To check the trigeminal nerve (CN V), assess both its motor and sensory functions. Here's a breakdown of the examination:
I. Motor Function Assessment:
- Jaw Clench: Ask the patient to clench their jaw tightly.
- Muscle Palpation: Palpate the masseter and temporalis muscles (located on the sides of the face/jaw) bilaterally. Observe for any asymmetry in size (volume) or tone (firmness). Asymmetry may indicate weakness on one side.
- Jaw Opening: Observe the patient as they open their mouth.
- Deviation Check: Look for any deviation (shifting) of the mandible (lower jaw) to one side. Deviation usually indicates weakness in the muscles on the same side as the deviation. For example, if the jaw deviates to the right, it suggests weakness on the right side.
- Resistance to Jaw Opening: Try to open the patient's jaw against their resistance. This assesses the strength of the muscles involved in keeping the jaw closed.
II. Sensory Function Assessment:
The trigeminal nerve has three sensory divisions:
- Ophthalmic (V1): Sensory innervation to the forehead, upper eyelid, and cornea.
- Maxillary (V2): Sensory innervation to the cheek, lower eyelid, upper lip, and maxillary teeth.
- Mandibular (V3): Sensory innervation to the lower lip, chin, mandibular teeth, and anterior two-thirds of the tongue.
To test the sensory function:
- Light Touch Sensation: Use a cotton swab or wisp of cotton to lightly touch each of the three divisions (V1, V2, V3) bilaterally. Ask the patient to close their eyes and indicate when they feel the touch. Compare sensation on both sides of the face. Test at several points within each division.
- Pain Sensation: Use a disposable safety pin or other similar object to gently prick the skin in each of the three divisions bilaterally. Ask the patient to close their eyes and identify the sensation as sharp or dull. Again, compare sensation on both sides and within each division.
- Corneal Reflex (Afferent Limb): Gently touch the cornea (clear front part of the eye) with a cotton wisp. The normal response is blinking. This tests the afferent (sensory) limb of the corneal reflex (trigeminal nerve). The efferent (motor) limb is controlled by the facial nerve (CN VII). This test should be performed with caution, especially in patients with corneal abnormalities.
III. Examination for Seventh Nerve Weakness (Facial Nerve):
- If you suspect seventh nerve (facial nerve) weakness, you can use a tongue blade as a reference.
- Alignment: Line up a tongue blade with the tip of the nose and the center of the mouth. This can help visualize any asymmetry in the mouth's position, which can be a sign of facial nerve weakness.
Summary:
Testing the trigeminal nerve involves assessing the motor function of jaw movement and the sensory function of facial sensation in its three divisions. This includes checking muscle strength, symmetry, and the patient's ability to perceive light touch and pain.
