APROACHING THE INTRACRANIAL STRUCTURES
PATIENT POSITIONING ON THE OPERATING TABLE
The proper position of the patient on the operating table is a very important step when undertaking surgery. It should allow the easiest performance of the surgical technique as required with no functional disturbances of the patient's physiology due to the prolonged positioning under general anaesthesia. Therefore, in general, we can indicate two essential groups of requirements for proper positioning: those which allow technical feasibility and others, which create the best intracranial physiological conditions for the patient and prevent complications from the long lasting positioning and anaesthesia. Regarding the first group of requirements, it is essential to foresee all technical steps and devices that will be required for the procedure as planned. These include the location and type of craniotomy, the needs of the microsurgical technique, as well as the presence of any additional equipment. It is preferred that the operative field be the uppermost part of the head during surgery; however, it is acceptable in some locations for it to be located laterally on the head after its final positioning. Microsurgery and safe handling of the patient by the staff during surgery require fixation of the head to avoid disturbances during micromanipulation.
For this purpose the three pin holder, which fixes the head within the outer table of the skull to the operating table by means of a special clamp armed with pins, is widely accepted.
Moreover, if the operative field is not strictly horizontal, it allows easier drainage of fluids from it during surgery (Figs. 1-4; 1-5).
Once the position of the skull is decided, it becomes necessary to complete the second group of requirements. As a rule, there should not be strains in different parts of the body when it is placed in the final position. Any major or requiring even minor force rotation, flexion or extension of the neck should be avoided. The same applies to the upper and lower girdle. Only then it can be decided which position of the body is to be chosen - supine, prone or lateral. The patient's head should be slightly superior to the right atrium to improve venous outflow and avoid congestion intracranially. Lifting the lower limbs will have the same benefit and will increase the circulating volume. These requirements will give additional angulation of the already positioned patient's body and are achieved by lifting the upper and lower parts of the operating table separately.
There are also certain general rules of positioning which have to be followed, such as allowing free chest respiratory movements, avoiding compression of superficial arteries, veins and peripheral nerves unprotected by muscles; for instance the most vulnerable cubital nerve at the elbow and the peroneal at the fibular neck.
Since the introduction of modern anaesthesia and perfection of intraoperative functional control, the semi-sitting and sitting positions of the patient during surgery became possible. It can be easily noticed, that these positions can be applied as an option in certain, but not all neurosurgical approaches (mainly posterior cranial fossa) instead of the horizontally positioned patient and some additional implications should be considered. The additional elevation of the head significantly decreases the venous and CSF pressures making the approach and drainage of fluid easier, but it may increase the risks of venous air embolism and it does also present additional requirements for cardiac and circulatory monitoring.
When the operation is in the frontal, fronto-parietal or fronto-temporal region the patient is placed supine, the head being slightly turned to the opposite side. If the anterior skullbase surface and sellar region have to be approached, the head should also be slightly extended. For approaches in the parietal, temporal and occipital regions the lateral position is usually the most convenient. In occipital approaches and in some lesions in the posterior fossa, especially in the cerebellopontine angle region lesions, the lateral position is combined with different degrees of rotation and flexion of the head (Fig. 1-6).
The prone position is that most commonly used in operations on the posterior fossa. In this position the patient's head is placed on a special headrest if not in a pin holder, and this should allow the anaesthetist to have a adequate access to the face. The shoulders, the chest and the pelvis should be supported only in their lateral parts, allowing the middle part of the chest and the abdomen to be free for respiratory movements.
The sitting position has great advantages in operations on the posterior cranial fossa, especially in cases of increased intracranial pressure, when venous pressure inside the cranial cavity is high (Fig. 1-7). The access to the fourth ventricle, the upper surface of the cerebellum and foramen magnum is easier. Because of the potential risks of air embolism and other complications, the "classical" sitting position is not very often applied, being substituted by the semi-seated position, in which the legs are well flexed. Several surgical technical rules should be followed when operating on a patient in the sitting position. It is important that the anaesthetist increases the central venous pressure at the dissection of every layer of the operative wound. The opened veins can at once be noticed and be occluded, the bleeding bone surface can be waxed or packed with oxidised cellulose. This increase of the central venous pressure should continue for not more than 3 - 4 min. It is advisable to maintain the ventilation with 10 mm H2O positive pressure at expiration.
