Neuroanesthesia and Neuromonitoring Flashcards

During spinal deformity correction requiring intraoperative SSEP monitoring, the anesthesiologist adds ketamine as an analgesic adjunct at a subanesthetic dose (0.3 mg/kg/hr infusion). Which of the following most accurately describes ketamine's effect on somatosensory evoked potentials?

Answer: Enhancement of cortical SSEP amplitude with minimal effect on latency

Ketamine is unique among anesthetics in that it enhances — rather than suppresses — cortical SSEP amplitude. This is attributed to its NMDA receptor antagonism, which differs mechanistically from the GABAergic suppression caused by propofol, volatile agents, and benzodiazepines. Ketamine has minimal effect on SSEP latency and is occasionally used as a deliberate adjunct when SSEP signals are of poor quality or difficult to obtain. This property also makes it safe for use in cases where other agents are suppressing signals to uninterpretable levels.

A patient with severe TBI (GCS 6) is being monitored using continuous pressure reactivity index (PRx). The PRx–CPP plot shows a U-shaped curve with a minimum PRx of 0.04 at CPP 76 mmHg (CPPopt). The current CPP is 62 mmHg with a real-time PRx of 0.38. Which statement most accurately characterizes this patient's cerebrovascular state?

Answer: Autoregulation is impaired at the current CPP; cerebral blood flow is pressure-passive and the patient is operating significantly below CPPopt

PRx is calculated as the moving correlation coefficient between arterial blood pressure and ICP. A PRx > +0.2–0.3 indicates pressure-passive (autoregulation-impaired) cerebral blood flow, while PRx < +0.2 suggests intact autoregulation. CPPopt is the CPP at which PRx is minimized (most negative or lowest positive). Here, the current CPP of 62 mmHg yields PRx 0.38, indicating impaired autoregulation with pressure-passive CBF. The patient is operating 14 mmHg below CPPopt (76 mmHg), suggesting that increasing CPP toward the optimal value could restore autoregulatory capacity and improve cerebrovascular regulation.

During awake craniotomy for eloquent-cortex glioma resection, direct cortical stimulation at 6 mA triggers a focal clonic seizure of the left hand that persists for 25 seconds after stimulation has stopped. The patient remains alert. Which of the following is the most appropriate IMMEDIATE intervention?

Answer: Direct irrigation of the exposed cortex with cold (4°C) Ringer's lactate solution

Cold saline or Ringer's lactate irrigation applied directly to the exposed cortex is the first-line treatment for intraoperative electrocorticographic seizures during awake craniotomy. The thermal effect rapidly depolarizes and silences focal cortical activity without affecting the patient's systemic consciousness or cooperation — which is essential to continue language and motor mapping. Systemic agents (benzodiazepines, propofol) would impair the patient's responsiveness, rendering the awake portion of the procedure uninterpretable. IV levetiracetam is used prophylactically but does not provide the immediate local termination required in this setting.

Transcranial Doppler (TCD) performed on day 7 following aneurysmal subarachnoid hemorrhage reveals: right MCA mean flow velocity 218 cm/s, right extracranial ICA mean flow velocity 46 cm/s. The patient has no new neurological deficits. What is the most precise interpretation of this TCD pattern?

Answer: Moderate cerebral vasospasm without symptomatic ischemia; the Lindegaard ratio confirms true luminal stenosis

The Lindegaard Ratio (LR) = ipsilateral MCA mean velocity ÷ extracranial ICA mean velocity = 218 ÷ 46 ≈ 4.7. An LR > 3 distinguishes true vasospasm from hyperemia (LR 6 to severe vasospasm. Although an MCA velocity of 218 cm/s might suggest severe spasm by velocity alone, the LR of 4.7 places this in the moderate category. The patient's lack of neurological deficits is consistent with vasospasm without infarction. This distinction is clinically critical: hyperemia would discourage vasopressor use, whereas moderate-to-severe vasospasm warrants aggressive hemodynamic optimization and close monitoring.

During open thoracoabdominal aortic aneurysm (Crawford extent II) repair, bilateral lower-extremity MEPs are lost abruptly 35 minutes after aortic cross-clamping. Bilateral SSEPs remain entirely unchanged throughout. The surgical team is preparing to close. What is the most accurate interpretation and appropriate response?

Answer: Anterior spinal cord ischemia is present; SSEPs are preserved because the posterior columns receive separate vascular supply via the posterior spinal arteries

MEPs assess the integrity of the corticospinal tracts in the anterior and lateral columns, which are perfused by the single anterior spinal artery (ASA). SSEPs assess the dorsal (posterior) columns, perfused by the paired posterior spinal arteries (PSAs). In thoracoabdominal aortic surgery, segmental intercostal and lumbar arteries (the Adamkiewicz artery in particular) supply the ASA. Their sacrifice creates anterior cord ischemia, resulting in MEP loss while SSEPs are preserved — the classic discordant pattern. This is a neurosurgical emergency requiring immediate action: increasing MAP, reducing CSF pressure, and considering intercostal artery reimplantation. Proceeding to closure risks permanent paraplegia.

Intraoperative electrocorticography (ECoG) during temporal lobe epilepsy surgery reveals no spontaneous epileptiform discharges after propofol-based anesthesia is reduced. The neurosurgeon requires pharmacological activation of the seizure focus to delineate the resection margin. Which agent, administered as a low-dose IV bolus, most reliably and selectively activates epileptiform activity for ECoG mapping?

Answer: Methohexital 20–30 mg — the only barbiturate with reliably epileptogenic (pro-ictal) properties at subanesthetic doses

Methohexital is pharmacologically unique among barbiturates in possessing epileptogenic rather than anticonvulsant properties at subanesthetic doses. Unlike thiopental or phenobarbital, which universally suppress neuronal excitability, methohexital activates epileptiform discharges and has been used for decades as a pharmacological activator during ECoG mapping in epilepsy surgery. A small IV bolus (20–30 mg) transiently reveals the ictal focus while reducing background noise, facilitating accurate resection margin delineation. Etomidate can also evoke myoclonic activity but is less selective. Ketamine produces non-specific high-frequency activity that may confound mapping. Remifentanil has no established role in seizure focus activation.