Single centre, retrospective study of patients receiving ICP monitoring during 2013. For each patient, the following measurements were recorded (A , B and L recorded bilaterally):
- ONSD 3mm behind the globe - maximum recorded (A)
- ONSD half way between the globe and the superior orbital fissure (SOF) - average recorded (B)
- Distance from the globe to the SOF (L)
- Anterior-posterior diameter of the foramen magnum (FM)
Optic nerve ratio (ONR) and ValX were calculated using equations 1 and 2, respectively (Figyre 1). The strength of the relationship between ValX and ICP was assessed using Pearson´s correlation coefficient (r). A receiver operating characteristic (ROC) curve was produced to assess the ability of ValX to predict ICP above 15mmHg. A subset was re-measured by a second assessor and interclass correlation coefficient (ICC) was used to assess inter-rater reliability.