BURR HOLE APPROACH
This is an essential basic technique for approaching the intracranial cavity safely. Performing a burr hole can be a separate procedure in certain conditions requiring such a limited approach, or a step in the techniques of craniotomy or craniectomy. Here it is considered as a separate procedure. Some of the goals of the burr hole technique can be achieved at present by "twist drill" or other smaller drill perforation techniques, especially in intracranial pressure monitoring or intraventricular drainage placements, but a burr hole also permits visual control over the dural and cerebral surface for better haemostasis.
Indications. Burr holes may be used for exploration in patients with suspected extracerebral collections and the drainage of those which are fluid, such as chronic subdural haematomas and hygromas, for the aspiration of intracerebral fluid collections such as abscesses, for needle biopsy of a cerebral tumour, or to aspirate intracerebral haematomas.
Anaesthesia. Burr holes may be made under local anaesthesia in a conscious, cooperative adult patient and in unconscious patients who are not restless. In all other circumstances general anaesthesia is recommended.
Surgical technique. After preparing the shaved skin surface with antiseptic solutions, an incision 2 - 2,5 cm long is done by transsecting all layers of soft epicranial tissues in one. A small self-retaining retractor is inserted and opened widely for control of bleeding and better visualisation. Any remaining bleeding is controlled by electrocoagulation while preserving the skin edges. The pericranium is incised in the same direction as the skin and detached to each side with a small periosteal elevator. Perforation of the bone is performed by electric or manual drill. Using a manual drill, the surgeons learns to sense a characteristic change of feeling in the drilling resistence after reaching and perforating the inner skull table. However, until they acquire this 'feeling', for safety of dura, beginners should frequently take out the blade to check to what depth it has penetrated, and once dura visible, to change it with the burr. An automatic drill will stop by itself reaching the dura. If bleeding appears from the bone, because of the opening of diploic vessels, haemostatic wax is pressed on the bleeding point. If the bleeding is evoked from an injury of the dura, the hemostasis is achieved by tamponade with cottonoids, soaked in saline, or, if this is not effective, with bipolar coagulation. The diameter of the bone opening must be 10 - 15 mm (and with a thicker cranial vault, even larger). A larger diameter provides better access for manipulation through the burr hole.
If the dura must be opened, it is elevated with a sharp hook and delicately incised with a small scalpel blade. Care must be taken to avoid injury to the underlying brain. Once the incision is done it can be extended crosswise. Any bleeding from the edge of the dura is controlled with bipolar coagulation.
Closure. When closing the wound, the dura may not be sutured, but may be covered with gel-foam. The galea is closed with a few interrupted sutures, the knots being inverted and the thread ends cut short. The skin is closed with interrupted sutures that will control all but major bleeding (Fig. 1 - 8).
Postoperative care and complications. The sutures can be removed on the fifth day; usually the dressing needs not be disturbed until that time. Complications after a burr hole procedure are usually rare. Intracranial haematoma is unusual unless bleeding from the dura or brain has not been properly stopped. However, as in all intracranial procedures, infection of the wound may have serious consequences.
SUPRATENTORIAL CRANIOTOMY
SKIN INCISIONS AND SCALP FLAPS
After optimal positioning of the patient and preparation of the scalp with antiseptic solutions, the skin incision is marked out with a sterile marking pen. When the patient is under general anaesthesia, as is common, a marking scratch can be made along the skin incision with a scalpel or a needle, affecting mainly the epidermis, until a tiny red line of blood is obtained; however, no drops should be seen. Transversal lines at a distance 5 - 6 cm along the skin incision are also marked. They indicate at closure the proper position of the scalp flap, avoiding its rotation or distension of the wound edges. Scalp incisions have most commonly horseshoe shapes, open to the cranial base and intended to preserve the neural and vascular supply (Fig. 1 - 9). In the frontal region the incisions have the same shape; however, for aesthetic reasons, they should appear on the forehead only exceptionally. To avoid visible scars in the frontal region incisions most often go along the anterior hairline in a coronal or bitemporal shape. Such a bilateral incision is very often followed by a standard unilateral craniotomy. The temporal muscle usually occupies one part of the region enclosed by the incision. It can serve in the following stage of the operation as a pedicle for the bone flap if it is required.
