How do you test and document the function of the 5th (trigeminal) cranial nerve?

Testing and Documenting the 5th Cranial Nerve (Trigeminal Nerve)

Clinical Examination Approach

Test the trigeminal nerve by systematically evaluating both its sensory and motor components across all three divisions (V1-ophthalmic, V2-maxillary, V3-mandibular), documenting specific findings for each branch and comparing symmetry between sides.

Sensory Testing

The trigeminal nerve provides sensory innervation to the face through three distinct divisions that must be tested separately 1, 2:

• V1 (Ophthalmic division): Test light touch and pinprick sensation over the forehead, upper eyelid, and cornea 3
• V2 (Maxillary division): Test sensation over the cheek, upper lip, and area below the eye to above the mouth 2
• V3 (Mandibular division): Test sensation over the lower jaw, lower lip, and chin 2

Key testing technique: Use a cotton wisp for light touch and a disposable pin for pain sensation, comparing responses bilaterally and asking the patient to report any differences in sensation 4, 5.

The corneal reflex specifically tests V1 sensory function (afferent limb) by lightly touching the cornea with a cotton wisp and observing for bilateral eye blink 3.

Motor Testing

The motor component of CN V innervates the muscles of mastication, specifically through the mandibular division (V3) 1, 2:

• Palpate the masseter and temporalis muscles bilaterally while the patient clenches their jaw, assessing for symmetry and bulk 4
• Observe jaw opening: Ask the patient to open their mouth against resistance; weakness causes deviation toward the affected side 2
• Test lateral jaw movement: Have the patient move their jaw side-to-side against your hand 5

Documentation Format

Document findings systematically for each component:

Sensory examination:

• "Light touch intact/diminished/absent in V1/V2/V3 distribution bilaterally"
• "Pinprick sensation intact/diminished/absent in V1/V2/V3 distribution bilaterally"
• "Corneal reflex present/diminished/absent bilaterally"
• Note any asymmetry between sides 4, 5

Motor examination:

• "Masseter and temporalis muscle bulk symmetric/asymmetric"
• "Jaw opening midline/deviates to right/left"
• "Jaw strength 5/5 bilaterally" or note weakness
• "Lateral jaw movements intact bilaterally" 2, 4

Common Pitfalls

Critical caveat: Subtle sensory deficits may be missed without careful bilateral comparison, as patients often cannot detect unilateral changes unless specifically compared side-to-side 5. Always test symmetrically and ask the patient to compare sensations between sides rather than simply asking if they "feel" the stimulus.

Motor testing pitfall: Jaw deviation can be subtle; observe carefully from directly in front of the patient during mouth opening, as deviation toward the weak side indicates ipsilateral motor dysfunction 2.

When Imaging is Indicated

If clinical examination reveals trigeminal nerve dysfunction, MRI is the preferred imaging modality for investigating the nerve 6. The American College of Radiology recommends high-resolution MRI with thin-cut sequences through the entire course of CN V from brainstem nuclei to peripheral branches 6. For trigeminal neuralgia specifically, 3-D heavily T2-weighted sequences combined with MRA are the standard approach 6.