Posterior Fossa Procedures
Most of the topics relevant to posterior fossa procedures ( Table
53-12
) have been discussed in the first half of the chapter and include
the sitting position and its cardiovascular effects and complications (quadriplegia,
macroglossia) (see the section "Positioning"), pneumocephalus (see "Pneumocephalus"),
and VAE and PAE (see "Venous Air Embolism"). These sections should be read in conjunction
with this segment. Use of the sitting position to facilitate surgery on the posterior
fossa increases the likelihood of all of these phenomena, although they are relevant
to nonsitting positions as well. This section will review the cardiovascular events
associated with direct stimulation of the brainstem and their possible implications
for postoperative management.
Brainstem Stimulation
Irritation of the lower portion of the pons, the upper part of
the medulla, and the extra-axial portion of the fifth cranial nerve can result in
a number of cardiovascular
perturbations. The former two areas are most often stimulated during procedures
on the floor of the fourth ventricle and the last area during surgery at or near
the cerebellopontine angle, for example, acoustic neuromas and microvascular decompression
of the fifth (tic douloureux), seventh (hemifacial spasm), or ninth (glossopharyngeal
neuralgia) nerves. The cardiovascular responses may include bradycardia and hypotension,
tachycardia and hypertension, bradycardia and hypertension, or ventricular dysrhythmias.
[311]
Meticulous attention to the ECG and a directly
transduced arterial pressure during manipulation in this region are necessary to
provide the surgeon with immediate warning of the risk of damage to the adjacent
cranial nerve nuclei and respiratory centers. Pharmacologic treatment of the dysrhythmias
that occur may serve to attenuate the very warning signs that should be sought.
Balloon compression of the trigeminal ganglion is another situation
in which dysrhythmias may occur. The procedure attempts to produce neurapraxia of
the fifth cranial nerve by the rapid inflation of a Fogarty-type balloon within Meckel's
cave.[312]
[313]
The balloon is introduced percutaneously through the cheek and beneath the maxilla.
The procedure is best accomplished with general anesthesia because both the entry
of the needle into Meckel's cave and the balloon compression (lasting several minutes)
are intensely stimulating. A relatively profound, though transient bradycardia will
occur and is, in fact, sought as confirmation of adequate compression. External
pacemaker pads have been advocated but are, in our experience, unnecessary.
Any irritation and injury of posterior fossa structures that may
have occurred during surgery should be taken into account when planning extubation
and postoperative care. In particular, procedures involving dissection on the floor
of the fourth ventricle entail the possibility of injury to cranial nerve nuclei
or postoperative swelling in that region, or both. Attention should be paid to the
fact that cranial nerve dysfunction, particularly nerves IX, X, and XII, can result
in loss of control/patency of the upper airway and that swelling of the brainstem
can result in impairment in both cranial nerve function and respiratory drive. The
posterior fossa is a relatively small space, and its compensatory latitudes are even
more limited than those of the supratentorial space. Relatively little swelling
can result in disorders of consciousness, respiratory drive, and cardiomotor function.
The anesthesiologist and surgeon should interact to make decisions regarding whether
extubation is appropriate and the location where postoperative observation should
take place (i.e., ICU or non-ICU).
Spontaneous ventilation was once advocated for procedures that
entailed a risk of damage to the respiratory centers. Spontaneous ventilation is
now rarely used because the proximity of the cardiomotor areas to the respiratory
centers should permit cardiovascular signs to serve as an indicator of impending
injury to the latter. It is our opinion that the respiratory pattern is more likely
to be a relevant monitor when the "threat" to the brainstem is the result of vessel
occlusion (as might occur with accidental interruption of perforating vessels during
vertebrobasilar aneurysm surgery[239]
) than when
it is due to
direct mechanical damage caused by retraction or dissection in the brainstem.
Various electrophysiologic monitoring techniques may be used during
posterior fossa surgery, including somatosensory evoked responses, brainstem auditory
evoked responses, and electromyographic monitoring of the facial nerve. The latter
requires that the patient not be paralyzed or have a constant state of incomplete
paralysis. Somatosensory evoked response monitoring imposes some constraints with
respect to the selection of anesthetics, as discussed in Chapter
38
.
