Number	Name	Origin	Action
I	Olfactory	Telencephalon	Smell
II	Optic	Diencephalon	Vision
III	Oculomotor	Mesencephalon	Somatic motor for eyeballs, iris, and ciliary body
IV	Trochlear	Mesencephalon	Somatic motor for eyeballs
V	Trigeminal	Metencephalon	Motor and sensory
VI	Abducens	Myelencephalon	Somatic motor
VII	Facial	Myelencephalon	Motor and sensory
VIII	Vestibulocochlear	Myelencephalon	Hearing and balance
IX	Glossopharyngeal	Myelencephalon	Visceral motor and sensory, general sensory
X	Vagus	Myelencephalon	Visceral motor and sensory, general sensory
XI	Spinal accessory	Myelencephalon	Visceral motor
XII	Hypoglossal	Myelencephalon	Somatic motor

The blood supply to the brain is derived from the carotid and vertebral arteries. The arterial component collectively joins at the circle of Willis (Fig. 37-4). The venous drainage is a series of valveless, bidirectional venous sinuses that communicate directly with the vertebral venous sinuses. The patterns of infectious spread and metastasis can be easily noted via the venous system that ranges from the pelvis to the cranium.⁷

FIG. 37-4 Cerebral vasculature. A, Principal arteries of brain. B, Circle of Willis and lateral view of cerebral circulation.

Cerebrum

The right and left hemispheres of the cerebrum are connected centrally by the corpus callosum, a broad band of nerve fibers. The cerebrum, also known as the telencephalon, occupies most of the area within the cranium and is arranged into superficial folds (called gyri) and furrows (called sulci), which are surgical anatomic landmarks. The outer cerebral cortex is the gray matter; the inner tissue is the white matter. Cranial nerve I originates here. Each hemisphere of the brain is divided into four anatomic lobes:

1. The frontal lobe lies within the anterior fossa.

2. The parietal lobe lies in the superior and anterior portion of the middle fossa.

3. The temporal lobe lies inferior to the frontal and parietal lobes within the middle fossa.

4. The occipital lobe lies posteriorly within the middle fossa.

The hypothalamus and thalamus, referred to as the diencephalon, also lie within the cerebrum to form the floor and lateral walls of the third ventricle. All afferent impulses, except smell, pass through here to the cerebrum. The optic chiasma forms the anterior border.

The diencephalon controls body temperature, emotion, hunger, thirst, sleep, and some hormones. Although it lies in the sella turcica outside the cerebrum, the pituitary body attaches to the inferior aspect of the hypothalamus.

The cerebral pedicles and the corpora quadrigemina form the midbrain, which is also referred to as the mesencephalon. The cerebral aqueduct runs through the full length of the structure. Cranial nerves III and IV originate here.

Brainstem

The brainstem lies anteriorly within the posterior fossa. It extends from the cerebral hemisphere to the base of the skull, where it merges with the spinal cord.

Pons and cerebellum

The pons and cerebellum form the metencephalon. The pons is a bridge between the cerebrum and the medulla. It lies anterior to the bilobed cerebellum and is the origin of cranial nerves V, VI, VII, and VIII. The cerebellum lies below the occipital lobes of the cerebrum, posterior to the brainstem, within the posterior fossa. It is about one fifth the size of the cerebrum. The cerebellum controls motion and equilibrium.

Medulla

The medulla oblongata forms the floor of the fourth ventricle and contains the origin of cranial nerves IX, X, XI, and XII. It contains the vital centers, such as cardiac, vasomotor, and respiratory function.

Four spaces within the brain are referred to as ventricles (Fig. 37-5). The lateral ventricles, one in each hemisphere of the cerebrum, drain into the foramen of Monro. The foramen of Monro opens into a central cavity, the third ventricle, which is connected by the aqueduct of Sylvius, with the fourth ventricle lying anterior to the cerebellum and posterior to the brainstem. CSF is a clear substance produced in the choroid plexuses, which are vascular extensions of the pia mater lining the ventricles. CSF is predominantly produced in the lateral ventricles and circulates through the subarachnoid space around the meninges covering the brain and spinal cord. The normal adult volume of circulating CSF is 125 to 150 mL. Obstruction of the CSF flow causes increased ICP.

