Neurosurgical Principles for the Non-Surgeon

All brain catastrophes are subject to numerous physiological cascades over time after the initial event, with each having typical timelines of recovery. Such phenomena as cerebral edema, intracranial pressure (ICP) elevation (intracranial hypertension), low cerebral perfusion, hyperthermia, hypoxia, and herniation are regularly observed in the intensive care unit (ICU) and practitioners must not only be able to recognize them early, but also intervene to correct them in the most expeditious and targeted manner to avoid death and disability.

Konakondla, et al (2019)

All such phenomena occasionally also occur outside the ICU, and outside hospitals but in the community and must be anticipated and accommodated for.

CNS contents by volume:

1. Brain + CSF (80-85%)
2. Cerebral Blood Volume (CBV) = 3-7%
3. CSF (5-12%)

Raised Intracranial Pressure (ICP) relates to a reduced Cerebral Perfusion Pressure (CPP)

• translocation of CSF to spinal sites, and increased absorption
• reduced CBV – the smallest fraction (percentage) by volume, but responds rapidly to reduce CO2

CPP = MAP – ICP

MAP/MCAP (mean arterial pressure) when supine;   MCVP = 2- 5 mm Hg higher

Aetiology – SOL, CSF retention,  CBV, oedema

Intracranial

• Head injuries: mass lesions + oedema
• Tumour: mass lesions +/- oedema
• Subarachnoid Haemorrhage (SAH) / Intracranial haemorrhage (ICH) / Post-ischaemia
• Hydrocephalus (pseudotumour cerebri)
• Infection

Systemic

• Hypertensive / metabolic encephalopathy
• Reye’s syndrome
• Infection
• Osmolar imbalance
• O₂ /  CO2
• Impaired cerebral venous drainage
  • Coughing / straining / physiotherapy / head down
  • JVP compression
  • Incorrect mechanical ventilation

Clinical Presentation

• ICP alone
  • Headache, vomiting, papilloedema
• Local brain distortion
  • Headache / vomiting / drowsiness
  • Decerebrate rigidity
  • Third Nerve palsy
  • Bradycardia, hypertension
  • Abnormal respiration
  • Impaired brainstem reflexes

Ischaemia

• Level of Unconsciousness / Unconscious
• Fixed dilated pupils
• Arterial hypertension
• Apnoea
• Absent brainstem reflexes

ICP Monitoring

• External Ventricular Drain (EVD)
• Subdural monitor – e.g. Richmond screw
• Extradural
• Parenchymal

Complications

• Infection: 2-7%
  • > 5 days
  • Leak
  • Compound fracture
• lntracranial haemorrhage

Intracranial Hypertension: Management

1. Mass lesion
   • Surgical decompression
   • Osmotic diuretics / hyperventilation / dexamethasone preoperatively
2. CSF retention
   1. CSF drainage
      • EVD
      • Reservoir
      • V-P shunt
3. CSF formation
   • Lasix
   • Acetazolamide (Diamox)
4. Cerebral Blood Volume (CBV)
   • MAP within limits of cerebral autoregulation
     • B-blockers often better than vasodilators
   • Head-Up to facilitate venous drainage
     • Especially if PEEP
   • Avoid large ⇑ CVP / intrathoracic pressure
   • Avoid coughing/ straining
   • Avoid extreme flexion / rotation of neck
   • Adequate analgesia before physiotherapy / endotracheal aspiration
   • Hyperventilation to PaC02 30-35 (35-40) with adequate muscle relaxation, analgesia and sedation to AWP, pain and avoid anxiety
   • Thiopentone
     • Cerebral vasoconstriction
     • For coughing and straining

Cerebral Oedema: Management

Cerebral oedema is an increase in the cellular water content of the brain due to vasogenic and/or cytotoxic causes.

1. Fluid restriction – prevent hyponatraemia and water overload while maintaining CVS stability and good renal function of 1.5-2 L/day (for 70 kg person)
2. Diuretics
   1. Osmolar – Mannitol
      1. 0.25 – 0.5 g/kg (or 1 g/ kg) to keep ICP < 20
      2. Stat or q 6-hourly for 24 hours
   2. Loop – frusemide
3. Steroids
   1. Especially with head tumours
   2. Head injury?
   3. lschaemia?
4. General measures
   1. Hyperventilation
   2. Cerebral Blood Volume (CBV)
5. Thiopentone – patient must be euvolaemic and asymptomatic
   1. 3 mg/kg loading ➔ 100-200 mg over 30-60 min
   2. Arterial line
   3. Swan-Ganz catheter
   4. Ventilation + ABGs
   5. ICP monitor
   6. Cardiac monitoring
   7. Temperature monitoring
   8. Brainstem AIVP
6. Surgery

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References

Konakondla, Sanjay; Brimley, Cameron J., Timmons, Shelly D. “Neurosurgical Physiology and Neurocritical Management of the Acute Neurosurgical Patient.” Operative Neurosurgery 17 (Supplement 1), August 2019: S17–S44. https://doi.org/10.1093/ons/opz090.