Ultrasonic Measurement Of Optic Nerve Sheath Diameter: A Noninvasive Surrogate Approach For Dynamic,Real-Time Evaluation Of Intracranial Pressure

Increased intracranial pressure?ICP?is an acute situation that is associated with poor clinical outcomes and high rates of mortality.Therefore,it is very important to make a timely diagnosis and treatment of increased ICP.Until recently,invasive ICP monitoring was the gold standard.Invasive techniques,though accurate and highly sensitive,require strict neurosurgical settings and cause side effects that must be managed such as hemorrhage,infection.Therefore,it is necessary to explore non-invasive monitoring methods.Recently,studies have reported that measuring the optic nerve sheath diameter?ONSD?using non-invasive imaging technologies provides alternative markers for early elevated ICP.The optic nerve is a part of the central nervous system,and is surrounded by dura,arachnoid,and pia mater;the optic nerve sheath is the continuation of the dura mater,and the subarachnoid space contains cerebrospinal fluid.Thus,any increase in the ICP in the subarachnoid space is transferred to the fluid in the optic nerve surrounding it.Studies have shown that ONSD in patients with elevated ICP is wider than that in patients with normal ICP.However,it remains unclear whether the acute reduction in ICP may lead to an immediate effect on ONSD.The increase of ICP can be divided into three levels:the pressure is slightly increased in the range of 15²0 mmHg?about 200²80 mmH₂O?;the pressure increased moderately in the range of 20⁴0 mmHg?about280⁵50 mmH₂O?;the pressure greater than 40 mmHg?about 550mmH₂O?is a severe increase.Generally,the slight-moderately increase of ICP is regarded as the threshold value of reducing ICP treatment in clinic.International guidelines have recommended transcranial Doppler?TCD?as an effective and powerful method to confirm the clinical diagnosis of brain death.Brain death is the final manifestation of extremely elevated ICP.However,the sensitivity of TCD to mild moderate ICP increases remains to be determined.In order to solve the above problems,we have carried out the following two parts of research:Part I Ultrasonic measurement of optic nerve sheath diameter:a non-invasive surrogate approach for dynamic,real-time evaluation of intracranial pressureAims:The current study aims to identify whether ultrasonographic measurement of ONSD could dynamically and sensitively evaluate the real-time ICP.Methods:All patients who required lumbar punctures?LP?for diagnostic purposes between August 2016 and January 2017 at the Department of Neurology were recruited.The ONSD measurements were performed approximately 5 minutes prior to and after LP.The operations were performed by two independent and experienced operators who were blind to each other's results.Two measurements were performed for each eye.The first measurement was performed in the transverse section and the second was performed in the vertical section,and thus,a total of 8 values were obtained.The final ONSD measurement value for every patient was derived from the average of the 8 values to reduce deviation.Spearman correlation analysis was performed to compare the measurements between two eyes and two observers.A Bland-Altman analysis was performed to determine inter-observer reliability.The Wilcoxon rank sum test was used to compare the differences in the ONSD before and after LP.The correlation between the ONSD and ICP was evaluated using Spearman correlation analysis.Changes in the ONSD??ONSD?and ICP??ICP?before and after LP were assessed.The correlation between?ONSD and?ICP was evaluated using Spearman correlation analysis.Results:A total of 84 patients?mean±standard deviation[SD]age,43.5±14.7 years;41[49%]men;18 patients with elevated ICP?were included in the study.Spearman correlation coefficient between the two observers was 0.779 and 0.703 in the transverse section,and 0.751 and 0.788 in the vertical section for the left and right eyes,respectively.A Bland-Altman analysis yielded a mean?SD?difference of 0.038?0.232?mm in measurements between the two observers.Limits of agreement?mean±1.96 x SD?were 0.493 and-0.417 mm.The?ONSD and?ICP of the 84patients ranged from 0 to 0.62mm and from 0 to180mmH2O,respectively.The median?IQR??ONSD and?ICP were 0.11?0.05–0.21?mm and30?20–40?mmH2O,respectively,for all participants.With a reduction in cerebrospinal fluid pressure,80?95%?subjects showed an immediate drop in ONSD;the median?IQR?decreased from 4.13?4.02–4.38?mm to4.02?3.90–4.23?mm?P<0.001?.Significant correlations were found between ONSD and ICP before LP?r=0.482,P<0.01?and between?ONSD and?ICP?r=0.451,P<0.01?.Conclusions:The study demonstrates that the ONSD decreases immediately following a reduction in CSF pressure,thereby confirming that ONSD reacts to ICP in real time.Our results,therefore,suggest that the measurement of ONSD is potentially a non-invasive approach for dynamic,real-time monitoring of ICP changes,especially in the early stages.Part II Reliability of Assessing Intracranial Pressure Using Transcranial Doppler and Ultrasonographic Measurement of Optic Nerve Sheath Diameter:A Prospective Observational Study.Aims:This study aimed to compare the reliability between ONSD and TCD in detecting potential ICP elevations.Methods:We performed a prospective observational study to compare the estimation of ICP via the measurements of TCD and ONSD.Patients who needed LP in the Department of Neurology were recruited from December 2016 to July 2017.The ONSD and TCD measurements were performed by two independently experienced observers who were blinded to each other's assessments,all of which were completed before LP.A Pearson correlation coefficient with a p-value was used to assess associations among ICP,TCD parameters,ONSD,and demographic variables,with r representing the correlation coefficient.We assessed differences in demographic variables,TCD parameters,and ONSD between the normal ICP group and elevated ICP group using an independent samples t-test.Results:Finally,165 participants?mean age,41.96±14.64 years;80 men;29 patients with elevated ICP?were included in this study.The mean ICP was 170±52 mmH₂O?range75-400mmH₂O?.Univariate analyses revealed that ICP was non-significantly associated with TCD parameters and significantly associated with ONSD?r=0.60,P<0.001?.The mean ONSD of the elevated ICP group was significantly higher than that of the normal ICP group?4.53±0.40 mm vs 3.97±0.23 mm,P<0.001?.The multivariate linear regression determined the difference between ICP and ONSD to be significantly different.Conclusions:Our results indicate that in the early stage of ICP,cerebral arterial hemodynamics assessed with TCD could not predict ICP with sufficient reliability;however,the use of ONSD may be a reliable predictive index for elevated ICP.