Special considerations in neurosurgery

Neurosurgical procedures are classified according to the anatomic location in the nervous system: brain and cranial nerves, spinal cord and nerve roots, or autonomic and somatic peripheral nerves. Regardless of the location of the surgical site, neural tissue is handled gently to minimize functional disability from surgical trauma. Hemostasis is a critical factor to sustain the vital functions of circulation and respiration. The visibility of structures in the surgical site also should be ensured.

Diagnostics

Magnetic resonance imaging

Magnetic resonance imaging (MRI) provides a three-dimensional image of the complex structures of the brain, revealing tumors and aneurysms. Magnetic resonance spectroscopy is a variant of MRI that can differentiate necrotic tissue from vital tissue after the application of radiation to reduce tumor size.

Computed tomography

Computed tomography (CT) creates a detailed image of the brain with or without contrast media; however, CT is not as accurate as MRI for diagnosis of brain tumor types. CT is useful for diagnosis of trauma and bleeding into surrounding tissues. Sometimes CT is used to observe for tumor recurrence.

Positron emission tomography

Positron emission tomography (PET) depicts the brain activity more than delineation of structure. It is used to distinguish living tissue from necrotic tissue and can be used to determine tumor grade after diagnosis. Interventional radiosurgery is facilitated by PET scan data.

Digital holography

Three-dimensional mapping is done with holograms. This investigational technique is currently being studied for application in brain mapping and tumor diagnosis.

Prognostics

The survivability of a brain tumor is determined by several factors:

• Malignant or benign

• Cell type and location

• Duration of cell growth

• Surgically treatable

• Tumor grade (grades IV and V are the worst)

• Patient age; age extremes have worse prognosis

• Ability to function

• Symptomatic conditions

Methods of hemostasis

The hemostatic methods commonly used by neurosurgeons for most procedures include the following.

Scalp clips

The scalp is highly vascular. Bleeding is controlled with Leroy or Raney clips placed over the edges of the wound with a special clip applier as the primary incision is made. These disposable clips are used as temporary wound pressure for hemostasis and are removed during closure of the wound.

Compressed absorbent patties, made of rayon, cotton, or polyester, rather than gauze sponges are used on fragile delicate neural tissues to absorb blood and fluids. They are also used for protection of wound edges and for hemostasis. Assorted sizes are moistened with normal saline solution, Ringer’s lactate solution, or topical thrombin and pressed out flat on a smooth surface that is easily accessible to the neurosurgeon. Although the patties have no loose fibers, they could pick up lint if placed on a towel.

The standard for sponge counts includes counting these patties. They are retrieved before the surgical site is closed. Patties have a radiopaque thread securely attached to each one. This reminds the surgeon that they are in the wound and facilitates their removal. Care is taken not to suction patties into the suction tip. Some of the patties are as small as ¼ inch square. All patties are counted, although they are detectable by x-ray. The strings should not be cut off.

Chemical agents

Chemical hemostatic agents may be used after resection to control bleeding from large vessels, sinuses, or the surface of a tumor bed. The scrub person moistens a gelatin sponge with normal saline solution or topical thrombin before handing it to the neurosurgeon. Microfibrillar collagen, supplied in fibers or knitted sheet form, is applied dry. The tips of tissue forceps should be dry to prevent this substance from adhering to them. Oxidized cellulose also is applied dry but is removed after hemostasis has been attained because it can interfere with wound healing.

Ligating clips

Clips are applied on larger vessels where electrocoagulation would be insufficient or its thermal effect would be hazardous. Some permanent clips, such as intracranial aneurysm clips, are specifically designed only for neurosurgical use. Titanium is preferred because it is nonmagnetic and causes less interference with diagnostic cranial studies, such as CT and MRI. Each type and size of clip requires a specific applier.

Electrosurgery

Monopolar electrosurgical unit (ESU) current is used to cut and coagulate tissue and small vessels of dermis and epidermis. Current is conducted through hemostatic forceps, fine smooth-tipped tissue forceps, or a metal suction tip to bleeding vessels. A combination suction-fulguration tip also may be used. The monopolar current passes through the patient’s tissues and returns to the generator via the return electrode.

