Somatic Nerve Blocks
Trigeminal Nerve Block
The two principal indications for trigeminal nerve block are trigeminal neuralgia and intractable facial cancer pain. Depending on the site of pain, these blocks may be performed on the gasserian ganglion itself, on one of the major divisions (ophthalmic, maxillary, or mandibular), or on one of their smaller branches.
The rootlets of cranial nerve V arise from the brain-stem and join one another to form a crescent-shaped sensory (gasserian) ganglion in Meckel’s cave. Most of the ganglion is invested with a dural sleeve. The three subdivisions of the trigeminal nerve arise from the ganglia and exit the cranium separately. (Figure 47–8A).
Gasserian ganglion block—Fluoroscopic guid-ance is mandatory for the performance of this pro-cedure ( Figure 47–8B). An 8- to 10-cm 22-gauge needle is inserted approximately 3 cm lateral to the angle of the mouth at the level of the upper second molar. The needle is then advanced posteromedial-ly and angled superiorly to bring it into alignment with the pupil in the anterior plane and with the mid-zygomatic arch in the lateral plane. Without entering the mouth, the needle should pass be-tween the mandibular ramus and the maxilla, and lateral to the pterygoid process to enter the cra-nium through the foramen ovale. After a negative aspiration for cerebrospinal fluid and blood, local anesthetic is injected.
Blocks of the ophthalmic nerve and its branches—In this procedure, to avoid denerva-tion-related keratitis, only the supraorbital branch is blocked in most cases (Figure 47–8C); the oph-thalmic division itself is not blocked. The nerve is easily located and blocked with local anesthetic at the supraorbital notch, which is located on the su-praorbital ridge above the pupil. The supratrochlear branch can also be blocked with local anesthetic at the superior medial corner of the orbital ridge.
Blocks of the maxillary nerve and its branches—With the patient’s mouth slightly opened, an 8- to 10-cm 22-gauge needle is inserted between the zygomatic arch and the notch of the mandible (Figure 47–8D). After contact with the lateral pter-ygoid plate at about 4-cm depth (position 1 in fig-ure), the needle is partially withdrawn and angled slightly superiorly and anteriorly to pass into the
pterygopalatine fossa (position 2). Local anesthetic is injected once paresthesias are elicited. Both the maxillary nerve and the sphenopalatine (pterygo-palatine) ganglia are usually anesthetized by this technique. The sphenopalatine ganglion (and ante-rior ethmoid nerves) can be anesthetized transmu-cosally with topical anesthetic applied through the nose; several cotton applicators soaked with local anesthetic (cocaine or lidocaine) are inserted along the medial wall of the nasal cavity into the area of the sphenopalatine recess. The sphenopalatine gan-glion blockade may be helpful for patients with chronic nasal pain, cluster headache, or Sluder’s neuralgia.The infraorbital branch of cranial nerve V passes through the infraorbital foramen, where it can be blocked with local anesthetic. This foramen is approximately 1 cm below the orbit and is usu-ally located with a needle inserted about 2 cm lateral to the nasal ala and directed superiorly, posteriorly, and slightly laterally.
Blocks of the mandibular nerve and its branches—With the patient’s mouth slightly opened (Figure 47–8E), an 8 to10-cm 22-gauge needle is inserted between the zygomatic arch and the man-dibular notch. After contact with the lateral ptery-goid plate (position 1 in figure), the needle is par-tially withdrawn and angled slightly superiorly and posteriorly toward the ear (position 2). Local anes-thetic is injected once paresthesias are elicited.
The lingual and inferior mandibular branches of the mandibular nerve may be blocked intraorally utilizing a 10-cm 22-gauge needle ( Figure 47–8F). The patient is asked to open the mouth maximally and the coronoid notch is palpated with the index finger of the nonoperative hand. The needle is then introduced at the same level (approximately 1 cm above the surface of the last molar), medial to the finger but lateral to the pterygomandibular plica (position 1 in figure). It is advanced posteriorly 1.5–2 cm along the medial side of the mandibular ramus, making contact with the bone (position 2). Both nerves are usually blocked following injection of local anesthetic.
The terminal portion of the inferior alveolar nerve may be blocked as it emerges from the mental foramen at the mid-mandible just beneath the cor-ner of the mouth. Local anesthetic is injected once paresthesias are elicited or the needle is felt to enter the foramen.
Complications of a gasserian ganglion block include accidental intravascular injection, subarachnoid injection, Horner’s syndrome, and motor block of the muscles of mastication. The potential for seri-ous hemorrhage is greatest for blockade of the maxillary nerve. The facial nerve may be uninten-tionally blocked during blocks of the mandibular division.