NBDHE

This chapter covers the cranial nerves with emphasis on the trigeminal and facial nerves, the arteries and veins of the head and neck, lymph node drainage and palpation, and the fascial spaces through which dental infections spread.

The trigeminal nerve is the main sensory nerve of the face and the target of every dental injection. It has three divisions.

V1 ophthalmic (sensory only) supplies the forehead, upper eyelid and nose. It is not anesthetized in dental procedures.

V2 maxillary (sensory only) leaves through the foramen rotundum into the pterygopalatine fossa and gives off the branches that matter for maxillary anesthesia:

V3 mandibular (sensory and motor) leaves through the foramen ovale. Its motor branches supply the muscles of mastication, mylohyoid, anterior digastric, tensor veli palatini and tensor tympani. Sensory branches:

The facial nerve exits through the stylomastoid foramen, passes through the parotid gland and divides into five motor branches: temporal, zygomatic, buccal, marginal mandibular and cervical. It also carries taste from the anterior two-thirds of the tongue through the chorda tympani and parasympathetic fibers to the submandibular, sublingual, lacrimal and nasal glands.

The common carotid artery divides into internal and external carotid arteries at about the level of the superior border of the thyroid cartilage; the carotid pulse is palpated just anterior to the sternocleidomastoid at this level. The internal carotid supplies the brain and eyes and has no branches in the neck. The external carotid supplies the face and oral structures. Its branches include the superior thyroid, lingual, facial, ascending pharyngeal, occipital, posterior auricular, superficial temporal and maxillary arteries.

The maxillary artery is the larger terminal branch and the most important for oral structures:

Greater palatine, nasopalatine (via sphenopalatine) and middle meningeal arteries.

Venous blood from the face drains mainly to the internal jugular vein through the facial and retromandibular veins. Two venous connections explain serious complications:

• The pterygoid plexus of veins surrounds the lateral pterygoid muscle near the maxillary tuberosity. Overinserting the needle during a PSA block can puncture it and produce a rapidly forming hematoma in the infratemporal fossa that bulges the cheek. The PSA depth for an average adult is about 16 mm with a short needle.
• The facial vein and the pterygoid plexus both communicate with the cavernous sinus inside the skull, through the ophthalmic veins and emissary veins. Head and neck veins generally lack valves, so blood (and infection) can flow in either direction. An infection from a maxillary tooth or the "danger triangle" of the face can therefore travel to the cavernous sinus and cause cavernous sinus thrombosis, a life-threatening condition.

Lymph nodes filter lymph before it returns to the blood. Superficial nodes include the occipital, retroauricular (posterior auricular), anterior auricular, superficial parotid, facial, submental and submandibular nodes. Deep cervical nodes run along the internal jugular vein; the superior deep cervical group includes the jugulodigastric node, which drains the palatine tonsils and posterior tongue. All head and neck lymph eventually reaches the deep cervical nodes, then the jugular trunk, then the thoracic duct (left side) or right lymphatic duct.

A primary node drains a region directly; a secondary node receives lymph from the primary node. A tender, enlarged node points the clinician toward the likely source.

Palpation technique: the patient sits upright with the head tilted slightly forward to relax the tissues. Use the pads of the fingers in gentle circular compression. Submental nodes are palpated with the patient's head tipped down; submandibular nodes by cupping the fingers under the mandible and pressing up and in while the patient tilts toward the side being examined; cervical nodes along and under the sternocleidomastoid. Record size, consistency, mobility and tenderness.

Fascial spaces are potential spaces between layers of fascia that fill with fluid or pus during infection. Location of a tooth apex relative to muscle attachments decides which space an infection enters.

Ludwig angina is a rapidly spreading cellulitis of the submandibular, sublingual and submental spaces bilaterally, usually from mandibular molar infection. The floor of the mouth rises, the tongue is pushed up and back, and the airway can close. It is a medical emergency.

• V2 branches (PSA, MSA, ASA, greater palatine, nasopalatine) supply maxillary teeth and palate; V3 branches (inferior alveolar, lingual, buccal, mental, incisive) supply the mandible.
• The facial nerve passes through the parotid gland; its five branches are temporal, zygomatic, buccal, marginal mandibular and cervical.
• The maxillary artery (from the external carotid) supplies the teeth; the pterygoid plexus explains PSA hematomas.
• Valveless veins connect the face and pterygoid plexus to the cavernous sinus, a route for life-threatening infection.
• Submental nodes drain mandibular incisors; submandibular nodes drain most teeth; hard, fixed, painless nodes need referral.
• Mandibular molar infections below the mylohyoid line enter the submandibular space; bilateral spread is Ludwig angina.