Related studies on measuring the normal values of optic nerve sheath diameter in healthy Chinese adults based on CT scans

The optic nerve enters the middle cranial fossa through the optic foramen in the orbital apex, and enters the brain through the optic chiasm and optic tract. The optic nerve sheath diameter (ONSD) surrounding the optic nerve is composed of dura mater, arachnoid membrane and pia mater². The space between dura mater and arachnoid was subdural space. The cerebrospinal fluid in the subarachnoid space is directly connected to the subarachnoid space between the arachnoid membrane and the pia mater. The change of cerebrospinal fluid pressure in the subarachnoid space will be transmitted in a very short time and reflected in the change of ONSD. Therefore, ONSD can be used as a non-invasive method for noninvasive monitoring of ICP³. There are relatively few studies on CT measurement of ONSD in the Chinese population, and there are almost no studies of this size. Compared with previous studies, this study increases the selection of the location of ONSD measurement, and also adds ONSD/ETD value, which will also be an important evidence for ICP monitoring.

Comparison of different imaging examinations

In a review that included almost all imaging methods to measure ONSD in healthy people⁶, the most accurate ONSD measurement was MRI, followed by CT, and the results of ultrasound measurement were slightly worse than the former two. MRI Has a high resolution of soft tissue and is relatively accurate in ONSD measurement, but its examination time is too long, the price is expensive, the use of maintenance equipment is not convenient for patients, and in some primary hospitals, there is even no MR Equipment, the above reasons limit the application of MR In patients with acute increased ICP. As the most convenient imaging equipment, ultrasound measurement mostly depends on the operator’s own technology, and the measurement results are also quite different. Moreover, it can not intuitively respond to the injury in the brain, and is rarely used in brain diseases. At present, there are relatively few studies on CT measurement of ONSD in healthy adults, but the role of CT in ICP monitoring cannot be ignored. CT is usually used as the first choice for ICP diseases caused by craniocerebral trauma, cerebral hemorrhage, cerebral hernia, and so on. It is also the first choice for the review of many brain diseases. CT has the advantages of fast scanning speed, fast three-dimensional reconstruction speed, slice thickness can be adjusted to 1 mm, and preoperative and postoperative evaluation. It has many advantages for the monitoring of craniocerebral diseases and increased ICP. In this study, we initially enrolled 60 patients and measured the Optic Nerve Sheath Diameter (ONSD) in both axial and sagittal positions. The results revealed that the differences in ONSD values measured at these two positions were not statistically significant. This indicates that ONSD measurements can be sufficiently performed in the axial position without the need for additional three-dimensional reconstruction. Furthermore, studies¹ have found no difference in ONSD measurements between the cerebral axial CT plane and the specifically reconstructed optic nerve axial plane, suggesting that dedicated CT reconstruction for ONSD measurement is unnecessary, which will reduce the post-processing time for CT scans. This approach will save time for patient treatment. Therefore, in subsequent parameter measurements, we directly used the 1 mm slice thickness CT images from the cerebral scan in the axial position for ONSD measurement, without performing additional three-dimensional reconstruction on the optic nerve.

In our study, we found that the measurement results of different imaging techniques were not the same. The reference value range of our measurement results at ONSD3mm was 5.28 (5.23–5.32) mm, which was lower than the reference ranges of 5.9 mm and 6.0 mm of other domestic researchers in the diagnosis of increased ICP^7,8, indicating that our measurement was relatively accurate, and at the same time, it was close to the ONSD3 mm range of normal people measured by Liu Yinlong⁹. Liu Chang¹⁰ measured the ONSD at 3 mm by MR And the result was 4.76 (4.72–4.80) mm. Wang Lijuan and Chen F measured the range of healthy adults by ultrasound as 3.46 (4.42–3.49) mm and 5.1 (4.7–5.4) mm respectively^11,12. Due to different imaging principles, there are also differences in measurement results. Therefore, normal reference ranges of different imaging methods are needed in clinical work.