At the next stage local anaesthetic can be injected. The majority of operations are performed under general anaesthesia, however, local infiltration (to 40 cc of local anaesthetic,
1 cc of 1% epinephrine is added) has not lost its value due to the action of the added epinephrine which considerably reduces the bleeding at the time of incision. Additionally, this infiltration is very helpful for the easy separation of galea from periosteum (pericranium). The infiltration of the skin with local anaesthetic is done in the following way: first the skin and the subcutaneous tissue along the course of the incision is injected until the surface acquires an 'orange rind' appearance. At the time of this infiltration the needle should not penetrate through the galea and usually strong resistance is felt, due to the firm attachment of the epicranial aponeurosis to the skin. After that, at different points along the skin incision, the needle is inserted beyond the galea and the supraperiosteal space (subaponeurotic areolar tissue) is copiously infiltrated over the complete operative field. This tissue is loose and the infiltration is easily done, without particular resistance. At the end, as a last stage, infiltration is done in the temporal muscle in the region of the craniotomy. This is accomplished by vertical penetration in one or two places and the needle is inserted until the resistance of the bone is felt.
The initial incision involves the skin, the hypodermic fatty tissue and galea aponeurotica.
The pericranium and the subaponeurotic areolar connective tissue remain intact. The incision is better performed in three to four steps along its full length for easier control of bleeding. At the time of the incision the surgeon and his assistant are firmly pressing the edges of the wound to the underlying bone to occlude the bleeders and to retract wound edges for better control in the depth. The galea is attached intimately to the skin and they can be retracted only together; if the galea is not cut, the wound will not open (Fig. 1-10). After completing every step of the incision, the pressing fingers are released one by one applying in their place haemostatic clips or artery forceps. When applying an artery forceps, it is better to occlude the bleeding artery itself, but when this presents difficulties, the galea should be fixed and stretched or pulled by the forceps, pressing the adjacent bleeders with it.
However, technically it is much easier to apply special haemostatic clips (for instance Raney clips) which have the same action (pressing the galea to the skin and by that occluding the blood vessels). The big vessels penetrate through the aponeurosis from the subaponeurotic areolar tissue, and there they are better controlled by coagulation. Once the incision and the haemostasis are completed, if a soft tissues flap was prepared, it is elevated, separating galea from periosteum (pericranium); this is the easiest layer to separate due to the areolar tissue (Fig. 1-11). The elevated soft flap is fixed by sutures or hooks on the galea, so that it will not move during surgery and thereby disturb subsequent manipulation. Then it is better to cover it by wet gauzes or towels.
BONE FLAPS
Bone flaps are planned before surgery and their limits are described clearly according the basic anthropological points of the skull. Size and limits depend on the needs of the surgeon with respect to viewing and manipulation spaces. With the advent of the surgical microscope the dimensions of craniotomies have become smaller. As the surgeon intends to perform certain techniques intracranially, he needs to guarantee sufficient space for safe manipulation and an additional angle for observing simultaneously the field. He should be able to sufficiently expose the lesion or provide space for brain retraction with the selected craniotomy size and location.
Every craniotomy is performed in a certain standard technical manner. It includes bone exposure at the preselected cutting line, positioning of burr holes, bone cutting and elevation of the flap. Incision of the periosteum is done over the presumed areas of burr holes and bone cutting. Basal burr holes often require incision of temporal fascia and muscles to expose the drilling and cutting places (Fig. 1-12). Eventual bleeding is controlled. Then one, two or more burr holes are produced. The placing of burr holes is decided according to the needs of the surgeon in introducing the bone cutting device and for better control under the flap before lifting it. Therefore burr holes should be placed where bone cutting starts and ends, when the flap reaches the skull base and where difficulties are or can be encountered in turning the bone
cutting line. Some burr holes play "key" roles for the proper shaping of the flap and thence for proper exposure. They are placed with regard to some anatomical structure laying below. Such burr holes are the pterionial, the sinus angle (transverse to sigmoid), burr hole in trigeminal neuralgia, the burr holes at the edge of the sagittal sinus and torcular and others. Their misplacement is a clear surgical mistake and special attention must be paid to determining their proper location. They usually stay on the path of the approach and have given ground to contemporary key-hole surgery. Other burr holes can vary slightly in their positions for purposes of easier performance of the craniotomy.
