Pathology of Progressive Supranuclear Palsy
Progressive supranuclear palsy is a tauopathy characterized by abnormal accumulation of hyperphosphorylated tau protein (specifically 4-repeat tau) in neurons and glial cells throughout the basal ganglia, brainstem, and frontal cortex, leading to neurodegeneration and cell death 1.
Microscopic Pathology
The defining pathological features include:
- Neurofibrillary tangles (NFTs) composed of hyperphosphorylated 4-repeat tau protein in neurons
- Tufted astrocytes - characteristic glial tau pathology unique to PSP
- Oligodendroglial coiled bodies - tau inclusions in oligodendroglia
- Neuropil threads - tau-positive neuronal processes 2, 3
These lesions distribute in neuroanatomically specific patterns, predominantly affecting:
- Globus pallidus
- Subthalamic nucleus
- Substantia nigra
- Brainstem tegmentum (especially midbrain)
- Superior cerebellar peduncle
- Thalamus
- Frontal cortex (variable involvement depending on clinical variant) 4, 3
Macroscopic Pathology
Gross examination reveals:
- Midbrain atrophy, particularly the tectum and tegmentum
- Loss of pigmentation in the substantia nigra reflecting dopaminergic neuronal death
- Atrophy of the subthalamic nucleus
- Atrophy of the superior cerebellar peduncle
- Variable cortical atrophy, especially frontal regions in certain variants 2, 5
Cellular Pathology
The neurodegeneration manifests as:
- Severe neuronal loss with corresponding gliosis in affected nuclei
- Dopaminergic neuron degeneration in the substantia nigra projecting to the striatum
- 4-repeat tau dominance - PSP shows predominantly four-repeat tau isoforms rather than three-repeat forms, distinguishing it from other tauopathies 4, 3
Pathological Variants
The anatomical distribution of tau pathology varies by clinical phenotype 3:
- Classic PSP-Richardson syndrome: Predominant involvement of basal ganglia, subthalamic nucleus, substantia nigra, and brainstem with limited cortical pathology
- PSP with cortical syndromes (frontotemporal dementia, corticobasal syndrome): Greater cortical tau burden
- PSP-Parkinsonism variants: More severe degeneration in cardinal nuclei (globus pallidus, subthalamic nucleus, substantia nigra) with less cortical involvement
Genetic Factors
The major genetic risk factor is the MAPT H1 haplotype, which may alter expression of specific tau protein isoforms, predisposing to abnormal tau accumulation 2, 6. Rare familial cases have been linked to mutations in the Bassoon (BSN) gene 4.
Key Distinction from Other Parkinsonian Disorders
PSP is a tauopathy, not a synucleinopathy 1. This fundamentally distinguishes it from Parkinson's disease (which shows Lewy bodies containing alpha-synuclein) and multiple system atrophy (which shows alpha-synuclein in oligodendroglia). However, approximately 70% of PSP cases show comorbid pathologies including Alzheimer-type changes, Lewy bodies, or TDP-43 pathology 7.
Clinical-Pathological Correlation
The pathological changes correlate with clinical features:
- Substantia nigra degeneration → parkinsonism and rigidity
- Midbrain tegmentum involvement → vertical gaze palsy
- Brainstem pathology → postural instability and falls
- Frontal cortex involvement → cognitive impairment and behavioral changes 2, 5
The severity and distribution of tau pathology determines the clinical phenotype and rate of progression, with PSP-Richardson syndrome showing the most aggressive pathology and shortest survival (5-7 years), while PSP-Parkinsonism demonstrates slower progression (8-12 years) 6.