Julia Purchla, MS4
Ultrasound Journal Club, Trauma Article, December 2019: Young, A., Guilfoyle, M., Donnelly, J. et al. Correlating optic nerve sheath diameter with opening intracranial pressure in pediatric traumatic brain injury. Pediatr Res 81, 443–447 (2017).
Background: In pediatric patients with severe traumatic brain injury (TBI), increased intracranial pressure (ICP) is associated with secondary brain injury in the form of limitations to cerebral blood flow and cerebral oxygen and thus, is a strong predictor of poor neurologic outcomes and associated with increased mortality. This makes diagnosing increased ICP crucial as it guides the clinician’s decision in placing an ICP monitoring device and can reliably allow the clinician to maintain adequate cerebral perfusion pressure (CPP) and oxygenation. The gold standard of monitoring ICP is placement of an invasive monitoring device, and the indications to do so are the following: (i) a Glasgow coma score of £8 in the presence of an abnormal CT head scan or (ii) with a normal head CT scan, but have >2 risk factors (SBP <90 mm Hg, decorticate or decerebrate posturing); (iii) multiple injuries with an altered level of consciousness; (iv) suspected to be at risk of elevated ICP. The traditional features of an abnormal head CT scan (midline shift, abnormal basal cistern and sulcal effacement, cerebral herniation, ventricular compression) are unfortunately unreliable to predict ICP as ICP is dynamic and can change acutely. An alternative to this is evaluating the optic nerve sheath diameter (ONSD). It is continuous with the meninges and encased by subarachnoid membrane, and in the setting of increased ICP, CSF builds up in the optic nerve sheath widening its diameter. This phenomenon can be seen within minutes of any acute changes in ICP. In adults, the measurement of the ONSD using ultrasound has been shown to correlate with increased ICP and has done so reliably however, this requires technical expertise. Alternatively, CT and MRI measurements of ONSD have been validated in adults, it has not been shown in the pediatric population. This study aims to review head CT scans of pediatric patients with TBI and correlate with ONSD with opening ICP at the time of surgery.
Methods: The data were collected retrospectively from records of severe TBI pediatric patients admitted to Addenbrookes Hospital Pediatric Intensive Care Unit (PICU) between January 2009 and December 2013. Patient demographics and 6-month follow-up data were obtained from electronic records. Information on admitting clinical features (e.g., Glasgow Coma Scale (GCS), hypotension and hypoxia) were obtained from the PICU discharge summary. CT findings were obtained from referring images to the Department of Neurosurgery, Addenbrookes Hospital.
Inclusion Criteria: (i) Confirmed traumatic brain injury (structural abnormality confirmed on CT or MRI), (ii) severe injury or failure to demonstrate significant early clinical improvement (i.e. poor neurology on sedation hold) or an injury which required close conservative management or neurological monitoring in PICU, (iii) patients requiring invasive monitoring of ICP and arterial blood pressure.
Patients were managed according to current TBI guidelines and the insertion of an intracranial monitoring device is part of routine clinical practice. Interventions were aimed at keeping ICP < 20 mm Hg using a tiered treatment protocol.
Two primary investigators of the study not involved in data collection and blinded to the patient’s condition independently measured bilateral ONSD from the initial admission CT head scan (all performed on a 16 section multi-detector row CT scanner in spiral mode with 1 mm section thickness). Each observer scrolled through the images to identify the level demonstrating a visual estimation of the largest diameter, the image was magnified, and the ONSD on each nerve was measured using electronic calipers, once per side. The ONSD was measured as previously described by Legrand et al. Briefly, the ONSD was measured 3 mm behind the insertion of the optic nerve into the globe, perpendicular to the long axis of the optic nerve using a digital viewer (Centricity PACS, General Electric Healthcare, Chicago, IL) with an electronic caliper. The mean ONSD was calculated between both sides, which was used for the analysis.
Primary Endpoint: To correlate measured ONSD with opening ICP at insertion of invasive monitoring probes
Results: 36 patients were enrolled. The inter-rater agreement for ONSD measurements as defined by single-score intraclass correlation was 0.91 (95% CI: 0.85– 0.94; P < 0.0001)[JP1] . The median ICP was 18 ± 10 mmHg (median ± IQR), the median right ONSD was 5.6 ± 2.5 mm and the left was 5.9 ± 3.2 mm. Both mean ONSD and maximum ONSD were significantly correlated with initial ICP (r = 0.712, P < 0.0001 and r = 0.713, P < 0.0001, respectively). Linear regression of ONSD against ICP, age, and sex found that only ICP was significantly associated with ONSD (P < 0.0001); there was no significant relationship between ONSD and age (P = 0.970) or sex (P = 0.340). Area under Receiver-operating characteristic (ROC) curve for both mean and max ONSD was 0.85 [JP2] (95% CI: 0.71–1.00). The optimal threshold of mean ONSD identified by maximizing the J statistic was 6.1 mm (sensitivity 77%, specificity 91%) and the optimal threshold of maximum ONSD was 6.3 mm (sensitivity 77%, specificity 87%[JP3] ). The threshold for 100% sensitivity for mean ONSD was 4.9 mm (specificity 26%) and for maximum ONSD was 5.2 mm (specificity 30%).
Statistical Analysis: Inter-rater reliability of ONSD measurement between investigators was quantified with the single-score intraclass correlation coefficient; agreement between investigators was assessed using a Bland-Altman plot[JP4] . The correlation between ONSD and ICP was evaluated with Pearson’s coefficient[JP5] . Classification of the initial ICP following insertion of monitoring was high (≥20 mmHg) or low (<20 mmHg). Receiver-operating characteristic curves [JP6] of patients’ mean and maximum ONSD against the dichotomized initial ICP were then constructed and the area-under- the-curve (AUC) [JP7] was determined. Generally, AUC > 0.8 is considered to indicate “good” performance. The optimal threshold of ONSD balancing sensitivity and specificity for predicting high ICP was calculated by maximizing Youden’s J statistic[JP8] ; the threshold for 100% sensitivity was also noted.
Pitfalls and weaknesses: An important limitation of this study is the small sample size.
The Takeaway: ONSD on CT can be used as a predictor of raised intracranial pressure [JP9] in the pediatric population with TBI. When pediatric patients present with an ONSD of over 6.1 mm following a TBI, ICP monitoring should be implemented[JP10] . For values lower than 6.1 mm, there remains a considerable risk of a patient having a raised ICP and as such treatment and management should be implemented considering the entire clinical picture.