Once the burr hole places have been determined they are drilled in such an order that the one which is expected to bleed most is done last.
The drilling is done in the manner already described in the section on burr hole techniques.
The bottoms of the burr holes are packed with topical haemostatics and cottonoids.
Bone cutting can be performed with the traditional Gigli saw or with the more recently designed air or electrically powered craniotomes. However, for both techniques it is essential to separate the dura from the inner table with dissectors, as the cutting instrument's edges and bone laminae can injure the dura (Fig. 1-13).
There are two options in removing the flap: to keep it attached to a pedicle of periosteum or temporal muscle during surgery, or to remove it and fix it again at the end of the surgery. If the flap is to be kept attached, the bone cutting line under the attachment will not be accessible and it is fractured after proper undercutting with Gigli saw, craniotome or nibbler (Fig. 1-16; 1- 17). The removed flap is either kept in a pot of saline or covered with wet gauze, and the temporal muscle or periosteum are retracted. Lifting the flap provokes bleeding and the first task that follows is to control it. Some craniotomies for special approaches may require the lifting of two flaps or drilling out additionally of some portions of the skull base.
OPENING OF DURA
The opening of the dura can be made in different ways: this depends on the place of craniotomy and the pathological process. It should be mentioned that the incision of the dura, in any direction whatever, does not evoke necrosis, as it is a fibrous structure and is rich in vascularization. The dura is opened after complete control of bleeding. It is caused either by visible dural vessels affected by bone flap lifting or by sources hidden under the rim of the craniotomy. Bleeding from exposed dura is controlled easily by bipolar coagulation. No extensive coagulation over larger areas is applied as that shrinks the dura and makes its closure difficult. Searching for bleeders under the rim of craniotomy is not efficient as it detaches more dura from the inner table and bleeding increases. Better results are obtained by inserting topical haemostatic agents and lifting the dura by duroperiosteal stitches. If bleeding is not intense the stitches can be put in after closure of the dura.
Dural incision should not cause additional cortical injury, which is prevented by lifting the dura with a hook when it is tense and with fine forceps when it is lax. The hook is inserted only below the external dural layer and gently pulled, while a fine blade incises along 2-3 mm both layers beside the hook until the arachnoid seen. Sometimes, if the arachnoid is torn there is a leakage of CSF. Bleeding is temporarily covered by a cottonoid, but it can occur despite the fact that vessels are carefully avoided (Fig. 1-18).
The incision lines are decided after some principles are considered. They should not cross dural sinuses and lacunae, they have to fold conveniently and should not reach close to or under the bone edges. The most simple pattern of incision lines has to be chosen, as well as that one associated with easier closure and better water-tightness. The incision is done with scissors having blunt tips and edges. A small cottonoid is inserted under the tip of the scissors, as the arachnoid and cortex need protection. Sometimes the protection of a narrow spatula is needed. Once the incision is 1 - 2 cm long and the edges can be lifted, traction sutures are inserted. They permit lifting the edges and safer continuation of the incision. When the incision is completed the flaps produced by the incisions are turned out of the craniotomy and covered by wet gauzes or cottonoids. Delicate bipolar coagulation can be used for bleeders of the dural edges (Fig. 1-19).
CLOSING A CRANIOTOMY
As with all surgical wounds, the craniotomy is closed strictly following the anatomical layers, in the reverse order of that adhered to when opening.