Relative ratio of measurement at 3 mm and 10 mm ONSD

In this study, different from previous studies, we selected two positions of ONSD for measurement, and the results showed that the average difference of measurement results at 3 mm and 10 mm was statistically significant (t = 31.78, P < 0.001). The value of ONSD at 3 mm was greater than that at 10 mm, which was consistent with the results of previous studies⁵. For ICP monitoring, the most stable results can be obtained by measuring the diameter at 10 mm from the eyeball⁵, because nystagmus, gaze deviation and involuntary eye movements after trauma or stroke are not affected or less affected at this depth, and the measurement is more accurate. In addition, during the measurement process, we found that the ONSD at 3 mm posterior to the eyeball of some subjects was relatively thick because it was too close to the eyeball, which may cause measurement bias. We inform patients to keep their eyes as still as possible because movement of the eyeball can also cause changes in the ONSD values.At present, there is still no relatively good measurement position, and multi-position measurement will increase the accuracy of ICP monitoring. In the future, researchers can include patients with confirmed increased ICP, multi-position measurement of ONSD, and further explore a more accurate measurement position.

Some researchers have proposed to use ONSD/ETD instead of ONSD alone for ICP monitoring, because it can reduce the normal variation of ocular phenotype and reduce the error caused by large differences in ONSD. It may be more sensitive and specific for the evaluation of increased ICP¹³. The value of ETD/ONSD obtained by CT measurement can also predict the malignant progression of stroke, the late malignant progression of increased ICP and midline shift^14,15, and more additional information can be obtained, all of which indicate that ONSD/ETD has good application value in ICP monitoring.

In our examination, we found that the difference in ETD (Eyeball Transverse Diameter) values between the two eyes was statistically significant (22.48 ± 0.85 vs. 22.45 ± 0.86, P = 0.017). We analyzed the possible reasons as follows. First, there can be natural anatomical variations between the two eyes in terms of size and structure, which can lead to differences in measurements such as the optic nerve sheath diameter (ONSD). Additionally, the dominant eye often exhibits slightly different characteristics compared to the non-dominant eye, including variations in optical properties and even subtle differences in eye movement patterns. These factors can contribute to the differences we observed in our measurements. Furthermore, individual differences in eye movement and fixation during the imaging process can also play a role. Even though participants are instructed to keep their eyes still, minor involuntary movements or differences in gaze direction between the two eyes can affect the measurement outcomes .

Racial differences

In our study, we found that different ethnic groups had different ONSD measurements. Compared with the results of previous studies in China, the normal reference range of ONSD in this study was consistent with the previous studies^7,8,9, indicating that our measurement results were more accurate, and our large sample size also made the subjects in this study more representative and the risk of bias lower. In addition, we summarized the ONSD3mm values of healthy people measured by multiple CT at home and abroad, and found that, for example, the measured values of American populations such as Canada and the United States were large, with the mean values above 6.0 mm^16,17; the mean ONSD values of Asian and African populations such as Iraq and Nigeria were slightly smaller than those of American populations^17,18,19. However, the population in Switzerland and other European regions is in the middle level, with an average value of about 5.4 mm^20,21. The above causes may be caused by potential genetic factors, living environment, social economy and other factors.

It should be noted that the above results are only a pooled analysis of multiple different studies, and the specific differences may be affected by a variety of factors, such as sample selection, measurement methods, and study design. Therefore, in the future studies, we need to design experiments more rigorously and expand the sample size to further explore the differences of optic nerve sheath and its influencing factors among different ethnic groups.

Gender differences

In this study, we found that there was a significant correlation between gender and ONSD, and the differences of ONSD and ONSD/ETD values between different genders were statistically significant, and the normal reference range of males was larger than that of females. This conclusion is consistent with the research conclusions of many imaging methods at home and abroad, including CT, MR And ultrasound measurement results^10,11,22,23. There may be differences in nerve fiber density due to gender¹⁰. In addition, we found from some literature that the skull of women is smaller than that of men. The research results of Graillon et al. analyzed the gender difference in the volume of the orbital cavity on the 3D reconstruction model, and the results showed that the volume of the orbital cavity of the male skull was statistically significantly higher than that of the female skull, and according to the volume of the orbital cavity, The accuracy of gender inference was 77.3%²⁴. Kaplanoglu and Andrades also found that the orbital diameter and volume of males were larger than those of females^25,26. Based on the skull and orbital anatomy, we speculated that gender differences in the eyeball would affect the ONSD value, and that males would be larger than females. Therefore, in clinical work, men and women should have their own reference ranges.

Limitations and future prospects

The following findings were found: ① The sample was selected from a single center; ② This study is a cross-sectional study, which has certain regional and time limitations; ③ Too few influencing factors were included in ONSD. In the future, multi-center studies with larger sample sizes are expected to be further standardized. This study only included adults, and future studies could also measure the optic nerve sheath in minors. In addition, future studies could also include patients with increased ICP to verify the accuracy of normal reference values.