Discuss the evolution of awake craniotomy and reasons for performing an awake procedure.

Perform preoperative evaluation and preparation and anesthetic technique for awake craniotomy.

Review the pathophysiology of malignant hyperthermia (MH).

Describe the triggers and presentation of MH.

Outline the steps to successfully recognize and treat an acute episode of MH.

Anesthesia for Awake Craniotomy
John C. Drummond, MD, Professor of Anesthesiology, University of California, San Diego, School of Medicine, and Staff Anesthesiologist, Veterans’ Affairs Medical Center, San Diego, CA

Evolution of awake craniotomy: anthropologists in South America have identified skulls that underwent trephination; appears patients survived long enough for bone edges to heal; Penfield described homunculus patterns (motor and sensory)

Success of awake craniotomy: presence of anesthesia provider unnecessary, therefore no excuse for physiologic harm in course of providing sedation and comfort; craniotomy technique intensely dependent on local anesthetic technique used by surgeon; anesthesia provider obligated only to monitor amount of local anesthetic used; scalp highly vascular and uptake rapid; bupivacaine more logical than lidocaine in terms of duration of action at pin sites, but toxicity causes problems; anesthesia provider should monitor dose of local anesthetic; even after good scalp infiltration, some patients have pain, particularly when dura (at base of, and under, temporal lobe) stimulated; additional local infiltration of dura commonly necessary

Rationale for awake craniotomy: to identify, by patient response, areas of brain critical for motor and sensory function or speech and memory; localization of seizure focus (concern that general anesthetic will suppress seizure responsiveness); placement of stimulators in deep brain locations in attempt to abolish movement disorders

Resections near critical areas: Rolandic sulcus (motor-sensory area) and Wernicke’s area (generates speech patterns; connected to Broca’s area anteriorly) located in temporal lobe region; surgeon uses bipolar stimulating device to locate speech arrest and motor responses (not uncommonly, anesthesia provider under drapes looking for motor response); areas of interest identified and marked on brain surface before resection undertaken (ability of patient to respond critical); portions of hippocampus and amygdala (located in medial temporal lobe) important in memory function; hippocampal resection on one medial temporal lobe generally well tolerated; when deep temporal lobe resection performed, neuropsychologist in operating room (OR)

Speech and memory: memory usually represented bilaterally; speech usually located in left hemisphere (limited to anterior portion during resection; more resection possible on nondominant [typically, right], hemisphere)

Movement disorders: likely to become more common indications for awake craniotomy; performing ablations or delivering electrical stimuli to various deep nuclei (eg, thalamic nuclei, subthalamic nucleus, substantia nigra, internal portion of globus pallidus) frequently interrupts tremors; involves aberrant reverberating circuit response; interruption of circuit frequently sufficient to interrupt abnormal movement; problem occurs because anesthetics readily suppress movements; anesthetic agents that interrupt tremor or interfere with electrophysiologic footprint and identification of nucleus of interest should be avoided

Anesthetic objectives: minimize discomfort associated with prolonged immobilization and entrapment in pin head holder; leave patient responsive for required testing; keep electroencephalography (EEG) and seizure inhibition to a minimum

Preoperative evaluation and preparation: education (explain process; patient must warn of cough, sneeze, or movement; anesthesia provider should know seizure type and be able to recognize aura [typically self-limited]); no anticonvulsant premedication if EEG localization of seizure focus planned; patient with seizure frequently undergoes subdural or epidural placement of electrode grids and video telemetry; allows correlation of clinical event to EEG

Anesthetic technique: nasal O2 (easier communication); reliable capnography; noninvasive monitors; urinary catheter; sniffing position (most important); various regimens include propofol and alfentanil; speaker uses pure propofol technique to avoid possible respiratory depressant synergism that can occur if mixed with other medications