Dural closure is done first, the intention being the production of a complete and watertight cover of the cerebral surface. Because the dura may have been turned and pulled for many hours, the dural flaps must be well repositioned. The properly oriented flap position is fixed with interrupted stitches. Then the incisions are closed with continuous or interrupted sutures, avoiding any minimal gapping and guaranteeing a watertight closure. Mild tension of edges is acceptable at closure. Gaps have to be repaired with plastic techniques. Several techniques are used routinely. Dural layers (they are two) can be split and the upper turned out of the dural flap to cover the defect of closure. If it is impossible, then a free flap of periosteum and supraperiosteal fibrous tissue, fascia lata or other commercially available materials for this purpose can be used. Whatever it is, closure should assure protection of the brain and prevention of CSF leakage (Fig. 1-20).
Before replacing the bone flap care is taken to perform haemostasis. Epidural bleeding is controlled with topical haemostatic agents and duroperiosteal stitches. We prefer to leave epidural drainage tubes under mild negative (suction) pressure in the great majority of supratentorial craniotomies, taking out the tube through a burr hole and aside from the epicra-nial tissues incision. The already positioned tube is fixed to the skin with a stitch. Then the bone flap is repositioned. Free flaps are fixed to the bone edges of the craniotomy by silk or wire.
Flaps attached to the periosteum or temporal muscle are turned back and also fixed. Periosteum is repaired with interrupted sutures. Temporal muscle is sutured if long and gaping incisions remain, but only to overcome the gap.
When muscle fascicles are only split and not transsected, sutures are rarely needed. Temporal fascia is closed with interrupted stitches. Before proceeding to the more superficial layer and attachment of the temporal muscle to the skull, stable bone flap fixation is assured (Fig. 1-21).
Epicranial flap is usually closed in two layers - galea and skin. Haemostatic clips and forceps are gradually removed and major bleeders controlled by coagulation. In conditions of significant diffuse bleeding a second suction drain can be left below the galea. The galea is sutured with inverted interrupted stitches avoiding any misorientation of the flap. Therefore at the beginning three to four stitches are placed at the points already marked at the skin before incision and only then is the entire layer completed. Galeal sutures are intended to properly reposition the galea and ensure coincidence of the edges of this layer for efficient fibrous union. Last to be sutured is the skin; this is also accomplished with interrupted stitches at a distance of 1 to 2 cm between sutures. If the bone flap is replaced, the dressing can be used to apply mild compression on the wound by using a stretching bandage.
POSTERIOR FOSSA CRANIECTOMIES
Approaches to the posterior fossa differ from the supratentorial ones because of the anatomical differences found in the area at all levels. Below the external occipital protuberance the skin is thicker, muscles are much larger, firmly attached to the bone and as a rule they cover all the surface of the approach through the bone. Therefore, the skull opening is deeper beneath the surface of the skin than in the common supratentorial approach. In these conditions attached flaps cannot be turned. Therefore, remaining options are a craniectomy or a free flap. The infratentorial space is a smaller compartment than the supratentorial and can require bone decompression at the end of surgery as well as merging of the bone opening with foramen magnum opening. Thick epicranial layers with good attachment to the skull and the increased risks of CSF leaks make very real the danger of placement of epidural suction tube drainage; hence, such drainage is hardly ever used. All these different conditions require different techniques in posterior fossa approaches.
Other specific characteristics are the presurgical planning of the approach and the choice of location for the craniotomy. The two main types of approach - the midline and the lateral, have proved to provide access to the majority of extra- and intraaxially located lesions as well as to a very extensive part of the internal skull base. Some essential anatomical reference points are used for the proper placement of craniectomies and planning of incisions. The positions of the external occipital protuberance, mastoid tips and midline are defined by palpation and better marked with a pen. Next the external projection of the posterior arch of Cl is established; it corresponds to the horizontal line through the mastoid tips (Fig. 1-22).
Infratentorial approaches differ according to which surface of cerebellum or brainstem needs to be reached. Lateral surface of cerebellar hemispheres, cerebellar peduncles, pontine and medullar surface can be approached only after hemispherial retraction; skull opening should reach the lateral border of the posterior wall of the posterior fossa where the medial wall of the petrous bone is in continuity to the condylar part of the occipital bone. Posterior hemispherial and vermian surfaces as well as the IVth ventricle are reached caudally after median and paramedian exposures through occipital squama with different width. Standard approaches have been designed for the needs of treatment in different types of posterior fossa lesions by the eminent pioneers of neurosurgery. Some of them have survived the severe verdict of our rapidly innovating times, with small modifications due to microsurgery. Because of their anatomical properties, they naturally can be either midline or lateral.
