Dermatomes of the Face
- A dermatome represents the area of skin that provides cutaneous innervation by fibers of a specific nerve. Most of the skin on the body is innervated by spinal nerves, but the face receives cutaneous innervation from a cranial nerve (known as the trigeminal nerve).
- The trigeminal nerve is known as the 5th cranial nerve (CN V) and is the largest cranial nerve in the body. It gives rise to 3 divisions or branches known as the ophthalmic division (V1), maxillary division (V2), and mandibular division (V3), which provide cutaneous (sensory) innervation to the skin on the face.
- Only the mandibular division carries motor fibers, which innervate muscles of mastication.
- As it relates to skin innervation of the face and neck there is also contribution from the cervical spinal nerves (C2, C3, C4). Note: The spinal nerve at C1 lacks any significant afferent input from the skin and thus is the only spinal nerve without any representation as a dermatome.
- Innervation of the skin includes: Forehead, scalp, upper eyelid, cornea, & dorsum of the nose.
- Cutaneous Branches of CN V1: Supraorbital n., supratrochlear n., lacrimal n., infratrochlear n., external nasal n.
- Clinical Pearls:
- Corneal Reflex: This nerve is tested during the "corneal reflex" which is an involuntary blinking of the eyes when the cornea receives either tactile, painful, or thermal input. The input travels down the afferent fibers, but it is the facial nerve (CN VII) that sends impulses down the efferent fibers to cause contraction of the orbicularis oculi muscle resulting in the blinking of the eye.
- Ophthalmic Herpes Zoster: The V1 division is most commonly involved in a herpes virus infection involving the trigeminal nerve. When an infection occurs along this division, vesicles can erupt along the branches of the nerve to also include involvement of the cornea of the eye. Recognizing this is clinically important as it can lead to painful corneal ulcerations and scarring. You should associate vesicles on the tip of the nose (Hutchinson's Sign) with ophthalmic involvement due to their shared innervation.
- Innervation of the skin includes: Lower eyelid & conjunctiva, lateral nose, anteroinferior nasal septum, cheeks, upper lip. It also innervates the mucosa of the maxillary sinus, premolar, canine, and incisor of the maxillary teeth.
- Cutaneous branches of CN V2: Infraorbital, zygomaticofacial, zygomaticotemporal
- Clinical Pearls:
- Infraorbital Nerve Block: When repairing skin of the upper lip and/or cheek, infiltration of area around the infraorbital nerve with lidocaine can provide local anesthesia and pain relief to the patient. This is done by injecting the anesthetic in the area of the infraorbital foramen as a result of elevating the upper lip and passing the needle through the area where the gingiva and oral mucosa meet. Aspiration prior to injection at this area is necessary since blood vessels also exit the infraorbital foramen along with nerve.
- Innervation of the skin includes: Lower lip, chin, anterior 2/3 of the tongue, anterior aspect of the external ear
- Motor innervation of muscles include: Of the three divisions, V3 is the only one with motor fibers. It innervates the muscles of mastication (medial & lateral pyerygoid muscle, masseter muscle, temporalis muscle)
- Cutaneous branches of V3: Auriculotemporal n., buccal n., mental n.
- Clinical Pearl:
- Submental Nerve Block: Infiltration of the area of the submental foramen will provide local anesthesia to the lower lip and mucosal membrane of the lower lip when repairing lacerations to this area. It is accomplished by injecting anesthetic in the area of the mental foramen, which can be reached by retracting the lower lip and passing a needle inferiorly between the gingiva and the oral mucosa between the premolars.
The division that passes anteriorly from the trigeminal ganglion to pass through the foramen rotundum at the base of the greater wing of the sphenoid bone.
The V3 division is the largest and exits the trigeminal ganglion inferiorly and passes through the foramen ovale.