What is the approach to a comprehensive central nervous system (CNS) examination?

CNS Examination Approach

A comprehensive CNS examination should systematically assess mental status, cranial nerves, motor function, sensory function, reflexes, coordination, and gait, with particular attention to localizing signs that distinguish peripheral from central pathology. 1

Mental Status and Cognitive Assessment

Begin with structured cognitive evaluation using validated instruments:

• Perform Montreal Cognitive Assessment (MoCA) rather than MMSE for initial screening, as MoCA demonstrates 88% classification accuracy with 78% sensitivity and 98% specificity for detecting cognitive impairment, while MMSE frequently misses early deficits 1
• Assess orientation, attention, executive function, language, visuospatial abilities, and memory domains 1
• Evaluate for acute changes in consciousness, behavioral alterations, and cognitive dysfunction that may indicate encephalitis or other acute processes 1
• Document insight and metacognitive awareness, as lack of insight is particularly common in frontotemporal dementia and helps differentiate from psychiatric disorders 1

Cranial Nerve Examination

Systematically test all 12 cranial nerves, understanding that peripheral/nuclear lesions produce ipsilateral symptoms while supranuclear lesions produce contralateral symptoms: 1, 2

CN I (Olfactory)

• Test each nostril separately with familiar odors 1

CN II (Optic)

• Visual acuity, visual fields by confrontation, fundoscopic examination 1

CN III, IV, VI (Oculomotor, Trochlear, Abducens)

• Assess pupillary size, shape, and light/accommodation reflexes 1
• Test extraocular movements in all directions 1
• Check for ptosis (indicates CN III palsy, not CN VI) 3
• CN VI palsy presents with limited abduction and horizontal diplopia without ptosis, while CN III palsy shows ptosis, "down and out" eye position, and possible pupillary dilation 3
• Evaluate for decreased velocity of saccades and smooth pursuit abnormalities 1

CN V (Trigeminal)

• Test facial sensation in all three divisions bilaterally 1
• Assess motor function via jaw opening/closing and masseter strength 1
• Check corneal reflex 1

CN VII (Facial)

• Distinguish peripheral from central lesions: peripheral CN VII palsy causes ipsilateral facial paralysis including forehead, while supranuclear palsy causes contralateral weakness sparing the forehead 2
• Test facial expression strength in upper and lower face 1

CN VIII (Vestibulocochlear)

• Assess hearing acuity and perform Weber/Rinne tests 1

CN IX, X (Glossopharyngeal, Vagus)

• Evaluate palate elevation and uvula deviation 1
• Test gag reflex 1
• Assess voice quality and swallowing 1

CN XI (Spinal Accessory)

• Test shoulder shrug (trapezius) and head turning (sternocleidomastoid) strength 1

CN XII (Hypoglossal)

• Assess tongue protrusion, strength, and fasciculations 1

Critical localization principle: Multiple cranial nerve involvement suggests specific anatomic locations—cavernous sinus affects CN III, IV, VI together; jugular foramen affects CN IX, X, XI together 1, 2

Motor Examination

Assess for both upper and lower motor neuron signs:

• Test muscle strength systematically in all major muscle groups using Medical Research Council grading 1
• Evaluate for parkinsonism: bradykinesia/akinesia, parkinsonian gait/posture, and rigidity (present in 25-80% of frontotemporal dementia cases) 1
• Check for tremor, dystonia, myoclonus, and apraxia 1
• Test for motor neuron signs including fasciculations, atrophy, and weakness 1
• Assess for primitive reflexes (grasp, snout, palmomental) 1

Sensory Examination

• Test light touch, pain, temperature, vibration, and proprioception 1
• Assess cortical sensory functions (stereognosis, graphesthesia, two-point discrimination) 1

Reflex Examination

• Test deep tendon reflexes bilaterally (biceps, triceps, brachioradialis, patellar, Achilles) 1
• Assess plantar responses (Babinski sign) 1

Coordination and Cerebellar Function

• Finger-to-nose and heel-to-shin testing 1
• Rapid alternating movements 1
• Assess for dysmetria, dysdiadochokinesia, and intention tremor 1

Gait and Station

• Observe gait pattern, including timed gait assessment 1
• Test tandem walking 1
• Perform Romberg test 1
• Evaluate for postural instability suggestive of progressive supranuclear palsy 1

Behavioral and Neuropsychiatric Assessment

• Screen for depression using validated scales (Geriatric Depression Scale or CES-D) 1
• Assess behavioral changes using instruments like Neuropsychiatric Inventory 1
• Evaluate functional abilities including instrumental activities of daily living 1

Critical Pitfalls to Avoid

Brainstem lesions produce complex patterns with ipsilateral cranial nerve deficits and contralateral motor/sensory deficits—do not assume simple localization 2

Isolated CN VI palsy can be a false localizing sign from increased intracranial pressure without direct nerve compression 2

Normal MMSE does not exclude significant cognitive impairment, particularly in frontotemporal dementia 1

When major neurologic signs or symptoms are present, obtain neuroimaging (CT/MRI with contrast) before lumbar puncture to exclude mass lesions or elevated intracranial pressure 1

Documentation Requirements

• Record vital signs including blood pressure, heart rate, temperature 1
• Document level of consciousness using standardized scales 1
• Note any asymmetries or focal findings 1
• Assess and document insight into deficits 1