MIDLINE APPROACHES
In general a midline approach is intended to expose sufficiently cerebellar vermis and tonsils, part of the hemispheres and the posterior columns of the uppermost spinal cord for safe manipulation. This is achieved by a bone removal of the mid-occipital squama, foramen magnum rim and very often the posterior arch of C1.
Incisions are performed along the midline of the posterior fossa with a length which permits, after retraction, sufficient exposure of bony structures - from the occipital protuberance to the spinous process of C2 at least (Fig. 1-23). The transverse dimension is planned according to the need to proceed laterally. Midline incisions are intended for penetration through the midline structures of soft tissues, which are the least vascular in the area, thereby significantly reducing blood loss. The external occipital protuberance and the midline of the fascia are identified by palpation after opening the skin and subcutaneous layer, control of subcutaneous bleeding and insertion of self-retaining retraction. A small transverse incision of the muscles immediately below the external occipital protuberance and medial part of the upper nuchal line is performed, if the planned craniectomy does not need to reach the transverse sinus. The muscle strip that remains attached to the bone facilitates the suturing at the end of the operation (Fig. 1-24). The identification of the midline is crucial and only after that is it incised caudally in the direction of the C2 spinous process if this is palpable. The bone of the occipital squama is reached first in its most superficial part - below the external occipital protuberance. Self retaining retractors are advanced and muscles are detached from bone with periosteal elevators (Fig. 1-25). With advancement of the incision the step is repeated again and so on several times until the posterior arch of the atlas can be reliably palpated. For better access it is sometimes necessary to expose the C2 spinous process at this stage thereby making its arch palpable. The incisions are done with electric cautery to reduce bleeding. When the occipital squama is completely exposed along the midline and the gap between squama and Ci has been well located, it is carefully approached (but not quite reached) with the midline incision. The midline is very cautiously incised over the posterior arch of Cl and the arch stripped of soft tissues at least 1 cm on both sides of midline with a small periosteal elevator. The posterior atlantooccipital membrane is dissected avoiding its perforation, the rim of foramen magnum is palpated and thereby the minimum requirements for bone opening are achieved. Any existing bleeding is controlled at this stage.
The midline approach is routinely made by craniectomy. The size of the craniectomy is preplanned and can vary, but as a rule, it is started by drilling two burr holes on both sides of the midline avoiding the dural venous collectors (cerebellar and foramen magnum sinuses).
The holes are placed at least 2.0 cm from the midline and at the mid-distance between the foramen magnum and the superior nuchal line (Fig. 1-26). Both burr holes are enlarged sufficiently and after gaining space the midline is crossed; one must of course expect venous bleeding at times. The enlargement continues cranially to the lower edge of the transverse sinuses and caudally until it reaches the foramen magnum, where venous bleeding is also commonly encountered. In this area, the atlanto-ocipital membrane is thoroughly dissected from the dura (Fig. 1-27). The foramen magnum rim opening is enlarged laterally to no more than 2 cm on each side. Following the dural plane, the inner surface of the Cl arch is dissected and removed with a fine nibbler or bone punch to an extent not more than 1.5 - 2 cm from the midline if its removal has been preplanned.
Dural and epidural bleeding is controlled by bipolar coagulation and topical haemostatic agents; bone bleeding is waxed. The dura is inspected for the position of dural sinuses and the control of intracranial pressure. Opening is undertaken only when intracranial pressure is within normal values; otherwise a disastrous brain prolapse, shifts and tentorial herniation can occur. Increased intracranial pressure is controlled by osmotic agents or ventricular puncture. The dura can be opened in many different ways, most commonly in a Y shaped or in a horseshoe form (Figs. 1-28; 1-29). The Y shaped opening of the dura avoids as much as possible dural sinuses and has conveniently turned flaps. Dural incisions have to be done on both sides of the midline over the hemispheres; both incisions converge to the midline at the dural fold corresponding to the foramen magnum. The midline is approached and transsected (it is the lower part of the falx cerebelli) only after the dural edges are well held by traction sutures. The lifting of dural edges will permit easy control of the expected bleeding from cerebellar and foramen magnum sinuses. The cerebellar sinus is best ligated; coagulation is ineffective (increased intravenous pressure opens the sealing by the bipolar coagulation) and shrinks the dura, making closure difficult. If bleeding is sudden, dural edges containing venous sinuses and lacunae can be clipped with big or giant temporary Sugita or Yasargil clips.
