Trajectories for frontal external ventricular drain placement: virtual cannulation of adults with acute hydrocephalus.

OBJECTIVES: External ventricular drains (EVDs) are commonly placed freehand using targeting landmarks unchanged since the pre-CT era; it is known to be an inaccurate procedure. To our knowledge, this is the first study to assess the geometric reliability of specific trajectories in a three-dimensional model.
DESIGN: Three-dimensional volume reconstruction of EVD trajectories in a Stealth Station S7.
SUBJECTS: Adults with a primary EVD sited for acute hydrocephalus secondary to spontaneous subarachnoid haemorrhage with CT angiography less than 24 hours previously.
METHODS: CT angiograms from 10 consecutive patients meeting the inclusion criteria were reconstructed. The surgical planning tool was used to construct three trajectories from Kocher's point: i) perpendicular to the skull (PTS) ii) towards the ipsilateral medial canthus coronally and the external auditory meatus sagitally (IMC) iii) towards the contralateral medial canthus coronally and the external auditory meatus sagitally (CMC). Their engagement with the frontal horn of the ipsilateral lateral ventricle (FILV) and distance from the ventricular wall and foramen of Monro were measured.
RESULTS: Mean supratentorial ventricular volume was 55.8 cc (range 35.2-83.4 cc). The IMC met the FILV in only one patient, on average missing the ventricular wall by 5.5±2.3 degrees (95% confidence interval). CMC and PTS met the FILV in 9 and 10 cases, respectively. Mean engagement was 16.3±5.1 mm (95% confidence interval) for PTS and 20.0±7.1 mm (95% confidence interval) for CMC. CMC and PTS gave significantly better engagement and aiming error margins than the IMC trajectory.
CONCLUSIONS: Despite its widespread use, the IMC trajectory performed poorly; PTS and CMC trajectories are more reliable ways of targeting the FILV when placing an EVD.