Cerebellum anatomy, function, diseases, and test
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The Cerebellum
1. Anatomy
Gross Structure
The cerebellum is a foliated ("leafy") structure attached to the brainstem, lying dorsal to the pons and medulla, in the posterior fossa. Despite weighing only ~10% as much as the cerebral cortex, its surface area is about 75% of the cortex due to extensive folding (folia). — Ganong's Review of Medical Physiology, 26th ed.
Two transverse fissures divide it into three lobes:
| Lobe | Boundaries |
|---|---|
| Anterior lobe | Anterior to the primary fissure |
| Posterior lobe | Between the primary and posterolateral fissures (largest) |
| Flocculonodular lobe | Posterior to the posterolateral fissure (phylogenetically oldest — archicerebellum) |
The vermis (midline strip) is divided into 10 lobules (I–X). The hemispheres flank it bilaterally.
Functional/Topographic Zones
| Zone | Region | Function |
|---|---|---|
| Vestibulocerebellum | Flocculonodular lobe | Balance, vestibular reflexes, eye movements |
| Spinocerebellum | Vermis + intermediate hemispheres | Gait, axial/limb coordination |
| Cerebrocerebellum | Lateral hemispheres | Planning and fine-tuning voluntary limb movements |
Midline structures control gait and axial musculature; lateral hemispheres control ipsilateral limb coordination. — Bradley & Daroff's Neurology
Cerebellar Peduncles
Three paired connections to the brainstem:
- Superior cerebellar peduncle (brachium conjunctivum) — mainly efferent: deep nuclei → red nucleus → thalamus → cerebral cortex (crosses at the inferior colliculus)
- Middle cerebellar peduncle — afferent only: contralateral pontine nuclei → cerebellum
- Inferior cerebellar peduncle — mixed: afferents from brainstem and spinal cord; efferents to vestibular nuclei
Cortical Layers (3 layers, 5 cell types)
| Layer | Key Cells |
|---|---|
| Molecular (outer) | Basket cells, stellate cells |
| Purkinje cell layer (middle) | Purkinje cells — the only output of cerebellar cortex (inhibitory to deep nuclei) |
| Granular (inner) | Granule cells, Golgi cells |
Input fibers:
- Mossy fibers → granule cells → parallel fibers → Purkinje cell dendrites (excitatory)
- Climbing fibers (from inferior olivary nucleus) → directly onto Purkinje cells (powerful excitatory drive)
Deep Cerebellar Nuclei (medial → lateral)
- Fastigial — receives vermis input; controls axial movement & balance
- Globose + Emboliform (= Interpositus) — intermediate zone input; limb coordination
- Dentate — lateral hemisphere input; voluntary movement planning
The cerebellar cortex → deep nuclei circuit is inhibitory; deep nuclei output is excitatory to thalamus and brainstem. — Adams & Victor's Principles of Neurology, 12th ed.
2. Function
The cerebellum does not initiate movement but acts as a comparator and error-corrector:
"The cerebellum monitors and modulates motor activities, responding to motor commands and inputs from sensory receptors of the joints, muscles, and vestibular system." — Bradley & Daroff's Neurology
Key functions:
- Coordination of movement (smooth, accurate trajectories)
- Planning and execution of movement
- Maintenance of posture and equilibrium
- Coordination of head and eye movements
- Motor learning — updates internal models by comparing intended vs. actual movement; dysfunction causes ataxia and improper motor predictions
- Non-motor/cognitive functions — connections to frontal, parietal, and temporal cortices underlie language, attention, and emotion; dysfunction here causes Cerebellar Cognitive Affective Syndrome (CCAS / Schmahmann syndrome)
The pathway is double-crossed: cerebellar nuclei → crossed thalamic projection → motor cortex → crossed corticospinal tract → spinal cord. Net result: a cerebellar lesion causes ipsilateral deficits. — Adams & Victor's
3. Diseases
Clinical Signs of Cerebellar Dysfunction (DANISH mnemonic)
| Sign | Description |
|---|---|
| Dysdiadochokinesia | Impaired rapid alternating movements |
| Ataxia | Wide-based unsteady gait |
| Nystagmus | End-gaze or bidirectional nystagmus |
| Intention tremor | Tremor that worsens on target approach (differs from resting tremor) |
| Scanning dysarthria | Slurred, monotone, staccato speech |
| Hypotonia | Decreased muscle tone |
Plus: dysmetria (overshoot/undershoot), decomposition of movement, rebound phenomenon, and titubation (nodding truncal tremor in midline lesions).