When the surgical field is dry, haemostasis is easily secured with ligatures or permanent titanuim clips, as the permanent clips are small and they do not obstruct vision and manipulation.
Then incision of the dura is completed along the spinal posterior midline. Dural flaps are lifted and, additional haemostasis is done (releasing the dural tension sometimes provokes epidural oozing). If needed, tension of dural flaps is restored by stitching them to the muscles during intracranial work (Fig. 1-30).
LATERAL APPROACHES
These approaches can vary in size and location. They give access to hemispherial lesions or retraction of the hemisphere. After the hemisphere retraction, the cisterns laterally and ventrally to the pons and medulla oblongata can be reached. This makes these approaches also convenient for lateral extraaxial lesions or intraaxial manipulation after lateral penetration. For hemispherial lesions, bone removal is performed over the lesion itself or the location of the intended cerebellar incision. To penetrate after cerebellar retraction, the bone opening should reach the lateral edge of the hemisphere. This corresponds to the inferior edge of the transverse sinus and the medial edge of the sigmoid. The transverse-sigmoid angle is located under the base of the mastoid. Therefore, cranial openings are oriented in accordance with the line connecting the external protuberance with the base of mastoid and the tip of the mastoid.
Straight or slightly curved incisions are used for lateral approaches at a certain distance from the midline and approximately parallel to it. The distance from the midline corresponds to the centre of the bone removal and its incision length - to its major diameter. If the lower edge of the transverse sinus has to be reached, 2-3 cm of the incision is cranial to the superior nuchal line. For reaching the medial edge of the sigmoid sinus, the incision should be able to expose the posterior mastoid surface (Fig. 1-31).
Incision is done along the preplanned distance and opening the subcutaneous layer is done with the same strokes to reach the fascia. Bleeding is controlled by bipolar coagulation and special attention is paid to the occipital artery, usually crossing the field from lateral to medial, and just below the fascia in the region near the superior nuchal line. The artery should be carefully coagulated along a few millimetres, otherwise it can bleed suddenly at any time during the next stage of the surgery. Cutting with the diathermy is efficient for the subsequent transsection of the fascia and muscles. Self-retaining retractors are positioned to maintain proper access to the depth of the wound. As in midline approaches, splitting and transsection of muscles proceeds from the nuchal line towards the skull base, in several steps. At each step, muscle is transsected, separated from the occipital squama with periosteal elevator and the self-retaining retractor advanced. The bone structures are palpated after every step to preserve the anatomical orientation and to ensure awareness on the part of the surgeon of the proper advancement of the incision. Little by little, the area of bone removal is exposed. Its exposure is completed with careful haemostasis. Craniectomy starts with a burr hole, placed within the preplanned bone removal, but away from the dural sinus margins and preferably in a thinner part of the occipital squama. The burr hole is enlarged by nibbling, advancing the craniectomy to the sinus margins and to the preplanned size. Special care is taken to nibble safely the margins of the dural sinuses if retraction of the hemisphere is intended. This leads at times to the exposure of mastoid cells, but they must be sealed with wax to prevent postoperative CSF leaks and infection. The mastoid emissary vein usually bleeds during separation of muscle from bone in its area and later - on during nibbling. On both occasions it is easily occluded with bone wax. The edges of the craniectomy are smoothed and bleeding is controlled. Some lateral bone exposures are done as craniotomies, inserting the craniotome through the burr hole and producing a free flap. The dura is opened with radial incisions, after ensuring that any abnormal rise of the intracranial pressure is controlled. Osmotic agents and even a lateral ventricular puncture may precede the dural opening if intracranial pressure was preoperatively increased. Dural flaps are turned out after incision and held by traction sutures. The incision should never reach the edges of the cra-niectomy as closure later on is extremely difficult, so after a central dural opening, the incisions run from the centre to the edges, taking care not to open a dural sinus or lacuna.
