What is a cranial nerve assessment and what are the functions of the 12 cranial nerves?

Cranial Nerve Assessment and Functions

A cranial nerve assessment is a systematic examination of the 12 pairs of cranial nerves that emerge from the brain and brainstem to evaluate their functional integrity, which is essential for proper diagnosis of neurological disorders affecting the head and neck region. 1, 2

Overview of Cranial Nerves and Their Functions

CN I: Olfactory Nerve

• Function: Sense of smell (special sensory)
• Assessment: Test ability to identify different scents (coffee, vanilla, etc.)
• Clinical significance: Anosmia may indicate frontal lobe tumors or head trauma 3

CN II: Optic Nerve

• Function: Vision (special sensory)
• Assessment: Visual acuity, visual fields, fundoscopic examination
• Clinical significance: Visual field defects may indicate intracranial lesions 1

CN III: Oculomotor Nerve

• Function: Eye movement (superior, inferior, medial), pupillary constriction, eyelid elevation
• Assessment: Pupillary light reflex, eye movements in all directions, ptosis evaluation
• Clinical significance: Pupil dilation and ptosis may indicate compression (aneurysm) 2

CN IV: Trochlear Nerve

• Function: Downward and lateral eye movement via superior oblique muscle
• Assessment: Test downward gaze when eye is adducted
• Clinical significance: Difficulty looking down and in; diplopia on downward gaze 3

CN V: Trigeminal Nerve

• Function: Facial sensation (ophthalmic, maxillary, mandibular divisions) and jaw movement
• Assessment:
  • Sensory: Light touch and pinprick sensation on forehead, cheek, and jaw
  • Motor: Jaw strength, lateral jaw movements
• Clinical significance: Facial pain, numbness, or weakness in jaw muscles 1, 4

CN VI: Abducens Nerve

• Function: Lateral eye movement via lateral rectus muscle
• Assessment: Test lateral gaze
• Clinical significance: Inability to abduct eye, medial strabismus 3

CN VII: Facial Nerve

• Function:
  • Motor: Facial expression muscles
  • Sensory: Taste (anterior 2/3 of tongue)
  • Parasympathetic: Lacrimal, submandibular, sublingual glands
• Assessment:
  • Test facial expressions (smile, frown, eye closure)
  • Taste on anterior tongue
• Clinical significance: Bell's palsy, facial weakness patterns (central vs. peripheral) 1, 4

CN VIII: Vestibulocochlear Nerve

• Function: Hearing and balance
• Assessment:
  • Auditory: Whisper test, tuning fork tests (Weber, Rinne)
  • Vestibular: Nystagmus, Romberg test
• Clinical significance: Hearing loss, vertigo, tinnitus 5

CN IX: Glossopharyngeal Nerve

• Function:
  • Sensory: Posterior 1/3 of tongue (taste), pharynx sensation
  • Motor: Stylopharyngeus muscle
  • Parasympathetic: Parotid gland
• Assessment: Gag reflex, swallowing, palate elevation
• Clinical significance: Dysphagia, loss of gag reflex 1, 3

CN X: Vagus Nerve

• Function:
  • Motor: Palatal and pharyngeal muscles
  • Sensory: Taste, pharynx/larynx sensation
  • Parasympathetic: Thoracic and abdominal viscera
• Assessment: Voice quality, palate elevation, gag reflex (with CN IX)
• Clinical significance: Hoarseness, dysphagia, uvula deviation 1, 3

CN XI: Spinal Accessory Nerve

• Function: Motor control of sternocleidomastoid and trapezius muscles
• Assessment:
  • Test shoulder shrug against resistance
  • Test head turning against resistance
• Clinical significance: Weakness in head turning or shoulder elevation 1, 4

CN XII: Hypoglossal Nerve

• Function: Motor control of tongue muscles
• Assessment: Tongue protrusion, movement side to side
• Clinical significance: Tongue deviation toward side of lesion, atrophy, fasciculations 1, 3

Systematic Approach to Cranial Nerve Assessment

1. Begin with observation:

   • Note facial symmetry, ptosis, nystagmus, or obvious deficits
   • Observe speech patterns and articulation

2. Proceed in numerical order (I through XII) for comprehensive assessment 6

3. Document findings with specific descriptions:

   • Compare sides (symmetry/asymmetry)
   • Note specific deficits rather than simply "intact" or "abnormal"
   • Grade strength on standard scale when applicable

Clinical Pearls and Pitfalls

• Multiple cranial nerve involvement often indicates lesions at the skull base or brainstem due to the close proximity of cranial nerve nuclei 1

• Imaging considerations: MRI is the gold standard for evaluating cranial nerves, while CT is better for assessing bony foramina and skull base 5

• Common pitfalls:

  • Failing to test all components of complex nerves (e.g., both sensory and motor functions of CN V)
  • Misinterpreting central vs. peripheral facial weakness patterns
  • Overlooking subtle deficits that may indicate early pathology

• Efficiency tip: While a complete assessment examines all nerves, focused testing based on presenting symptoms can be appropriate in specific clinical scenarios 4

By systematically assessing all cranial nerves, clinicians can accurately localize neurological lesions and guide appropriate diagnostic imaging and treatment decisions.