Bipolar current is frequently used for more precision in coagulation of tiny vessels without damage to surrounding tissue. The bipolar current does not travel through the patient’s tissue to a return electrode.

Lasers

Argon, CO₂, potassium titanyl phosphate (KTP), neodymium: yttrium aluminum garnet (Nd:YAG), and tunable dye lasers are selectively used in neurosurgery in conjunction with the operating microscope, endoscopes, and stereotaxis.⁸^,¹² Each type has benefits, but all have definite limitations.

Depending on the type of laser and the location and type of tumor or vascular lesion, the laser may vaporize, shrink, or coagulate tissue. Precise hemostasis, minimal damage to contiguous structures, and visualization of effects are definite advantages of laser surgery for removal of brain and spinal cord tumors. Lasers are also used to assist in microvascular anastomoses.

Ultrasonic aspiration

Although not technically a method of hemostasis, ultrasonic emulsification and aspiration devices fragment and remove tissue with minimal bleeding. The emulsification process assists in hemostasis of small vessels. The technique can be used to debulk or remove benign tumors in relatively inaccessible areas of the brain and intramedullary spinal cord. The high-frequency sound waves of the ultrasonic probe fragment the tumor while sparing adjacent structures, such as nerves and blood vessels. The tumor is emulsified and removed using suction. Various settings on the instrument allow the surgeon to adjust for removal of firm or calcified tumor or soft masses.

Interventional neuroradiology

With fluoroscopy, a team consisting of a neurosurgeon and a neuroradiologist may insert a percutaneous transfemoral catheter into a strategic point in the intracranial circulation that feeds an arteriovenous malformation, an aneurysm, or a vascular occlusion. A substance such as silicone or isobutyl 2-cyanoacrylate or a detachable microballoon or coil is injected to effectively embolize the lesion or its major deep-feeding arteries. This facilitates surgical resection of the lesion by minimizing potential hemorrhage. The intravascular procedure may be done preoperatively or intraoperatively.

Adjuncts to visibility

Neural tissues should be as clean, dry, and visible as possible without damaging them. Visibility is enhanced by the following procedures.

Irrigation

Most wounds are irrigated frequently with normal saline solution or Ringer’s lactate solution. The scrub person should keep a bulb syringe filled and ready for use. The solution should be maintained at close to body temperature or cooler to prevent vasodilation and excessive bleeding.

The tip of the bone drill burr should be irrigated during use to prevent overheating the tissues (Fig. 37-6).

FIG. 37-6 Irrigation at the tip of the drill is important to prevent overheating the tissues. Eye protection is needed to prevent splash of bone chips and solution.

Suction

Suction is necessary to evacuate blood, CSF, and irrigating fluid from the surgical site so that the neurosurgeon can identify structures. Necrotic tissue, pus, or cystic matter also may be aspirated. A Frazier tip is usually used. Caution is taken to avoid applying vacuum directly on healthy neural tissue, especially brain tissue. This tissue is protected by compressed absorbent patties. Suction should be available for all neurosurgical procedures.

Retractors

A variety of self-retaining retractors, such as Leyla or Greenberg bed-mounted styles, is used to retract scalp or skin and muscles. Dura mater is usually retracted with traction sutures. Blunt, malleable, flat, and spoon-shaped spatulas are used to retract brain tissue. Because visibility of structures is critical, a retractor with a fiberoptic lighting system incorporated into it may be used, especially for intracranial procedures.

Headlight

A fiberoptic headlight is used by most neurosurgeons for supplemental lighting in the surgical site.

Endoscope

A side-viewing fiberoptic endoscope may be used to enhance visibility at obscure angles in otherwise visually inaccessible areas. This endoscope is particularly useful for identification of lesions in the sella turcica, cerebral aneurysms, and intervertebral discs, for example.¹² An endoscope may be used for placement of electrodes for stimulators or a catheter for radioisotopes.

