Research trends in optic nerve sheath diameter monitoring – A bibliometric study

INTRODUCTION Intracranial pressure (ICP) monitoring is an interesting subject relating to sentinel surveillance in neuroanaesthesia. The continuity of cerebrospinal fluid along the optic nerve within a dural sheath and the corresponding ease of visualisation on sonography have captivated attention for conferring non-invasive diagnostic utility.[1] In a systematic review involving traumatic brain injury (TBI) patients, Xu et al. plot a sensitivity and specificity of 85% and 88%, respectively, for ultrasonography-assessed optic nerve sheath diameter (ONSD) compared to invasive ICP measurements.[2] Moreover, there has been considerable enthusiasm for applying ONSD monitoring beyond the traditional horizons of acute neurosurgical care.[3,4] Hence, we conducted a bibliometric analysis to explore the research trends in ONSD monitoring. METHODS The PubMed database was searched for the keyword 'optic nerve sheath diameter'. Metadata from January 1960 to December 2023, consisting of 1170 publications, was downloaded as a text (.txt) file. We subsequently used the VOSviewer version 1.6.20 (Leiden University Centre for Science and Technology Studies, Leiden, The Netherlands) to analyse the visualisation pattern of the weighted total link strength (network, overlay and density), the keyword co-occurrence and the clustering of keywords. Research hotspots in ONSD were analysed using the co-occurrence of author keywords in the VOSviewer. A minimum keyword occurrence of five was set as the limit, yielding 75 keywords from 1276 author keywords. RESULTS There has been a sustained rise in PubMed articles with the keyword 'optic nerve sheath diameter', particularly from 2008, with 2023 witnessing the highest number (n = 159). Network visualisation analysis of the 75 selected keywords was derived using the VOSviewer [Figure 1a]. The circle size in the hotspot portrays the frequency of keyword occurrence, with the interconnecting line thickness representing the co-occurrence of keywords. 'Intracranial pressure, Ultrasound, Ultrasonography, Optic nerve, Traumatic brain injury, Intracranial hypertension, Optic nerve sheath, ONSD, Idiopathic intracranial hypertension and Ocular ultrasound' were among the top 10 keywords identified in relation to the author keyword 'optic nerve sheath diameter'. The co-occurrence of keywords among the eight clusters using the VOSviewer, their colour coding and the broad theme they deal with were identified [Figure 1b]. The cluster density visualisation analysis and the overlay analysis of the co-occurred keywords for the last ten years (2014–2023, during which most of the publications in this field originated) are depicted in Figure 2a and b. The network analysis with a filter of authors with at least five publications on ONSD found 59 authors [Figure 2c].Figure 1: (a) Network visualisation analysis (weighted-to-total links) using VOSviewer of the author keywords related to PubMed search keyword optic nerve sheath diameter. (b) Illustrating individual author keyword clusters (in no specific order of frequency) and colour-coded as identified by VOSviewer. ICP = intracranial pressure, ONSD = optic nerve sheath diameter, POCUS = point-of-care ultrasonography, TBI = traumatic brain injuryFigure 2: (a) Cluster density visualisation. (b) Overlay visualisation analysis (weighted to links) of author keywords related to PubMed search keyword optic nerve sheath diameter (blue–green–yellow transition denoting recency of co-occurred keyword). (c) Network visualisation analysis of the related authors on VOSviewerDISCUSSION Keywords provide insights into the content of an article. Authors choose keywords wisely while preparing an academic draft to achieve an appropriate indexing that ensures ample dissemination and citation of the concerned article. Bibliometric analysis using these keywords can help understand the emerging frontiers in a topic. The top 10 co-occurred keywords and ONSD point to its ever-increasing acclaim for non-invasive ICP measurement in neurosurgery.[1,2] Nevertheless, the utility of ONSD to predict intracranial hypertension in diverse medical conditions outside of TBI cohort is signified by the unanimous presence of keywords like 'point-of-care', 'ultrasonography' and 'ICP' in majority of the clusters, along with the scattered presence of keywords like 'hyponatremia', 'pseudotumor cerebri', 'hydrocephalus', 'stroke', 'multiple sclerosis' and 'optic neuritis'. ONSD monitoring is widely applied for neurological outcome prognostication [cluster 3 in Figure 1b]. A broadened ONSD (>5.5 mm) was associated with poor neurological outcomes in the pooled data evaluated by Zhang et al.[5] among post-cardiac arrest patients (Odds ratio [OR]: 15.62, 95% confidence interval [CI]: 5.50-44.34 [P < 0.001]); and was also the case for stroke patients in the Xu et al.[6] cohort (mean [standard deviation] ONSD 5.87 (0.86) mm among the poor outcome group versus 5.21 (0.69) mm in the favourable outcome group [P < 0.001; OR: 2.83, 95% CI: 1.94-4.15]). Furthermore, Maissan et al.[7] highlighted that the dynamic ICP variations could be mapped in real-time by ONSD imaging in the neurointensive care unit [cluster 6 in Figure 1b]. ONSD monitoring has also acquired importance in non-neurological intensive care scenarios, as evidenced in clusters 2, 3 and 4. A systematic review by Bajwa et al.[3] estimated ONSD in pre-eclampsia, with measurements reaching as high as 6.93 (0.22) mm in patients with severe pulmonary congestion. Das et al.[8] delineated an ONSD of 5.4 (4.9–5.7) mm versus 4.6 (4.1–5.3) mm (P = 0.01), respectively, in paediatric acute liver failure with and without hepatic encephalopathy. Among patients with hyponatraemia, Demir et al.[9] found ONSD higher than optimal by 0.546 (0.068) mm versus 0.448 (0.081) mm (P < 0.05) in symptomatic and asymptomatic patients, respectively. An observational study by Peng et al.[10] assessing the diagnostic utility of an ONSD decline to predict post-dural puncture headache made for the newest addition in this arena, yet it did not feature in the 75 keywords analysed, likely as a keyword co-occurrence limit of five was preset. Simultaneously, indexing the measurements to the eyeball transverse diameter (ETD) features as a novel diversification to enhance the monitoring precision,[11] as elucidated in keyword clusters 5 and 6. Herein, Youm et al.[11] attribute a higher power to the ONSD/ETD ratio for predicting raised ICP (area under curve = 0.897) and correlate a differential predictive ability for ONSD measurements including and excluding dura mater (AUC = 0.855 versus 0.783, respectively). Of note, Kim et al.[12] used computed tomography to measure ONSD, ensuring a better imaging resolution and potentially eliminating the blooming artefacts in ultrasonography, the increasing recognition of which is reflected in cluster 4 of our keyword network visualisation. The discovery of negligible author networks [Figure 2c], excluding the high-volume research output from the group that popularised ONSD in 2008, bears testimony to the fact that diverse authors obtained findings indigenous to their settings. The overlay visualisation network [Figure 2b] found that the research appeal in this topic has shifted towards neurocritical care, stroke and emergency department, utilising corroborative parameters like ETD and Doppler-derived pulsatility index. However, our study was limited by including all the PubMed titles irrespective of the category (original/reviews/correspondences). This could have resulted in the repetition of keywords relating to the widely debated themes. CONCLUSION Bibliometrics have exciting trends to reveal regarding the clinical application of optic nerve sheath diameter for non-invasive intracranial pressure monitoring, noticing recent significant expansion within and outside the domain of neurosciences, and that too, accompanied by an improved focus on technical aspects of measurements. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.