Egyptian Board Neurosurgery: Hydrocephalus, CSF Dynamics and Skull Base Surgery

Two themes are tested together: the physics of CSF and the corridors to the skull base with their complications.

CSF dynamics

Production is 0.3 to 0.35 mL/min, roughly 500 mL daily, mainly by choroid plexus; absorption is at arachnoid granulations driven by a pressure gradient. Normal adult intracranial pressure is about 10 to 15 mmHg. Hydrocephalus is communicating when absorption fails or venous pressure is high, and non-communicating when there is obstruction. Shunt options are ventriculoperitoneal, lumboperitoneal and ventriculoatrial. Endoscopic third ventriculostomy suits aqueductal stenosis and third ventricular outflow obstruction.

Clinical syndromes

Normal pressure hydrocephalus: gait apraxia, urinary incontinence and cognitive decline, with ventriculomegaly out of proportion to sulcal enlargement. The DESH pattern shows a callosal angle under 90 degrees, tight high convexity sulci and widened sylvian fissures. A high-volume tap test or lumbar drain supports shunting, and gait improves first.
Complications: shunt malfunction with headache, vomiting and upgaze palsy; overdrainage with subdural collection, slit ventricle and postural headache; infection, usually Staphylococcus epidermidis, treated by removal and antibiotics.
Idiopathic intracranial hypertension: young woman with obesity, headache, pulsatile tinnitus and papilloedema. MRV excludes venous sinus thrombosis. Acetazolamide and weight loss first; shunt or optic nerve sheath fenestration for progressive visual loss.

Skull base

Corridors: endoscopic endonasal for pituitary, planum and clivus; pterional or orbitozygomatic for anterior and middle fossa; Kawase for petroclival; retrosigmoid for cerebellopontine angle; far lateral for the foramen magnum.
Transsphenoidal surgery risks CSF leak, diabetes insipidus in the first days and SIADH later, plus hypopituitarism.
Typical lesions: chordoma, T2 bright clival tumour; chondrosarcoma, petroclival with calcification; esthesioneuroblastoma; meningioma.
CSF leak: confirm with beta-2 transferrin and repair in layers.

Worked example

A 72-year-old with gait deterioration, mild cognitive change and ventriculomegaly undergoes a tap test. Improvement in gait after large-volume drainage justifies a shunt, whereas no change argues against it.

Traps

Diagnosing normal pressure hydrocephalus from imaging alone.
Attributing late postoperative hyponatraemia to diabetes insipidus when it is SIADH.
Shunting an obstructive hydrocephalus without considering third ventriculostomy.
Operating on a prolactinoma before checking prolactin.

Peripheral Nerve and Plexus Surgery

Expect anatomy recall in plexus format, a traction injury scenario, and timing rules that decide between observation, graft and transfer.

Brachial plexus anatomy

Roots C5 to T1 form upper, middle and lower trunks; each divides, then forms lateral, posterior and medial cords giving terminal branches. The upper trunk (C5-6) is most often injured in traction, producing Erb palsy with the arm adducted, internally rotated and the wrist flexed. Lower trunk injury (C8-T1) produces Klumpke palsy with intrinsic hand weakness and Horner syndrome. Root avulsion is preganglionic: Horner syndrome, preserved sensory nerve action potentials, and denervation of paraspinal muscles. Avulsion cannot be repaired directly and requires nerve transfer.

Timing and technique

Sharp laceration: explore and repair early.
Closed injury: allow for Wallerian degeneration, which takes about 3 weeks, then use EMG and clinical review; explore at 3 to 6 months if there is no recovery.
Options run from neurolysis and direct repair to cable grafting, conduits for short gaps, and transfers: Oberlin ulnar fascicle to biceps, double fascicular transfer, intercostal to musculocutaneous, spinal accessory to suprascapular.
A Tinel sign advancing distally indicates regeneration.

Entrapment neuropathies

Carpal tunnel: median nerve, nocturnal pain, thenar wasting, positive Phalen and Tinel signs; release the flexor retinaculum.
Cubital tunnel: ulnar nerve with clawing of the fourth and fifth digits and a positive Froment sign; decompress, with or without transposition.
Guyon canal: pure hand ulnar deficit, sometimes after hook of hamate fracture.
Common peroneal nerve at the fibular neck: foot drop after prolonged leg crossing or knee surgery.
Radial nerve in the spiral groove: wrist drop.

Worked example

A motorcyclist falls and cannot abduct or externally rotate the shoulder, with intact hand function: an upper trunk injury. EMG at 3 to 4 weeks confirms denervation. If there is no recovery by 3 to 6 months, nerve transfer rather than prolonged observation is offered.