The argon laser can be used through a ventriculoscope for intraventricular obliteration of the choroid plexuses and for intravascular treatment of lesions and neoplasms. The Nd:YAG laser can be used through a flexible or rigid neuroscope to vaporize a cyst. A straightforward, rigid, zero-degree scope may be used for electrocoagulation or laser obliteration of tissue and to obtain a CT-assisted stereotactic biopsy.⁸^,¹²

Neuroguide intraoperative viewing system

The NeuroGuide intraoperative viewing system provides capabilities for viewing structures through a tiny opening in the brain for diagnostic and therapeutic procedures with minimal trauma to tissues.

Operating microscope

The operating microscope provides an intense light and magnification for visualization of intracranial structures. Microsurgery permits removal of tumors and vascular lesions that are otherwise inaccessible or inoperable. The CO₂ laser may be used through a micromanipulator.

The microscope also is used for some spinal procedures and peripheral nerve repair. A fiberoptic camera attached to a microscope projects the surgical site onto a video monitor screen. This provides a clear view for the assistant and scrub person. In lieu of use of the microscope, the surgeon may wear binocular loupes to magnify the surgical site. Great care is taken not to bump or jar the instrument table or microscope during the procedure.

Patient care considerations for craniotomy

The patient’s fear of an inability to function independently as a result of intracranial surgery is paramount, either consciously or subconsciously. The brain is the core of one’s being. Many neurosurgical patients do not undergo premedication, which allows neurologic assessment before induction of anesthesia or during a procedure performed with the patient under local anesthesia. The circulating nurse should be sensitive to the patient’s fears and offer reassurance. Psychologic support is especially significant for a patient who is awake.

Patient preparation

Preparation of the patient in the OR usually begins with clipping of hair with electric clippers. Hair on the head is considered the patient’s personal property. When all of it is removed, it is saved. Hair removal and its disposition are documented on the patient’s chart. Demarcation of the desired outline for the incision may be made on the scalp after the skin preparation and before draping. A sterile disposable skin marker is available (Fig. 37-7).

Prevention of peripheral nerve and circulatory damage requires that the circulating nurse check pressure points on the patient’s body during prolonged procedures. Some microneurosurgical procedures take 10 hours or longer to complete. Gel pads or a foam mattress similar in configuration to an egg crate should be used if the patient is supine or prone on the OR bed.

Patient positioning

The standard OR bed is used. A specialized head holder or frame is used. The incision and the type of procedure determine the head holder that is needed to position the patient. The basic support unit of a neurosurgical head positioning device attaches in place of the headpiece on the standard OR bed. The Mayfield headrest or skull clamp fits into the basic frame to stabilize the head for cranial procedures.

The Mayfield headrest conforms to the contours of the patient’s head like a horseshoe. The circular or horseshoe-shaped headrest equalizes weight distribution around the patient’s face when in the prone position.

The Mayfield skull clamp provides stability that is necessary for microsurgical procedures and is desirable during lengthy procedures. This eliminates the risk of pressure-related complications around the face or eyes with the patient prone or in a lateral position. Three pins on the skull clamp partially penetrate the outer table of the skull. Pins on one side may be spring-loaded (Fig. 37-8) to join the skull and the clamp into one rigid mechanical unit. When pin fixation is used, antibiotic ointment is applied around each fixation pin to form an airtight seal. This reduces the risk of air embolism and infection.⁷ The circulating nurse should be familiar with the desired neurosurgical positions and the headrests, skull clamps, and attachments for each.

FIG. 37-8 Mayfield skull clamp stabilizes head for right frontotemporal craniotomy. Single pin on left and two spring-loaded pins on right penetrate outer table of skull.

Supine position

The supine position is used most commonly for approaches to the frontal, parietal, and temporal lobes within the anterior and middle fossae. A lateral position may be preferred for some of these surgical procedures, such as a unilateral approach to the right or left temporal lobe.

Prone position

The patient is anesthetized in a supine position on the transport cart and turned prone onto the OR bed. A detailed description of prone positioning is found in Chapter 26 of this text. The eyes are lubricated, and the lids are taped closed for protection. The patient’s transport cart should be immediately available in the event of the need for cardiac resuscitation. The patient needs to be placed supine in a rapid manner, and the nearby cart facilitates this maneuver.

The prone position is used to access the occipital lobe. It may also be used for a suboccipital approach; however, many neurosurgeons prefer a sitting position to approach the posterior fossa.