Epidural bleeding, including that from the emissary vein area, is controlled by topical agents.
SUPRA-INFRATENTORIAL APPROACHES
Two types of lesions require this type of approach, those which are superficial, but under the dura, over and under the tentorium, and those which are in the depth, and require a much wider angle of view and manipulation between delicate neural and vascular structures.
All approaches open supra- and infratentorially or middle and posterior cranial fossae with or without modifications enlarging the exposure on the account of the petrous bone. The bone bridge, left in previous times, is no longer left for protection of the lateral sinus, as refined and delicate bone removing instruments have been introduced. The extent of bone removal is planned according to the needs of width, angle and direction of the approach.
These approaches can consist of three parts or separate exposures: middle cranial fossa (or supratentorial occipital area, posterior cranial fossa and an optional petrous bone approach.
The three exposures can form a common dural field. This field is more often a lateral approach
- involving lateral suboccipital and temporal or temporooccipital areas with or without the petrous bone, but it can be also medial - occipital, supra- and infratentorial (Fig. 1-34; 1-35).
The supratentorial part is always a craniotomy reaching the temporobasal plane or continuing towards the inion - the upper margin of the transverse sinus. Its limits in anterior, posterior and superior directions depend on which part of the temporal and/or occipital cortex has to be retracted and which part of the free tentorial edge has to be reached. Important points for the temporal and occipital basal cortical surface retractions are the eminentia arcuata (corre-sponding to the superior semicircular canal), the superior petrosal sinus and the straight sinus.
The eminentia arcuata provokes a natural indentation on the cortex permitting easy retraction to the interpeduncular cistern. The superior petrosal sinus indicates the posterior edge of the superior petrosal surface and therefore the natural continuity of the supra- and infratentorial spaces along the skull base. The superior petrosal sinus can be transected in some combined approaches, allowing further complete splitting of the tentorium. The straight sinus is an important path determining structure position in the posterior, close to the midline approaches, where complete tentorial division is impossible.
The infratentorial part is done as a cra-niectomy or by extending the flap from supra-tentorial area. The area of special attention for craniectomy or bone flap lifting is the transverse sinus, where proper dural separation from the inner table is crucial. Burr holes, placed on both sides of the sinus allow the introduction of a blunt dissector to separate sinus wall from the bone before cutting it with the craniotome (Fig.1-36).
Bone flap is lifted in one and two pieces, and the temporal part can remain attached to the temporal muscle.
In lateral approaches the supra-infratentor-ial craniotomy can be extended in the middle and posterior fossa by additional petrous bone removal extradurally (Fig. 1-37). For the middle fossa a small flap mainly including the roof of the tympanic cavity can be helpful for increasing the angle of subtemporal access. Another option is presented by a posterior petrosectomy, drilling an exposure to the intrapetrosal internal carotid and internal acoustic meatus. A posterior fossa approach can be also enlarged, most often by a partial mastoidectomy, for purposes of presigmoid dural exposure. This is achieved by drilling out the posterolateral mastoid cells, carefully skeletonizing the labyrinth and the Fallopian canal. The internal acoustic meatus can be widely exposed from a posterior direction. Additional partial mastoidectomy completely exposes the dural sinus angle with the walls of the lateral, sigmoid and superior petrous sinuses. This allows the division of the superior petrous sinus and Labbe vein drainage.
Although this sinus division seems technically feasible, any decision to undertake it should be weighted against some complications, mainly associated with brain venous drainage impairment. Therefore a detailed angiographic study should precede the surgical planning, and the step is taken only when its benefits are obvious.
Closures in supra-infratentorial craniotomies don't differ from those in the other types of craniotomies, following the above mentioned layer supra- and infratentorially. However, special care is taken for the bone protection of sinus walls which have been exposed, guarding aginst their accidental coincidence with bone gaps. We also prefer leaving epidural drains and careful fixed dura to the edges of the craniot-omy, as these approaches are associated with long lasting intracranial work and massive CSF drainage.
