Intracranial Operations

A local anesthetic for intracranial operations should ideally have a duration of 3 to 5 hours, because the procedure usually lasts that long. Lidocaine or ropivacaine without epinephrine produces anesthesia for about 1 hour; however, when administered with 1:200,000 epinephrine, it may last 3 or more hours. Epinephrine also helps to control bleeding in this highly vascular area and slows the absorption of local anesthetic. The blood level rises more slowly, lessening the likelihood of systemic toxicity. Indeed, Hunter13 stated that 0.25% lidocaine with epinephrine 1:400,000 induced effective anesthetic of the scalp. Lidocaine, bupiv- acaine, or ropivacaine may be regarded as the agents of choice for local anesthetic in neurosurgery, depending on the duration.

The sensory innervation of the scalp and cranium is via the trigeminal and cervical nerves. The forehead is supplied by the supraorbital and supratrochlear branches of the frontal nerve and by the zygomaticotemporal nerve, which originates in the second division of the trigeminal nerve. The temporal region receives its innervation from the zygomaticotemporal nerve and the auriculotemporal nerve, a branch of the third division of the trigeminal nerve. The parietal and occipital regions take their innervation from the greater and lesser occipital nerves derived from the cervical nerves. Because these nerves converge toward the vertex and become subfascial on a line encircling the head just above the ear and passing through the occiput and glabella, it is possible to anesthetize the scalp of the operative field by making subcutaneous and subfascial injections of 0.5% to 1.0% lidocaine along this line.13 Where the skull is covered by muscle, these layers must be infiltrated to obtain complete anesthesia. The dura is pain-sensitive only at the base of the skull. The brain and cerebellum are insensitive. Sensory innervation of the dura at the base of the skull is supplied by several means ( Figs. 24-1 and 24-2 ):

1. The meningeal branch of the vagus serves the dura of the posterior fossa, especially the region of the lateral and occipital sinuses.

2. The recurrent branch of the ophthalmic nerve supplies the tentorium cerebelli.

3. A recurrent branch of the maxillary nerve separates before it leaves the skull and supplies the middle fossa.

4. A recurrent branch of the mandibular nerve accompanies the middle meningeal artery through the foramen spinosum and divides into an anterior branch, which supplies the greater wing of the sphenoid and adjacent dura, and a posterior branch, which supplies the mucous membrane of the mastoid air cells.

It is often difficult to cut a bone flap without pulling on or tearing across a branch of the middle meningeal artery, which results in the stretching of nerve fibers accompanying this structure. The brain is tethered within the skull by arteries and nerves that traverse the subarachnoid space. Retraction of the brain

Figure 24-1 Innervation of the scalp and cranium. (From Labat G: Regional Anesthesia, Its Technique and Clinical Application. Philadelphia, WB Saunders, 1923.)

for surgical exposure may produce pain from tension on these structures. This effect is especially true for the posterior fossa, wherein lie the cranial nerves carrying sensory impulses from the head and neck.™

The foregoing considerations make gasserian ganglion block helpful for operations involving the base of the skull and adjacent dura mater. The ganglion lies in Meckel's cave, which is an invaginated fold of dura in continuity with cerebrospinal fluid situated at the apex of the petrous temporal bone at the junction of the middle and posterior cranial fossas. It is bounded medially by the cavernous sinus containing the carotid artery and the third, fourth, and sixth cranial nerves. Superiorly is the inferior surface of the temporal lobe, and posteriorly the brainstem. Any of these structures could be damaged by the anesthetic needle entering the middle cranial fossa via the foramen ovale. Because

Figure 24-2 Area of analgesia after blocking of the nerves of the scalp. A subcutaneous and subfascial injection of local anesthetic on line A-A[1] produces analgesia in the frontal, parietal, and occipital regions. (From Labat G: Regional Anesthesia, Its Technique and Clinical Application. Philadelphia, WB Saunders, 1923.)

the ganglion is partially bathed in cerebrospinal fluid, local anesthetic injected into the area might spread to produce an effect on remote parts of the central nervous systems

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