Parkinson's disease and Parkinson-plus syndromes
Classical idiopathic Parkinson's disease is caused by $\alpha$-synuclein accumulation, with up to 90% loss of substantia nigra neurons by the time clinical symptoms appear. To assess dopaminergic deficit, functional methods are most sensitive: PET (PET) and SPECT (SPECT). Structural MRI in primary parkinsonism is usually normal and is primarily used to rule out focal lesions (e.g., vascular ischemia) and specific atypical syndromes.
Parkinson-plus syndromes have characteristic radiological markers:
- Progressive supranuclear palsy (PSP): selective midbrain atrophy forming the hummingbird sign (hummingbird sign) on sagittal slices.
- Multiple system atrophy (MSA): pronounced atrophy of the pons. In the cerebellar variant (MSA-C), FLAIR and T2 show hyperintensity in the middle cerebellar peduncles and pons ("hot cross bun" sign). In the parkinsonian variant (MSA-P), signal intensity increases along the lateral margins of the putamen.
- Dementia with Lewy bodies (DLB): unlike Alzheimer's disease, hippocampal volumes on MRI remain normal, while dopamine SPECT shows abnormalities.
Infectious and prion causes of dementia
A wide spectrum of infections can lead to rapidly progressive cognitive decline. It is important to differentiate them at early stages.
- Creutzfeldt-Jakob disease (CJD): a prion disease manifested by subacute dementia and myoclonus (CSF shows elevated tau protein and 14-3-3 protein). The most sensitive MRI sequence is DWI. In early stages, diffusion restriction is visible in the neocortex and striatum (putamen) due to microvacuole formation. Involvement is often bilateral.
- HIV encephalitis: the virus affects the brain directly. FLAIR shows ill-defined symmetric white matter hyperintensity. The key sign is the sparing of subcortical U-fibers.
- Progressive multifocal leukoencephalopathy (PML): an opportunistic infection caused by the JC papovavirus in immunocompromised patients. The virus destroys oligodendrocytes, causing massive demyelination. MRI shows lesions in the subcortical white matter (bright on T2, dark on T1) with a cortical "stripping" sign. Contrast enhancement is mild or absent.
Toxic, metabolic, and autoimmune encephalopathies
Central nervous system lesions in these disorders often have specific locations:
- Wernicke encephalopathy: associated with vitamin B1 deficiency (often in alcoholism). MRI (T2/FLAIR) shows hyperintensity in the medial thalami, mamillary bodies, and periaqueductal gray matter. Microbleeds are possible (visible on T1).
- Vitamin B12 deficiency: diffuse involvement of the deep and periventricular white matter of the hemispheres with a spared brainstem. In the spinal cord, the posterior and lateral columns are affected (without contrast enhancement).
- Leukodystrophies and genetic syndromes: typically present with strictly symmetric white matter lesions. An exception is mitochondrial diseases (e.g., MELAS), where lesions can be asymmetric. In FXTAS genetic syndrome in men over 50, hyperintensity of the middle cerebellar peduncles is typical.
- Autoimmune limbic encephalitis: can be paraneoplastic (associated with lung cancer) or non-paraneoplastic (antibodies to NMDA, VGKC). Coronal FLAIR slices show edema and hippocampal hyperintensity, sometimes involving the basal ganglia (as in Hashimoto's encephalopathy).
Other degenerative diseases and white matter lesions
Most white matter lesions in elderly patients are of vascular origin. However, other pathologies must also be considered. For example, when suspecting Alzheimer's disease, classical T2-weighted images may be insufficient. Gradient-echo imaging (T2-GRE*) reveals hundreds of small lobar microbleeds (hemosiderin "blooming" effect) due to concomitant amyloid angiopathy.
Rare non-parkinsonian movement disorders include:
- Huntington's disease: manifested by caudate nucleus atrophy.
- Neurodegeneration with brain iron accumulation (NBIA): T2* signal drop in the basal ganglia.
- Wilson's disease: midbrain hyperintensity on T2.
Role of MRI in epilepsy
Although epilepsy is classified separately (by seizure type, level of consciousness, and brain onset), neuroimaging methods play a critical role in finding its substrate. MRI is indicated for all patients routinely, except for benign childhood and genetic generalized forms. The main goal is to find a structural brain anomaly generating pathological activity (MRI sensitivity in drug-resistant patients reaches 85%). In urgent situations (trauma, acute focal symptoms), CT remains the method of choice.
Briefly: answers to topical questions
What changes are detected on MRI in Creutzfeldt-Jakob disease (CJD)?
In Creutzfeldt-Jakob disease, MRI reveals bilateral involvement with a combination of neocortical and subcortical findings.
