Central Nervous System Causes of Chronic Imbalance
The primary CNS causes of chronic imbalance are cerebellar ataxia (from neurodegeneration, inflammatory/infectious, or neoplastic processes affecting the cerebellum and brainstem) and sensory/proprioceptive ataxia (from spinocerebellar tract, dorsal column, or dorsal root ganglia involvement). 1
Cerebellar Ataxia
Cerebellar ataxia represents loss of motor coordination due to cerebellar dysfunction and manifests as disequilibrium with loss of muscle coordination. The key anatomic structures involved are:
- Cerebellum (morphologic atrophy and signal alterations)
- Brainstem (concurrent involvement common)
Etiologic Categories
The inherited and acquired causes include:
- Inherited forms: Autosomal dominant spinocerebellar ataxias (SCAs), autosomal recessive, congenital, mitochondrial, and X-linked types 1
- Sporadic adult-onset: Toxic/nutritional, immune-mediated, infectious, degenerative, and genetic reclassifications 1
- Inflammatory/neoplastic: Require contrast-enhanced imaging to distinguish from degenerative causes 1
Critical diagnostic point: Conventional neuroimaging may be completely unremarkable in early disease despite significant clinical symptoms. Patterns of parenchymal injury on imaging may suggest specific ataxia subtypes, but substantial overlap exists among etiologies. 1
Sensory/Proprioceptive Ataxia
This represents loss of coordination from impaired position sense rather than cerebellar dysfunction. The hallmark clinical feature is worsening with eyes closed (positive Romberg test). 1
Anatomic Substrates
CNS structures involved include:
- Spinocerebellar tracts
- Dorsal columns of spinal cord (cervical cord atrophy is a diagnostic finding) 1
- Dorsal root ganglia
Specific Syndromes
- Spinocerebellar ataxias with spinal involvement: SCAs can manifest with sensory ataxia when spinal structures are affected 1
- Cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS): Combined cerebellar ataxia and sensory neuropathy contributing to disequilibrium 1
- Autoimmune sensory ataxic neuropathies: Cause disequilibrium without cerebellar or brainstem involvement 1
Diagnostic Approach
MRI head without IV contrast is the preferred initial imaging modality for evaluating both cerebellar and sensory ataxia. 1 However, this approach has important limitations:
When to Add Contrast
Lack of contrast impairs ability to distinguish degenerative from inflammatory/infectious causes of cerebellar ataxia. Add contrast when there is concern for inflammatory or neoplastic etiology. 1
Advanced Imaging Techniques
DWI/diffusion-tensor imaging and spectroscopy show promise for:
- Detecting early changes before morphologic atrophy appears
- Distinguishing between ataxia subtypes 1
Spinal Imaging
MRI cervical and thoracic spine is indicated when sensory ataxia symptoms are disproportionate to clinical findings, as cervical spinal cord atrophy occurs in several degenerative ataxia forms. 1
Common Pitfalls
Normal initial MRI does not exclude cerebellar ataxia: Early disease may show no morphologic changes on conventional imaging 1
Overlapping imaging features: Different ataxia subtypes can appear identical on imaging, requiring clinical correlation and genetic/laboratory testing for definitive diagnosis 1
Missing spinal involvement: In sensory ataxia, focusing only on brain imaging misses critical spinal cord pathology 1
Coexisting vestibulopathy: Many cerebellar ataxia patients have concurrent central or peripheral vestibular dysfunction that confounds the clinical picture 1
Additional CNS Considerations
While the guidelines focus on ataxia syndromes, other CNS causes of chronic imbalance in elderly populations include normal-pressure hydrocephalus and cerebellar ataxia from various degenerative processes. 2 The combination of multiple risk factors is common in older adults, requiring systematic evaluation of all potential contributors.