Técnicas de Craneotomía: Abordaje y Posicionamiento de la Fosa Posterior

94 Specific Craniotomies
94.1 Posterior fossa (suboccipital) craniectomy
94.1.1 Indications
To g a i n a c c e s s t o t h e c e r e b e l l u m , c e r e b e l l o p o n t i n e a n g l e ( CPA) , t o o n e v e r t e b r a l a r t e r y , p o s t e r i o r
brainstem, fourth ventricle, pineal region, or, using extreme lateral posterior fossa approach to the
antero-lateral brain stem. See paramedian (p.1447) and midline (p.1450) suboccipital craniectomies
for details.
94.1.2 Position
Options
Position options include:
1. sitting position: see below
2. lateral oblique (p.1446): patient three-quarters oblique (almost prone),
3. semi-sitting
4. supine with shoulder roll, head almost horizontal
5. prone
6. Concorde position: prone, thorax elevated, neck flexed and tilted away from the side on which
the surgeon will be standing
Sit t ing posit ion
Used less frequently than in the past because of associated com plications and acceptable alternative
positions (except for some specific circumstances). However, some experts feel that the risks of the
sitting position have been greatly overstated.1
Ad va n t a g e s
1. improved drainage of blood and CSF out of surgical site
2. enhanced venous drainage which helps reduce venous bleeding and also ICP
3. easy ventilation due to unencumbered chest
4. patient’s head may be kept exactly midline, aiding operator orientation, and reducing risk of
kinking of vertebral arteries
Disadvant ages/risks
1. possible air embolism (see below)
2. fatigue of operators hands
3. increased surgical risks from placement of CVP catheter (required to treat possible AE): e.g.
pneumothorax with subclavian vein catheterization, thrombosis
4. risk of post-op hematoma at the operative site may be increased since potential venous bleeders
may remain occult while the patient is sitting, but may manifest when patient returns to a hori-
zontal position post-op. However, one study found no such increased incidence2
5. risk of post-op subdural hematoma: 1.3% of p-fossa cases3
6. possible brachial plexus injury: prevent this by not allowing patient’s arms to hang at the side.
Instead , fold th em across abdom en
7. midcervical quadriplegia4,5: presumably due to flexion myelopathy.6,7,8 Th e com b in at ion o f t h e sit t in g
position with hypotension9or neck flexion with possible compression of the anterior spinal artery, ±
cervical bar, and elevation of the head thus reducing the arterial pressure m ay all contribute
8. sciatic nerve injury (piriformis syndrome)10: prevent this by flexing patient’s knees (reduces
tension on sciatic nerve)
9. extent of post-op pneumocephalus is more pronounced, and may increase the risk of tension
pneumocephalus11; see Pneumocephalus (p.887)
10. venous pooling of blood in the LEs under anesthesia may cause relative hypovolemia and should
be counteracted by binding the LEs prior to positioning
11. decreased cerebral blood flow due to lower hemodynamic arterial pressure12
Air e m b olism (AE): A p ot e n t ially fat al co m p lica t ion o f a n y op er a t ion w h e n an op en in g t o a ir o ccu rs
in a non-collapsible vein (e.g. diploic vein or a dural sinus) when there is a negative pressure in the
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vein (e.g. when the head is elevated above the heart).13 Air is e n tr a in e d in t h e ve in a n d ca n b ecom e
trapped in the right atrium of the heart which may impair venous return causing hypotension. May
also produce cardiac arrhythmias. Paradoxical air embolism can occur in the presence of a patent
foram en ovale14 or pulmonary AV fistula, and may produce ischemic cerebral infarction.
Greater negative pressures occu r in the sitting position due to th e extrem e elevation of th e h ead,
but AE can occur in any operation with the head elevated higher than the heart. Incidence: a wide
range has been quoted in the literature, and depends on the monitoring method used: ≈7–25% inci-
dence with the sitting position using Doppler monitoring is an estimate.3
For op erat ion s w ith a significant risk of AE, a right atrial CVP line is recommended (to aspirate
air), and monitoring for air embolism; options include: transesophageal echo (the most sensitive),
precordial Doppler monitoring. (Although technically the risk of air embolism includes any case
where the head is higher than the right cardiac atrium, practically it is limited to cases where the
head of the bed is ≈>30° which is mostly limited to the sitting position for posterior fossa tumors.)
