A 62-year-old man presents with progressive tremor at rest, rigidity, and bradykinesia over the past 3 years. Neuroimaging and laboratory investigations are unremarkable. What is the most common pathological hallmark found in Parkinson disease?

Explanation

## Pathological Hallmark of Parkinson Disease **Key Point:** Lewy bodies are the cardinal pathological feature of Parkinson disease, found predominantly in the substantia nigra pars compacta (SNpc), locus coeruleus, and other brainstem nuclei. ### Lewy Body Composition Lewy bodies are intracytoplasmic inclusions composed of: - **Alpha-synuclein** (primary component) - Ubiquitin - Neurofilament proteins - Other associated proteins ### Distribution in Parkinson Disease | Region | Significance | |--------|-------------| | Substantia nigra pars compacta | Most affected; causes dopamine depletion | | Locus coeruleus | Early involvement; norepinephrine loss | | Dorsal motor nucleus of vagus | Autonomic dysfunction | | Substantia innominata | Cognitive decline | | Cerebral cortex | Lewy body dementia in advanced PD | **High-Yield:** The loss of dopaminergic neurons in the SNpc (up to 60–70% neuronal death) due to Lewy body pathology directly correlates with the motor symptoms of PD. ### Clinical Correlation **Clinical Pearl:** Lewy body pathology can extend beyond the SNpc in PD, explaining non-motor symptoms such as autonomic dysfunction, mood disorders, and cognitive decline in advanced disease. ### Braak Staging of PD Pathology Lewy body pathology spreads in a predictable pattern: 1. Dorsal motor nucleus of vagus → brainstem 2. Substantia nigra and locus coeruleus 3. Mesocortical and neocortical regions This rostral progression explains the evolution from motor to non-motor symptoms. **Mnemonic:** **LBPD** = **L**ewy **B**odies in **P**arkinson **D**isease — the defining neuropathology.