Disease Classification by Onset
| Acute (hours–days) | Subacute (weeks–months) | Chronic (months–years) |
|---|---|---|
| Stroke (infarct/hemorrhage) | Paraneoplastic (anti-Yo, anti-Tr, anti-VGCC) | Inherited ataxias (SCAs, Friedreich's) |
| Drug toxicity (alcohol, phenytoin, lithium, barbiturates) | Alcoholic/nutritional (B1, B12, B6, E deficiency) | Multiple sclerosis |
| Viral cerebellitis (varicella, EBV) | Anti-GAD65 autoimmune ataxia | Hypothyroidism |
| Posterior fossa subdural hematoma | Gluten ataxia (anti-gliadin) | Chiari malformation |
| — | Lyme disease | Dandy-Walker syndrome |
Source: Harrison's Principles of Internal Medicine, 22nd ed. (Table 450-1)
Key Specific Diseases
Friedreich Ataxia (autosomal recessive, GAA repeat in FXN gene)
- Onset < 25 years; progressive gait ataxia, limb ataxia, dysarthria
- Associated: areflexia, posterior column sensory loss, pes cavus, scoliosis, cardiomyopathy
- Square-wave jerks on eye exam
Spinocerebellar Ataxias (SCAs) — autosomal dominant
-
40 subtypes; SCA1, SCA2, SCA3 (Machado-Joseph), SCA6 are commonest
- SCA2: slow saccades; SCA3: saccadic pursuit, parkinsonism
- Caused by CAG repeat expansions (polyglutamine diseases) or other mechanisms
Multiple System Atrophy (MSA-C) — sporadic; cerebellar + autonomic + parkinsonian features
Paraneoplastic Cerebellar Degeneration — anti-Yo (ovarian/breast), anti-Tr (Hodgkin), anti-VGCC (SCLC); often the presenting manifestation of occult cancer
Posterior Fossa Stroke — focal, ipsilateral cerebellar signs; can cause life-threatening herniation
4. Neurological Tests for Cerebellar Function
Bedside Examination
| Test | Technique | What It Detects |
|---|---|---|
| Finger-to-nose (FNT) | Patient touches their nose then the examiner's outstretched finger (moved each time) | Dysmetria, intention tremor |
| Heel-knee-shin (HKS) | Supine patient slides heel from knee down shin of opposite leg | Lower limb cerebellar coordination |
| Rapid alternating movements (RAM) | Fist-open-fist or index finger tapping on distal thumb as fast as possible | Dysdiadochokinesia |
| Romberg test | Eyes open vs. closed standing — cerebellar ataxia present with eyes open (unlike sensory ataxia which worsens only on eye closure) | Distinguishes cerebellar from sensory ataxia |
| Tandem gait | Walking heel-to-toe | Midline (vermis) ataxia |
| Gait observation | Wide-based, irregular stride, veering | Gait ataxia |
| Rebound test | Sudden release of isometric resistance | Hypotonia, poor check reflex |
| Eye movement exam | Assess fixation, smooth pursuit, saccades | Nystagmus, dysmetric saccades, square-wave jerks |
"Asymmetry of cerebellar testing signs points toward a focal cerebellar lesion on the ipsilateral side." — Rosen's Emergency Medicine
Laboratory and Imaging Workup
Blood tests:
- Vitamins B1, B12, E (deficiency-related ataxia)
- Thyroid function (hypothyroidism)
- Paraneoplastic antibody panel (anti-Yo, anti-Tr, anti-Ri, anti-VGCC, anti-GAD65)
- Antigliadin / anti-transglutaminase antibodies
- Genetic panel for hereditary ataxias
CSF: Useful for inflammatory/paraneoplastic workup
MRI brain (modality of choice):
- Cerebellar atrophy (chronic/hereditary)
- Infarct or hemorrhage (DWI/FLAIR)
- Mass lesion, abscess, or demyelinating plaques
Genetic testing:
- CAG/GAA repeat expansion analysis (SCA1/2/3/6, Friedreich's)
- Whole exome/genome sequencing (WES/WGS) for undiagnosed cases
Diagnostic Workflow for Ataxia
Diagnostic approach: distinguish secondary (known cause) from primary/genetic ataxia. For genetic cases, determine whether family history is present and whether onset was before or after age 45. — Bradley & Daroff's Neurology in Clinical Practice
Key clinical pearl: Cerebellar lesions cause ipsilateral incoordination (unlike UMN lesions which cause contralateral deficits). Midline/vermis lesions → predominantly gait and truncal ataxia. Hemispheric lesions → limb dysmetria and intention tremor ipsilateral to the lesion.
Sources: Costanzo Physiology 7th ed.; Ganong's Review of Medical Physiology 26th ed.; Bradley & Daroff's Neurology in Clinical Practice; Adams & Victor's Principles of Neurology 12th ed.; Harrison's Principles of Internal Medicine 22nd ed.; Neuroanatomy through Clinical Cases 3rd ed.
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