Diagnosis and treatm ent:
AE sh ou ld b e su sp ect e d in an y o p er at ive ca se in w h ich t h e su rgica l sit e is h igh e r t h an t h e h e a r t
when there is any unexplained hypotension or decrease in EtCO
2.16
●transesophageal echocardiography (TEE). Bubbles can be seen on the 2D echo display
pros: considered the most sensitive monitoring modality
cons: significant false positive rate, expensive, invasive, requires experience and vigilance
●precordial doppler U/S: probe may be placed over 2nd to 4th intercostal space either to right or
left of sternum, or posteriorly between the scapula and spine. AE is heralded by a change in sonic
intensity and character at first by a superimposed irregular high-pitched swishing sound, and
then as more air is entrained so called “mill wheel”or machinery sounds dominate
pros: the most sensitive of the non-invasive techniques
cons: di cult in morbidly obese patients and in certain patient positions (e.g. prone or lateral),
interference from other sounds in the OR, requires vigilance
Th e e a r liest in d icat ion of AE m ay be a r ise in t h e e n d -t id al n it rogen (re qu ires m ass-sp ect r o m e ter on
monitor), then a fall in the end tidal pCO
2occurs. Machinery sounds in the precordial Doppler also
suggest AE. Hypotension may develop. Measures shown in Table 94 .1 sh ou ld be im m ed ia tely
instituted.
La t e r a l o b li q u e p o s it i o n
AKA “park bench”position.
●Axilla r y ro ll for t h e d ow n sid e a r m (se e Fig. 9 4.1) (or, p osit ion th e p at ien t so th at t h e d ow n sid e
arm extends over the edge of the table and is held in place by a sling formed by the Mayfield table
attachment with copious padding)
●Up p er ar m su pp ort ed on p illow s or tow els (avoid usin g a Mayo stan d w h ich rest ricts th e abilit y to
laterally tilt the OR table during surgery)
●Ad h esive t a p e t o gen t ly p u ll d ow n o n t h e u p p er sh ou ld er
●Br in g t h e p at ie n t’s back as close to the side edge of the table as possible (usually limited by the
travel of the head clamp) to bring the patient closer to the surgeon
●Elevate th or a x 1 0-15°
●Tilt t h e ve r t ex of t h e h ead t ow ar d s t h e floor (se e b elow )
●Optional spinal drainage (usually for large tum ors)
●Pillow between the legs
●Secure pat ien t w it h ad hesive tap e over p ads so t h e table can be “airplaned”(rolled) during the
operation
Ta b le 9 4 .1 Tr e a t m e n t f o r a i r e m b o l is m
1. find and occlude site of air entry, or else rapidly pack wound with sopping wet sponges/laps and wax bone
edges
2. lower patient’s head if at all possible (30° or less from horizontal)
3. jugular venous compression (bilateral best; second choice: right only)
4. rotate patient LEFT side down (attempt to trap air in right atrium)
5. aspirate air from right atrium via CVP catheter
6. ventilate patient with 100% O2
7. discontinue nitrous oxide if used (may expand AE)15
8. use pressors and volume expanders to maintain BP
9. PEEP is ineffective in preventing or t reating AE; m ay increase the risk of paradoxical AE13
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Fo r a cce ss t o t h e p o ru s a cu st icu s o r m o re ca u d a lly
(e.g. for vestibular schwannomas; not necessary for microvascular decompression for trigeminal
neuralgia).
Get th e shoulders out of th e w ay by flexin g th e n eck as m u ch as possible w hile m ain tain ing pat-
ent airway (aided by use of non-kinking wire-reinforced ET tube, so-called “armored tube”). The
upper shoulder is retracted caudally by adhesive tape (avoid excess traction which may injure bra-
chial plexus).
Head positioning
A Mayfield h ea d -clam p is p lace d w it h t h e sin gle p in on t h e sid e of t h e le sio n , sligh t ly a n te r io r to a
true-lateral on the skull (Fig. 94 .2.). Th e h ead is th en rot at ed 2 0–30° face-down from the
horizontal.
