Discussion: Optic Nerve Sheath Diameter on Computed Tomography Scans Reflects Elevated Intracranial Pressure in Patients with Craniosynostosis

There has been a great deal of study regarding the importance of intracranial pressure (ICP) in the management of craniosynostosis. The incidence, implications, and even the diagnosis of intracranial hypertension have all been heavily debated. Consensus has been hard to achieve, in no small part because of the difficulty in measuring this pressure. Generally speaking, measuring ICP requires an invasive procedure to place the probe. In lieu of this, indirect findings have been used as a proxy. Perhaps the most widely used is funduscopy to detect papilledema; however, this is plagued by low sensitivity.1 In this study, Romeo and colleagues used low-dose, noncontrast computed tomographic (CT) scans to measure optic nerve diameter.2 Increasing diameter was used as an indication of increased ICP. One hundred thirty-two patients undergoing cranial vault remodeling were enrolled. This included primary and secondary reconstructions. In addition to measuring optic nerve diameter on the preoperative scan, at the time of induction, every patient underwent spectral domain–optical coherent tomography. This is a test that acts much like an ultrasound of the retina.3,4 In previous studies, these measurements correlated quite well with direct ICP measurements5,6 but require pupillary dilation and sedation in young children. After this, at the time of craniotomy for calvarial reconstruction, the ICP was measured using subdural catheterization. On final analysis, it was found that maximum optic nerve sheath diameter (ONSD) was significantly larger in patients with ICP greater than or equal to 15 mmHg than in those with a diameter less than 15 mmHg. Analysis of ONSD cutoffs generally showed increasing specificity and decreasing sensitivity with larger sheath diameter thresholds. ONSD of 5 mm or greater demonstrated 85% sensitivity and 30.9% specificity for ICP greater than or equal to 20 mmHg, whereas a cutoff of 8 mm had a 2.6% sensitivity and 100% specificity. In other words, increasing the threshold of ONSD leads to underdiagnosis of raised ICP, but all patients who demonstrated ONSD of 8 mm or greater had intracranial hypertension. When the authors correlated ONSD with age in an algorithm, both sensitivity and specificity increased substantially. It was also found that ONSD measurements were more accurate for detecting elevated ICP in primary and nonsyndromic cases. This is unfortunate, as the concern for increased ICP is greater in secondary cases and in patients with multisuture or syndromic craniosynostosis. The vast majority of patients with a diagnosis of craniosynostosis will undergo surgery, regardless of ICP derangements. It is during the follow-up of these patients postoperatively that the question of increased ICP is often raised. In a 2018 study by Pellicer and colleagues,7 a cohort of 383 patients with craniosynostosis was retrospectively evaluated for headaches and intracranial hypertension. Elevated ICP was diagnosed if the patient developed papilledema or had elevated intracranial or lumbar pressures by direct measurement. Although overall, 6% of the patients required revision surgery secondary to intracranial hypertension, the rate was 13% in syndromic patients, 5.8% in multisuture synostosis, and only 2.7% in single-suture nonsyndromic synostosis. Of note, 33% of the entire cohort complained of headaches postoperatively, resulting in a positive predictive value of this symptom for ICP of less than 10%. Although the specificity of papilledema in the diagnosis of raised ICP is approximating 100%, the sensitivity of this test is less than 50%.1 It is in the group of patients with no evidence of papilledema but with worrisome complaints in the postoperative setting that this measurement has its greatest utility. Certainly, the value of this measurement is not just in patients undergoing cranial vault reconstruction but also in the general pediatric population. Children presenting to pediatricians with worrisome clinical signs and symptoms of increased ICP who undergo a CT scan would benefit from evaluation of optic nerve sheath diameter before significant clinical decisions are made. It is important to note that there are new techniques being developed to assess intracranial pressure, including deep learning approaches using ICP calculation from commonly obtained measurements such as arterial blood pressure, photoplethysmography, and electrocardiography.8 However, with our current level of technology, this easy-to-obtain CT-derived measurement should prove useful in helping to evaluate patients with suspected intracranial hypertension. DISCLOSURE The author has no financial interest to declare relation to the content of this Discussion or of the associated article.