Traps

Performing EMG too early.
Missing preganglionic avulsion suggested by Horner syndrome.
Confusing radiculopathy with a peripheral nerve lesion.
Delaying exploration of a penetrating injury.

Neurocritical Care, Perioperative Management and Complications

This domain tests the routine decisions that prevent secondary injury: ventilation targets, sodium handling, prophylaxis and recognition of specific syndromes.

General measures

Ventilation: PaCO2 35 to 40 mmHg, PaO2 above 80, oxygen saturation above 94 percent; lung-protective settings in acute lung injury.
Head up 30 degrees, avoid neck line compression, control fever aggressively, and keep glucose between 110 and 180 mg/dL.
ICP monitors: external ventricular drain for pressure measurement and drainage, or parenchymal monitor without drainage; zero at the tragus and follow clamping protocols.
Mechanical thromboprophylaxis early; pharmacological prophylaxis after the aneurysm is secured or the intracranial lesion is stable.

Sodium disorders after brain injury

Central salt wasting and SIADH both cause hyponatraemia after subarachnoid haemorrhage. Salt wasting shows hypovolaemia, high urine sodium, high urine output and raised BNP, and is treated with volume and hypertonic saline. SIADH shows euvolaemia and is treated with fluid restriction. Restricting fluid in salt wasting causes vasospasm and infarction.
Diabetes insipidus: urine output above 200 mL/hour with low specific gravity and rising sodium; replace with half-normal saline and give desmopressin.
Correct sodium by less than 10 mmol/L in 24 hours to avoid osmotic demyelination.

Specific syndromes

Neurogenic stunned myocardium and Takotsubo after subarachnoid haemorrhage: troponin rise with apical ballooning; avoid aggressive diuresis and treat as cardiac dysfunction, not infarction.
Non-convulsive status should be excluded by EEG in a comatose patient who does not improve as expected.
Withdrawal of life-sustaining treatment follows a formal brain death protocol with documented cause, absent brainstem reflexes and a positive apnoea test, performed and documented twice.

Worked example

Day 5 after poor-grade subarachnoid haemorrhage, sodium is 128 mmol/L with urine output of 250 mL/hour and low central venous pressure. This is central salt wasting; treat with normal saline and hypertonic saline, not with fluid restriction.

Traps

Fluid restriction in central salt wasting.
Correcting hyponatraemia too fast.
Treating fever as benign in a brain-injured patient.
Missing non-convulsive seizures in a comatose patient.

Basic Surgical Sciences, Ethics, Research and Evidence-Based Practice

This section is calculable and therefore high yield: test characteristics, risk measures, study design and consent.

Test characteristics and risk

Sensitivity is true positives over true positives plus false negatives; specificity is true negatives over true negatives plus false positives. Positive and negative predictive values depend on prevalence.
A highly sensitive test is useful to rule out disease; a highly specific test rules it in.
Relative and absolute risk reduction differ: reducing risk from 2 to 1 percent is a 50 percent relative reduction but a 1 percent absolute reduction, giving a number needed to treat of 100.
A p value is the probability of the observed data, or more extreme data, if the null hypothesis were true; it is not the probability that the null is true. A confidence interval expresses precision, and a risk ratio interval crossing 1 is not significant.
Type I error is rejecting a true null, alpha usually 0.05; type II error is failing to reject a false null, beta; power is 1 minus beta.

Study design

Randomised trial: randomisation controls confounding, analysed by intention to treat.
Cohort: incidence and relative risk. Case-control: odds ratio for rare disease. Cross-sectional: prevalence. Systematic review and meta-analysis rank highest for therapy, with GRADE used to rate certainty.

Ethics and law

Valid consent requires capacity, which means understanding, retaining, weighing and communicating a decision; plus voluntariness and disclosure of material risks.
Emergency treatment proceeds under necessity; incapacitated adults are treated in their best interests.
Research ethics rests on the Declaration of Helsinki and the Belmont principles of respect for persons, beneficence and justice, with ethics committee approval and genuine equipoise.
Declare conflicts of interest, protect confidentiality and disclose complications honestly.

Worked example

A shunt infection test has sensitivity 90 percent and specificity 95 percent; prevalence is 10 percent. In 1,000 patients there are 100 infected and 900 not: 90 true positives, 10 false negatives, 855 true negatives and 45 false positives. Positive predictive value is 90 of 135, about 67 percent; negative predictive value is 855 of 865, about 99 percent. A positive result needs confirmation; a negative result is reassuring.

Traps

Confusing relative with absolute risk reduction.
Quoting a p value as the probability the null is true.
Judging a test by sensitivity alone when prevalence is low.
Treating statistical and clinical significance as identical.