Propofol sleep-wake intervals: begin with induction dose of propofol, 1 mg/kg (patient typically apneic for few seconds only; able to breathe spontaneously reasonably quickly); then, infuse with propofol, 100 µg/kg and titrate up or down as necessary; monitor capnography so patient breathing spontaneously but generally unresponsive to voice; sleep 1—surgeon performs local anesthetic infiltration, applies pins, and places urinary catheter; patient awakened and allowed to remain awake during placement of drapes (knows what to expect during next awake period; gets used to manipulation); repeat propofol induction and infusion procedure (pin head holder then locked down); sleep 2—surgeon performs craniotomy and dural reflection; stop propofol infusion and allow patient to awaken (perform surface mapping or EEG recording); sleep 3—once areas to be resected are identified, anesthetize patient; once resection complete, patient reawakened (if EEG monitoring necessary, electrodes reapplied to verify elimination of seizure focus; if speech mapping occurs, no second phase); sometimes when resection completed, patient not reawakened (but with EEG, verify no residual focus); sleep 4—anesthetize for closure; nasal airway not recommended (may induce coughing); propofol leaves high-amplitude, β frequency EEG footprint for 10 to 15 min after discontinuation

Provocative maneuvers: include methohexital (0.3 mg/kg as bolus) or etomidate (0.1 mg/kg); will not render patient apneic for significant length of time; both agents slightly proconvulsive and may be answer to increasing seizure focus

Supplementing propofol technique: alfentanil—speaker never uses; remifentanil—for patient uncomfortable during awake phases; give in dose of 0.01 to 0.1 µg/kg per minute (start low and increase to avoid sudden apnea); dexmedetomidine—kinetics (necessity to load; time required to achieve level) have discouraged speaker from using as add-on supplement, although many anesthetists infuse constantly at beginning; used extensively for functional localization (eg, speech arrest, stimulus at side of face, forearm twitch when stimulating surface); memory testing requires acuity; also used for identification of seizure focus; deep brain stimulation and electrophysiologic localization; study in patients with Parkinson’s disease indicates dexmedetomidine, given in dose that elicits lethargic response to name spoken in normal tone, does not interfere with either tremor (abnormal movement) or EEG footprint

Other approaches: reports of good success with low-dose propofol and patient-controlled administration; other reports of use of awake-asleep-awake techniques (patients secured with tape and not in pin head holder); another study involved use of endotracheal tube that allows instillation of local anesthesia into glottis to avoid coughing; patients in pin head holder; extubate leaving tube exchanger (with pin holes for local anesthetic instillation) in place

Malignant Hyperthermia
Daniel I. Sessler, MD, Professor and Chair, Department of Outcomes Research, Cleveland Clinic, Cleveland, OH

History: malignant hyperthermia (MH) first described in humans by Denborough in 1961; animal model developed by Nelson in 1966 (porcine stress syndrome reported in 1953 at annual meeting of Hungarian Veterinary Society); caffeine/halothane contracture test developed by Kalow and Britt in 1970; prevention and treatment by dantrolene recognized by Harrison in 1975

Muscle pathology: MH affects only skeletal muscles (cardiac muscle not directly affected); entire syndrome related to calcium control; neuromuscular junction and contractile elements of muscle normal; only voltage-gated calcium release and calcium-mediated calcium release from sarcoplasmic reticulum affected by disease; dihydropyridine receptor abnormal and releases too much calcium in susceptible individual; that calcium triggers enormous release of calcium from sarcoplasmic reticulum; leads to initial contraction, followed by contracture

Epidemiology: incidence—rare (average anesthesia provider will see one case in lifetime); more common in men; rare at extremes of age; susceptibility—autosomal dominant; variable penetrance and expressivity (susceptible patients often fail to trigger); prevalence of genetic defect may be ≤1 in 200; associated with minor myopathies —statistical associations only; include central core disease, Duchenne’s muscular dystrophy, and King-Denborough syndrome

Triggers in humans: include succinylcholine and volatile inhalation anesthetics; no evidence to support stress as trigger (swine triggered by stress; original disease called porcine stress syndrome); psychotropic drugs trigger neuroleptic malignant syndrome, but not MH

Clinical presentation: previous uneventful experience with triggering anesthetics typical; hallmark presentation combined metabolic and respiratory acidosis; blood gas most useful monitor in suspected MH; often (but not always) accompanied by red blotchy venous cyanosis; may be accompanied by rigidity (resembling rigor mortis; occurs in more severe cases); hyperthermia does not always occur (relatively early and reliable sign)

