How does an external ventricular drain (EVD) work?

How an External Ventricular Drain Works

Basic Mechanism and Purpose

An external ventricular drain (EVD) is a catheter surgically inserted through the skull into the brain's lateral ventricles to drain cerebrospinal fluid (CSF) and measure intracranial pressure (ICP), thereby reducing elevated pressure and preventing secondary brain injury. 1, 2

The fundamental principle involves creating a controlled pathway for CSF to exit the ventricular system when normal drainage is impaired or when ICP becomes dangerously elevated. 3

Physical Components and Setup

The EVD system consists of:

• A ventricular catheter inserted through a burr hole in the skull into the lateral ventricle, typically via a frontal approach 1, 2
• A drainage collection system positioned at a specific height relative to the patient's head 3
• A pressure transducer that continuously measures ICP 2
• A drainage bag that collects the CSF 4

The height of the drainage system relative to the patient's head determines the pressure threshold at which CSF will drain—this is a critical adjustable parameter. 3

Mechanism of CSF Drainage

The EVD works through pressure equalization: when ICP exceeds the height of the drainage system (measured in mmHg or cmH2O), CSF flows out of the ventricles through the catheter into the collection bag. 3

• The pressure equalization (PE) ratio quantifies how effectively the system reduces ICP: it is calculated as (pre-drainage ICP - post-drainage ICP) divided by (pre-drainage ICP - EVD height) 3
• Patients with CSF outflow obstruction (such as hydrocephalus from intraventricular hemorrhage) typically have high PE ratios (0.86 ± 0.36), meaning they respond very well to drainage 3
• Patients with cerebral swelling (such as traumatic brain injury) have lower PE ratios (0.43 ± 0.31), indicating that pressure reduction is limited because the problem is brain tissue expansion rather than CSF accumulation 3

Dual Function: Monitoring and Treatment

The EVD serves two simultaneous purposes:

1. ICP monitoring: The catheter provides continuous, real-time measurement of ventricular pressure, which is the gold standard for ICP assessment 2
2. Therapeutic drainage: By removing CSF, the EVD reduces intracranial volume and thereby lowers ICP according to the Monro-Kellie doctrine 1, 2

The system can be toggled between these functions—closed for continuous pressure monitoring or open for drainage. 3

Clinical Applications

Primary Indications

• Intraventricular hemorrhage (IVH) with hydrocephalus: EVD placement prevents obstructive hydrocephalus and maintains cerebral perfusion pressure 1
• Traumatic brain injury with elevated ICP: EVD drains CSF when ICP exceeds safe thresholds (typically >20-25 mmHg) 5, 2
• Acute hydrocephalus from any cause: The drain bypasses blocked CSF pathways 1, 6

Enhanced Efficacy with Adjunctive Treatments

When combined with intraventricular fibrinolysis (IVF) for IVH, the EVD becomes significantly more effective:

• IVF reduces catheter occlusion rates from 37.3% to 10.6% 1
• Mortality decreases from 40.9% without IVF to 22.4% with IVF (OR 0.39, p<0.00001) 1
• Good functional outcomes improve from 38.3% to 47.2% 1
• Complete clot resolution occurs 3 days faster with IVF 1

Surgical Technique Considerations

Bolted catheters are superior to tunneled catheters for infection prevention and mechanical stability:

• Bolted EVDs reduce infection risk compared to tunneled catheters (OR 0.60, p<0.001) 1
• CSF leakage occurs in only 3.2% of bolted catheters versus 36% of tunneled catheters 1
• Antibiotic-coated catheters are superior to both silver-impregnated and uncoated catheters (p<0.001) for preventing ventriculitis 1

The insertion should ideally occur in a single-bed ICU room when placed outside the operating room, as this reduces infection rates from 34% to 13% compared to multiple-bed rooms. 7

Complications and Tissue Effects

Infection Risk

• EVD-related infections occur in 8.3-30% of cases, with device-associated infection rates of 10.4 per 1000 drainage days 7, 4
• Coagulase-negative Staphylococcus accounts for 62% of infections, followed by Enterococcus species (19%) 4
• Infection risk increases significantly with drainage duration beyond 5-7 days 1
• CSF leakage for >1 day dramatically increases ventriculitis risk (21.1% versus 0% for <1 day) 1

Mechanical Brain Injury

EVD insertion causes predictable tissue damage:

• Small hemorrhages with hyperdense cores surrounded by hypodense edema occur in 21% of patients (33/155), with average initial volume of 8.16 mL increasing to 15 mL after 10 days 8
• All patients develop small hypodense areas (average 0.38 mL) around the catheter representing both intracellular and extracellular edema 8
• These lesions typically do not cause neurologic deterioration or ICP elevation 8
• Risk factors for hemorrhagic complications include arterial hypertension, coagulation abnormalities, and multiple EVD insertions 8

Critical Management Parameters

Drainage Strategy

The EVD is typically set at 15 mmHg threshold in the Trendelenburg position for patients with CSF hypotension, or at standard heights (10-15 cmH2O above the external auditory meatus) for routine ICP management. 1, 3

Duration Considerations

• Median drainage duration is 7 days (range 1-44 days) 4
• Catheters should be removed as soon as clinically possible, preferably before 5 days, to minimize infection risk 1
• Patients with EVD-related infections have significantly longer ICU stays (21 versus 11 days, p<0.01) and hospital stays (28.5 versus 20 days, p<0.01) 4

Shunt Dependency

Approximately 14.9-15.8% of patients require permanent CSF shunting after EVD treatment, particularly those with pan-ventricular hemorrhage (Graeb score >3) or hematoma in the third and fourth ventricles. 1 Importantly, IVF does not reduce shunt dependency (OR 0.99, p=0.98). 1

Special Populations

ECMO Patients

In patients on extracorporeal membrane oxygenation, EVD insertion is a high-risk procedure due to systemic anticoagulation requirements and coagulopathy, but may be considered in selected patients at imminent risk of death from intraventricular hemorrhage with hydrocephalus. 1

Premature Infants

For posthemorrhagic hydrocephalus in premature infants, there is insufficient evidence to recommend specific infant weight or CSF parameters to direct timing of shunt placement, though EVD or ventricular reservoirs are commonly used as temporizing measures. 1