked form)
- Complex I genes: NDUFS1, NDUFS2, NDUFS4, NDUFAF6
- Complex IV genes: SURF1, COX10, COX15, SCO1
- Complex V genes: ATP5F1A (MTATP1)
- Pyruvate dehydrogenase genes: PDHA1, PDHB, DLAT
- Coenzyme Q genes: COQ8A (ADCK3), COQ9
- Assembly factors: Various complex-specific factors
- MT-ATP6: ATP synthase subunit 6
- MT-ND1, MT-ND5, MT-ND6: Complex I subunits
- MT-CO1, MT-CO2, MT-CO3: Complex IV subunits
- MT-CYB: Cytochrome b (Complex III)
The underlying pathophysiology involves impaired mitochondrial energy metabolism:
- Reduced ATP production: Impaired oxidative phosphorylation
- Increased reactive oxygen species: Mitochondrial dysfunction increases ROS
- Apoptotic pathways: Activation of cell death programs
- Neuroinflammation: Microglial activation
- Metabolic crises: Episodes of severe decompensation
Leigh syndrome typically presents in:
- Infancy: Most common (6-18 months)
- Early childhood: Can present up to age 5-6
- Adolescence/adulthood: Rare, milder variants
Early signs often include:
- Developmental delay: Failure to meet milestones
- Hypotonia: Low muscle tone
- Feeding difficulties: Poor suck, difficulty feeding
- Lethargy: Unusual tiredness
- Vomiting: Recurrent episodes
- Loss of previously acquired motor skills
- Progressive spasticity
- Ataxia and incoordination
- Dystonia
- Hypotonia (in some)
- Apneustic breathing: Abnormal breathing pattern
- Central hypoventilation: Respiratory failure
- Episodes of respiratory crisis: Often during illness
- Ophthalmoplegia: Eye movement paralysis
- Optic atrophy: Vision loss
- Nystagmus: Involuntary eye movements
- Strabismus: Misaligned eyes
Episodes of acute metabolic decompensation:
- Triggers: Illness, stress, fasting
- Features: Lethargy, vomiting, acidosis
- Outcome: Often leads to neurological deterioration
- Lactic acidosis: Elevated blood and CSF lactate
- Seizures: Can occur, particularly during crises
- Cardiomyopathy: In some genetic forms
- Peripheral neuropathy: In some variants
- Growth failure: Poor weight gain
The diagnosis is suspected based on:
- Progressive neurodegenerative course: Characteristic onset and progression
- Bilateral symmetric brain lesions: On MRI
- Elevated lactate: In blood or CSF
- Family history: May suggest inheritance pattern
MRI findings are characteristic:
- Bilateral symmetric lesions: In brainstem, basal ganglia, cerebellum
- T2 hyperintensity: Abnormal signal in affected regions
- Basal ganglia involvement: Putamen, caudate, globus pallidus
- Brainstem lesions: Particularly in midbrain and medulla
- Cerebellar involvement: In some cases
Characteristic patterns by genetic subtype:
- SURF1: White matter spongiform changes
- MT-ATP6: basal ganglia lesions often with diffuse cerebral involvement
- Lactic acidosis: Elevated blood lactate (2-15 mmol/L)
- Elevated CSF lactate: Even when blood lactate normal
- Abnormal organic acids: Urine analysis may show pattern
- Ketones: Elevated during metabolic crises
- Pyruvate dehydrogenase activity: In PDH-deficient forms
- Complex I-V activities: In OXPHOS deficiencies
- Fibroblast testing: Can confirm some forms
Genetic testing provides definitive diagnosis:
- Targeted panels: Mitochondrial disease gene panels
- Whole exome sequencing: Often identifies causal variant
- Mitochondrial genome sequencing: For mtDNA mutations
- Whole genome sequencing: In complex cases
May show:
- Ragged-red fibers: In some forms
- Reduced complex activities: On enzyme histochemistry
- Abnormal mitochondria: On electron microscopy
¶ Treatment and Management
No cure exists. Management focuses on:
- Supportive care: ICU-level support during crises
- Metabolic interventions: Bicarbonate for acidosis
- Seizure control: Anticonvulsant medications
- Nutritional support: IV fluids, feeding as needed
- Dietary modifications: Ketogenic diet in some forms
- Coenzyme Q10: Supplementation in some cases
- L-carnitine: For carnitine deficiency
- Thiamine: May help in PDH deficiency
- Sodium bicarbonate: For chronic acidosis
- Physical therapy: Maintain function
- Occupational therapy: Daily activity support
- Speech therapy: If swallowing difficulties
- Nutritional support: May require gastrostomy
- Respiratory support: May require BiPAP or ventilator
- AAV vectors: For nuclear-encoded genes
- Mitochondrial gene therapy: Novel approaches being developed
- Allotopic expression: Mitochondrial gene replacement
- EZH2 inhibitors: Being studied
- NAC and cysteine prodrugs: For glutathione deficiency
- Bypassing OXPHOS defects: Metabolic intermediates
- Neural stem cells: In development
- Mesenchymal stem cells: Being studied
Pre-implantation genetic diagnosis options:
- Mitochondrial replacement therapy: For mtDNA mutations
- Embryo selection: For couples at risk
Leigh syndrome typically shows:
- Progressive decline: Over months to years
- Plateau periods: May have periods of stability
- Episodic crises: Leading to stepwise deterioration
- Variable rate: Some forms slower than others
Prognosis varies by genetic form:
- Most severe: Death within 2-3 years of onset (common)
- Milder variants: Survival into adolescence or adulthood
- MT-ATP6: Often survive to adulthood with support
- Age of onset: Earlier onset often worse
- Genetic form: Specific variant influences course
- Residual enzyme activity: Higher activity often better
- Treatment response: Ketogenic diet helps some
- Supportive care quality: Affects outcomes
The neuropathological hallmark is bilateral, symmetric necrotizing lesions:
- Spongiform changes: Vacuolization of neuropil
- Neuronal loss: Death of neurons in affected regions
- Astrocytic gliosis: Proliferation of astrocytes
- Capillary proliferation: New blood vessel formation
- Microglial activation: Inflammatory response
Commonly involved:
- Basal ganglia: Putamen, caudate, globus pallidus
- Brainstem: Midbrain, pons, medulla
- Cerebellum: Particularly deep nuclei
- Spinal cord: Often involved
- Dorsal root ganglia: May be affected
Leigh syndrome is the most common mitochondrial disorder:
- Incidence: 1 in 30,000-40,000 births
- Carrier frequency: Higher in consanguineous populations
- Accounts for: ~30% of childhood mitochondrial disease
Cases reported worldwide with:
- Founder mutations: In specific populations
- Higher in: Regions with consanguinity
- Autosomal recessive: ~75% of cases
- Mitochondrial (maternal): ~20-25%
- X-linked: ~5% (mostly PDHA1)
Several models have been developed:
- Mouse models: Ndufs4 knockout, Surf1 knockout
- Zebrafish models: For high-throughput screening
- Drosophila models: For genetic studies
- Induced models: iPSC-derived neurons
¶ Understanding Pathogenesis
Current research focuses on:
- Mechanisms of selective neuronal vulnerability
- Role of metabolic crises in lesion formation
- Neuroinflammation in disease progression
- Relationship to other mitochondrial diseases
Key areas include:
- Gene therapy for specific genetic forms
- Small molecule approaches to bypass OXPHOS defects
- Neuroprotective agents
- Metabolic modulators
Priorities include:
- Disease progression markers
- Treatment response biomarkers
- Pre-symptomatic detection