Expected consequences: pulmonary—tachypnea; arterial oxygenation remains normal or near normal; myocardium—normal; catecholamine levels increase 20-fold during MH crisis; cardiac rate and stroke volume increase to supply skeletal muscle in extreme hypermetabolic mode; ventricular arrhythmias occur, due to increased catecholamine levels; renal—oliguria occurs from breakdown of myoglobin; hepatic—hyperkalemia from glycogen use; disseminated intravascular coagulation—occurs in late stages before premorbid state

Treatment: discontinue triggering drugs (ie, volatile anesthetics)—do not treat rigidity with succinylcholine (or with nondepolarizing muscle relaxant); hyperventilate—therapeutic; use of 100% O2 not as important as hyperventilation; use minute ventilation of 16 to 18 to reduce CO2 levels); do not change anesthesia machine, soda lime, or tubing; give dantrolene—2.5 mg/kg intravenously (mannitol included in preparation; dissolves in water; requires significant time to mix); make diagnosis and institute treatment early (dantrolene prevents muscle contraction, but does not reverse contracture)

Dantrolene: diphenylhydantoin; relatively long half-life; metabolized to equally active drug; limited solubility; decreases release of calcium in muscle cells (specifically treats pathology of MH); antiarrhythmic properties; “fairly nontoxic” (occasional muscle weakness); repeat dose (≤10 mg/kg) if patient not responding; allow 30 min for initiation of effect (primary monitor during MH crisis should be arterial blood gases)

Active cooling: generally low priority during MH crisis (highest priorities discontinuation of volatile anesthetics, hyperventilation, and administration of dantrolene); forced-air cooling, circulated water, and bladder lavage mostly ineffective (after 40 min, core temperature reduced by only few tenths of degree); ice water immersion most effective but not available without preparation in routine OR; speaker discourages cooling as focus; “just controlling temperature without controlling the muscle pathology would not save people anyway”

Caffeine/halothane contracture test: remains only validated test for MH; available in ≈8 centers in North America; patient must go to center to have test; requires piece of fresh thigh muscle (usually under lateral femoral cutaneous nerve block, not volatile anesthetic); North American protocol differs slightly from European protocol in interpretation of results; answer either clearly positive, clearly negative, or equivocal; high sensitivity, not specificity

Monitoring during crisis: arterial blood gases (primary monitor); ventilate to reduce respiratory acidosis and likelihood of arrhythmia; check urine for myoglobin (give fluids to maintain renal function); check serum potassium (treatment usually not necessary; initially high, then low); plasma creatine kinase (CK) correlates with severity (useful in distinguishing MH crisis from other crisis; does not increase instantly; peak concentration occurs 18 hr after MH crisis; sample every 6 hr for 24 hr)

Safe elective anesthesia: premedication to decrease stress and nervousness; regional technique acceptable (use any type of local anesthetic, including amide anesthetics); balanced general anesthesia acceptable (including opioids, propofol, ketamine, nitrous oxide, and etomidate; avoid succinylcholine and volatile anesthetics); allow mild hypothermia

Preparation of anesthesia machine: “don’t worry about it very much”; volatile anesthetics soluble in rubber (eg, ventilator), but not in plastic; however, accumulation minimal; flush out anesthesia machine during time required to bring patient to OR (must be done properly; place breathing bag on Y-piece connector and turn on ventilator [eg, 10-L flow] to circulate gas throughout entire anesthesia machine)

Masseter muscle rigidity: not MH, but related; high risk (50%) for susceptibility to MH; relatively common; defined by tightly clenched teeth and mouth unable to be opened while rest of body in complete relaxation; administration of additional succinylcholine detrimental; pathophysiology unclear (probably type of extreme fasciculation); clinical relevance that it indicates patient may have MH; discontinue triggering drugs until blood gases obtained (also reasonable to cancel surgery, particularly if large procedure)

Summary: triggers—succinylcholine; volatile anesthetics; presentation—tachycardia; increased end-tidal CO2 ; rigidity; red blotchy venous cyanosis; hyperthermia; combine respiratory and metabolic acidosis; treatment— discontinue triggering agents; hyperventilate; dantrolene