What are the key symptoms and neurological signs used to localize a central nervous system lesion?

CNS Localization: Symptoms and Signs

Localizing CNS lesions requires systematic recognition of specific symptom patterns that correspond to distinct neuroanatomical regions, from the cerebral cortex through the brainstem to the spinal cord, with each level producing characteristic combinations of motor, sensory, and cranial nerve deficits.

Fundamental Localization Principles

Cranial neuropathy can result from pathology affecting nerve fibers at any point from the CN nucleus to the end organ, requiring detailed knowledge of CN anatomy coupled with careful neurological examination for proper clinical localization 1.

Key localization concepts include:

• Pathologic processes affecting the brain, CN nuclei, or nerve fiber tracts are often associated with multiple neurologic symptoms 1
• Because of close proximity of many CN nuclei and segments, some lesions may result in multiple cranial neuropathies 1
• Lower motor neuron signs and dermatome defects suggest peripheral nerves; sensory dissociation points to the spinal cord; cranial nerve deficit with contralateral extremity deficit suggests brainstem etiology 2
• Higher level organized dysfunction points to the cerebrum; ataxia during intentional movements indicates cerebellar involvement; sudden unintended movements suggest basal ganglia disorders 2

Brainstem Localization

Midbrain Lesions

• Oculomotor nerve (CN III) involvement produces ipsilateral ptosis, mydriasis, and eye movement deficits 1
• Trochlear nerve (CN IV) involvement causes vertical diplopia worsening on downward gaze 1

Pontine Lesions

• Pontine lesions produce alternating syndromes because CN VI courses through the corticospinal tracts within the pons, resulting in contralateral hemiparesis with ipsilateral cranial nerve deficits 3
• Frequently accompanied by ipsilateral facial paralysis because CN VII curves over the CN VI nucleus 3
• Classic pontine syndromes causing variable involvement of CN V-VIII include Millard-Gubler syndrome, Foville syndrome, locked-in syndrome, and facial colliculus syndrome 3
• Ischemic and hemorrhagic infarcts are the most frequent cause of acute brainstem syndromes affecting cranial nerves V-VIII 3

Medullary Lesions

• Ipsilateral facial sensory loss occurs if the spinal trigeminal nucleus is involved 3
• Dysphagia and dysarthria occur when CN IX and X nuclei are affected 3
• The glossopharyngeal nerve (CN IX) arises in the medulla and is responsible for elevation of the palate; its involvement causes dysphagia 3
• Lateral medullary lesions affect vestibular structures, causing vertigo, nystagmus, and ataxia 3

Spinal Cord Localization

Cervical Cord

• Multiple discrete focal lesions with cigar-like appearance on sagittal imaging and wedge-shaped on axial imaging 1
• Lesions typically small, less than 2 vertebral segments, involving less than half the cord 1
• Location typically cervical-thoracic, peripheral region, lateral and posterior columns with central grey matter not spared 1
• Spondylotic myelopathy is the most common cause of chronic extradural compression in the cervical spine 4

Red Flags for Alternative Diagnoses

• Longitudinal extensive transverse myelitis affecting 3 or more vertebral segments suggests NMOSD 1
• Leptomeningeal/root enhancement suggests neurosarcoidosis 1
• Lesions involving only grey matter suggest NMOSD, infections, or ischemic myelopathy 1

Cerebral Hemisphere Localization

Periventricular Lesions

• Ovoid/round shape, at least 3mm along main axis, asymmetric distribution 1
• Location abutting lateral ventricles without intervening white matter 1

Juxtacortical/Cortical Lesions

• Location touching or within the cortex 1
• Deep white matter lesions separated from cortex should be excluded 1

Infratentorial Lesions

• Location in brainstem, cerebellar peduncles and hemispheres; contiguous to cisterns or floor of fourth ventricle 1
• Surface of pons and pontine trigeminal root entry zone 1
• Uni- or bilateral paramedian location in medulla oblongata 1

Leptomeningeal Involvement

Symptoms and signs are related to specific CNS areas involved and are typically multifocal 1.

Most Frequent Manifestations

• Headache 1
• Nausea and vomiting 1
• Mental changes 1
• Gait difficulties 1
• Cranial nerve palsies with diplopia or visual disturbance (CN VI, III, IV, II) and hearing loss (CN VIII) 1
• Radicular signs including weakness, voiding and cauda equina problems 1
• Focal or irradiating (radicular) neck and back pain 1

Critical Considerations

• Some symptoms are related to increased intracranial pressure due to CSF circulation disturbances and can be rapidly alleviated by CSF drainage 1
• Bladder, sexual and bowel dysfunction are possibly underreported and should be explored at presentation 1
• A detailed neurological examination using a standard evaluation form should be carried out at diagnosis 1

Primary CNS Lymphoma Localization

Common Sites

• Frontal lobe and other brain hemisphere areas (38%) 1
• Thalamus or basal ganglia (16%) 1
• Corpus callosum (14%) 1
• Periventricular regions (12%) 1
• Cerebellum (9%) 1
• Meninges (16%) 1

Common Neurological Symptoms

• Focal deficits including hemiparesis and ataxia (70%) 1
• Neuropsychiatric symptoms or personality changes (43%) 1
• High intracranial pressure (33%) 1
• Seizures (14%) 1
• Headache, confusion, cognitive dysfunction and lethargy 1

Critical Diagnostic Pitfalls

• CT scanning should NOT be routinely ordered as initial imaging for posterior fossa ischemic strokes, as sensitivity is as low as 10% 3
• False-negative DWI occurs with very small ischemic brainstem infarcts; thin-section coronal DWI detects nearly 25% more acute brainstem infarcts than standard axial DWI 3
• In patients with multiple cranial neuropathies, imaging is useful to exclude intraparenchymal process; CSF analysis and serologic studies may be indicated for extraparenchymal causes 1
• MRI is the standard modality for imaging CNs, with 3.0T preferred over 1.5T for superior signal-to-noise ratios and spatial resolution 1
• Use of intravenous contrast is imperative for evaluation of cranial neuropathy with MRI 1