94.1.3 Paramedian suboccipital craniectomy
In d ica t io n s
1. access to the cerebellopontine angle (CPA)
a) CPA tumors, including:
●vestibular schwannoma
●CPA m e n in gio m a
●epidermoid
optional
lumba r
drain
stabilizing
tape (over
pads)
shoulder
tape
pillow between
the legs
10-15
axillary
roll
°
Fig . 9 4 .1 La t e r a l o b liq u e ( “park bench”) position
©2001 Mark S Gre e nberg, M.D.
All rig hts re s e rve d.
Unauthorized use is prohibited.
30°
Fig . 9 4 . 2 Position of head and headholder
for right suboccipital craniectomy (looking
down on top of patient’s head)
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b) microvascular decompression
●trigeminal neuralgia
●hemifacial spasm
●miscellaneous: geniculate neuralgia, glossopharyngeal neuralgia
2. lesions of one cerebellar hemisphere:
a) tumors: metastases, hemangioblastomas…
b) hemorrhage within cerebellar hemisphere
3. access to vertebral artery
a) aneurysms: PICA, vertebrobasilar junction
b) vertebral endarterectomy
4. access to antero-lateral brainstem tumors (extreme lateral p-fossa approach)
a) foramen magnum tumors, including: chordomas, meningiomas
Position, skin incision, craniectomy, approach…
See list of altern at ives (p. 1445). See lateral obliqu e position (p. 1446).
Skin incision
Li n e a r ( p a r a m e d ia n ) in c is io n s
Access t o CPA. For m icr ova scu lar d e com p r e ssion s an d small CPA t u m o r s, a lin e ar in cisio n p r ovid es
adequate exposure and involves less trauma to overlying muscles, and may be easier to get water-
tight closure than with midline incision. For all of the following, the linear skin incision is located
5mm medial to the mastoid notch (a palpable landmark,Fig. 94 .3 ):
1. “5–6-4”incision (incision placed 5 mm m edial to mastoid notch, extending from 6 cm above
notch to 4 cm below). High enough to expose transverse sinus:
a) for approach to fifth nerve: microvascular decompression for trigeminal neuralgia
2. “5–5-5”incision (5 mm m edial, extending 5 cm up to 5 cm dow n), used for approach to seventh/
eighth nerve complex:
a) microvascular decompression for hemifacial spasm
b) small vestibular schwannoma
3. “5–4-6”incision (5 mm m edial, extending 4 cm up to 6 cm dow n): used for approach to lower
cranial nerves:
a) glossopharyngeal neuralgia
“Hockey-stick”incision
Useful for cerebellar hem ispheric lesions as well as for larger CPA lesions w here getting th e m uscles
out of the way will facilitate maneuvering instruments about the posterior fossa.
In cision is m ade in th e m id lin e startin g at ≈C2 sp in ou s p r oce ss, p ro ce e d in g su p er io r ly t o ju st
above the inion, and then laterally to just beyond the mastoid tip (Fig. 9 4 .4 ). A sh or t o p t io n al cau -
dal curve may be made laterally to further remove the muscle from the operative field.
Cr a n ie ct o m y
La n d m a r k s
Th e locat ion of t h e in fer ior m argin o f t h e t ra n sver se sin u s is qu ite a ccu r at ely e stim at ed at t w o fin -
ger-breadths above the upper limit of the mastoid notch (usually just above the superior nuchal
line). This should be the upper limit of the skull opening.
Fo r m icro vascu la r d e co m p re ssio n
Cr a n ie ct o m y ≈2cm diameter placed in the angle between transverse and sigmoid sinuses.
Fo r sm a ll t u m o rs (< 2.5 cm )
Cr a n ie ct o m y ≈4cm diameter placed in the angle between transverse and sigmoid sinuses.
Fo r la rg e t u m o r s
A la rge r cr an iect om y m ay be n ee d e d , t h e size of w h ich is lim it e d b y:
1. transverse sinus superiorly
2. foramen magnum inferiorly (which may be opened as prophylaxis against tonsillar herniation in
the event of p-fossa edema post-op)
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"Fra zie r Burr Hole "
for ve ntric ulos tomy
(see text) transverse sinuses
mastoid process
sigmoid sinus
mastoid notch
linear skin incision
craniectomy
inion
superior
nuchal line
Fig . 9 4 .3 Param edian suboccipital craniectomy
"hockey s tick"
incision
Fig . 9 4 .4 “Hockey-stick”skin incision
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