- FLAIR sequence — moderate signal hyperintensity in multiple neocortical areas (discontinuous pattern) and in the striatum (especially the putamen / putamen).
- DWI sequence — abnormal signal is significantly more prominent in early stages due to microvacuoles causing diffusion restriction (may disappear in late stages).
At disease onset, involvement may be unilateral. Contrast administration is useful to rule out other pathologies.
What are the characteristic radiological markers of progressive supranuclear palsy (PSP) on MRI?
The main radiological marker of progressive supranuclear palsy on MRI is marked midbrain atrophy (mesencephalon).
- Hummingbird sign (hummingbird sign) — on mid-sagittal slices, the brainstem with an atrophic midbrain resembles a bird with a beak.
- "Penguin silhouette sign" (penguin sign) — a specific tegmental shape phenomenon.
- Indirect signs of atrophy — reduced anteroposterior diameter (less than 14 mm), concave midbrain contour, and a decreased midbrain-to-pons area ratio.
Enlargement of the third ventricle, cerebellar peduncle atrophy, putamen (putamen), frontal and parietal lobe atrophy are also observed. Signal enhancement of the superior cerebellar peduncles (on FLAIR), globus pallidus (globus pallidus), and red nucleus (nucleus ruber) is noted.
Which MRI changes are characteristic of vascular parkinsonism?
Vascular parkinsonism on MRI is characterized by vascular lesions, primarily focal lesions of the basal ganglia.
- Vascular-ischemic lesions — detected in the basal ganglia and other parts of the brain.
- Absence of "strategic zone" involvement — there is a mismatch between the nature and severity of clinical manifestations and the localization and extent of vascular damage.
Detection of vascular lesions on MRI with normal tracer uptake on DAT-SPECT rules out primary idiopathic Parkinson's disease.
What MRI features are pathognomonic for corticobasal degeneration?
Sources do not specify strictly pathognomonic MRI features for corticobasal degeneration, but characteristic changes are described. MRI diagnostics reveal asymmetric parietotemporal brain atrophy. Additional clinical signs (dysautonomia, gaze palsy, cerebellar and pyramidal signs) assist in differential diagnosis, but are not pathognomonic for this disease either.
What are the radiological criteria and MRI features characteristic of normal pressure hydrocephalus (Hakim-Adams syndrome)?
The primary MRI feature of normal pressure hydrocephalus (Hakim-Adams syndrome) is marked enlargement of the cerebral ventricles. This neuroimaging presentation must be evaluated in conjunction with the classic clinical triad, which includes urinary incontinence and cognitive impairment. The diagnosis is confirmed by a positive tap test, characterized by clinical improvement after cerebrospinal fluid removal.
What are the neuroimaging features of Wilson's disease on brain MRI?
Brain MRI in Wilson's disease (hepatolenticular degeneration) reveals evidence of copper accumulation within central nervous system structures.
- Midbrain — signal hyperintensity is observed on T2-weighted images.
- Subcortical nuclei — specific copper deposition is visualized.
These changes are accompanied by decreased blood ceruloplasmin levels and the presence of Kayser-Fleischer rings on ophthalmological examination.
To memorize the MRI features of atypical parkinsonism: The bird peeks at the bun. Bird — PSP (hummingbird sign in the midbrain), Bun — MSA (hot cross bun sign in the pons).
Class and exam questions
How to differentiate dementia with Lewy bodies (DLB) from Alzheimer's disease on MRI?
In Lewy body dementia, the hippocampi typically remain of normal volume on structural MRI, whereas Alzheimer's disease is characterized by their marked atrophy.
Which MRI sequence is mandatory when prion disease (CJD) is suspected?
It is critically important to use diffusion-weighted imaging (DWI). It allows for the detection of restricted diffusion due to microvacuoles in the cortex and striatum at the very earliest stages of the disease.
What is the main radiological distinction between HIV encephalitis and PML?
In HIV encephalitis, the pathological process in the white matter typically spares the subcortical U-fibers. In contrast, in progressive multifocal leukoencephalopathy (PML), these fibers are actively destroyed, producing the cortical "ribboning" sign.
What is the role of neuroimaging in typical Parkinson's disease?
Structural MRI is performed not to confirm idiopathic parkinsonism, but to rule out secondary causes (e.g., ischemic lesions in the basal ganglia) and features of Parkinson-plus syndromes.
What else to study on this topic
- Specifics of SPECT and FDG-PET applications in neurodegenerative disorders.
- Differential diagnosis of autoimmune and infectious limbic encephalitis (including herpes simplex encephalitis).
- ILAE 2017 classification of epileptic seizures.
- Radiological presentation of rare leukodystrophies and metabolic white matter disorders.
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