An idiopathic increase of intracerebral tension is a clinical setting of unknown etiology which is characterized by elevated CSF pressure more than 20 mmHg in the absence of underlying neurological abnormalities or cranial structural lesion [3]. The disease should be diagnosed early to avoid serious permanent changes in vision which may end up to blindness [9].
The gold standard for diagnosis of IIH is monitoring and measurement of the opening CSF pressure through the lumbar puncture technique, yet it is an invasive method with variable complications. Moreover, the different sites of insertion and different types of devices make it difficult to accurately diagnose and monitor idiopathic increase of intracranial tension (ICT) [10]. Several studies used the CSF pressure cut-off value of 20 mmHg (25 cm CSF fluid) for the diagnosis of raised ICP [11].
Empty sella turcica, distension of the optic nerve sheath, posterior globe flattening, and optic nerve tortuosity were significantly associated with IIH [12].
This study revealed that there was a strong correlation between the measured ONSD in the 3D DRIVE and the increased ICP more than 20 mmHg which is a cornerstone for diagnosing IIH, with a significant difference between the mean ONSD in the patients with increased ICP and the control healthy volunteers (mean = 5.81 ± 0.33 mm and 4.95 ± 0.45 mm, respectively, P value < 0.001).
To our knowledge, this was the first study which used axial thin cut 3D DRIVE sequence in measuring the ONSD, and we used 3D DRIVE as the inherited high image quality of this sequence with higher spatial and contrast resolution to allow better assessment of fluid-filled structures or those surrounded by the CSF, which gives a more accurate measurement of the ONSD, and as the ONSD measurement was of low value (in millimeters), the 3D DRIVE will be more specific for the diagnosis of increased intracranial hypertension; another advantage of the 3D DRIVE sequence is the availability of the sagittal reformatted images which allow visualization of the posterior eye globe flatting and optic nerve head protrusion in a second plan in addition to the axial plan.
Other studies also concluded a positive relationship between the ONSD and the increased ICP of more than 20 mmHg, either using an MRI technique or through US or CT techniques [4, 13,14,15,16,17,18,19,20].
A study done by Geeraerts et al. [4] also used MRI in the measurement of ONSD, yet an axial fat-suppressed T2 WI turbo spin-echo sequence was obtained on a 3-T MRI unit with relatively large slice thickness (4 mm) and interslice spacing (5 mm), yet in our study, the use of thin cut 3D DRIVE sequence with 1.4/0.7-mm slice thickness and gap allowed better demarcation of the optic nerve sheath and more accurate assessment of ONSD.
Lim et al. [13] used CT for the measurement of ONSD, yet the main disadvantage of this study was the radiation exposure.
Many studies used US for measuring the ONSD catching the advantage of US as a bedside test with less cost [15,16,17,18,19,20]. However, the main disadvantage of US is that the US is an operator-dependent technique so its measurement may be affected by the inexperience of the examiner, the other disadvantages are the poor penetration of US beam, the artifacts from the tissues under observation, the bad cutting plane, and the low spatial resolution, all these disadvantages may explain the difference in ONSD value using the MRI and the ultrasound.
The optic nerve sheath diameter was measured 3 mm behind the posterior aspect of the ocular globe, with the axis of measurement perpendicular to the nerve axis. Several studies used the same method of measurement [4, 13, 17, 18], yet the ONSD was measured 1 cm anterior to the optic foramen on an axial T2 MRI sequence in a study done by Shofty et al. [20] who compared the ONSD with IIP in pediatric patients with an idiopathic increase of intracerebral tension.
In our study, two operators, with 5 years head and neck experience, used to measure optic nerve sheath diameter in both 3D DRIVE and T2 WI sequences, the 3D DRIVE showed significantly less interobserver variability (mean of difference in 3D DRIVE and T2 WI sequences was 0.01 ± 0.08 mm versus 0.08 mm ± 0.1, respectively, P value < 0.001), and this low interobserver variably was attributed to the inherited high image quality of 3D DRIVE sequence with higher spatial and contrast resolution allowing better assessment of fluid-filled structures which give a more accurate measurement of the ONSD with higher confidence. This value was also better than the value stated by Geeraerts et al. [4] who showed that the interobserver variability mean difference was 0.11 ± 0.17 mm, yet in Geeraerts et al.’s [4] study the interobserver variability was analyzed on 22 out of 74 participants (12 out of 36 normal volunteers and ten out of 38 patients with elevated ICP) and axial proton density/T2-weighted fat-suppressed sequence was used for measuring the ONSD.
In our study, the optimal ONSD cut-off value for diagnosing high ICP more than 20 mmHg in 3D DRIVE sequence was > 5.31 mm, with 94.12% sensitivity, 82.35% specificity, and 88.24% accuracy; on the other hand, the optimal cut-off value of the ONSD measured in the T2 WI sequence was of higher value (> 5.6 mm), yet with less sensitivity, specificity, and accuracy (88.2%, 64.71%, and 76.47%, respectively), and at lower cut-off value of > 5.31 mm, the sensitivity was increased to become similar to that of ONSD cut-off value measured in 3D DRIVE sequence yet with significant reduction of specificity down to 52.94%. The better accuracy of 3D DRIVE contributed to the cisternographic effect of the 3D DRIVE sequence and its higher contrast resolution which allow more precise discrimination and more accurate measurement of ONSD, as it avoids the optic nerve sheath blurring seen in the T2 WI sequence which becomes more apparent when image zooming was performed during ONSD measurement.
A higher yet comparable cut-off value was also noted in different studies. Geeraerts et al. [4], who compared patients with post-traumatic intracranial hypertension with healthy volunteers, revealed that optimal ONSD cut-off value was 5.82 with sensitivity of 90%, which increased at lower cut-off value of 5.3 mm to become 100% at the cost of reduced specificity reaching 50%; the difference in the cut-off values between our studies was attributed to the different MRI sequence and magnet strength as well as the studied patients’ pathologies, as Geeraerts et al. [4] measured the ONSD in patients who suffered from traumatic brain injury with elevated ICP rather than patients with IIH; also, he measured ONSD on a 3-T MRI unit using fat-suppressed T2 WI turbo spin-echo sequence instead of 3D DRIVE sequence. Lim et al. [13] reported 89.9% sensitivity and 80% specificity of ONSD cut-off value of 5.5 mm yet the ONSD was measured in post-traumatic adults with high ICP using CT rather than an MRI scan.
Many studies using ultrasonographic examination were also used to measure the cut-off value for diagnosing intracranial hypertension. Jeon et al. [14] who used ultrasound to measure ONSD in patients requiring external ventricular drainage concluded that the best ONSD cut-off value for diagnosing intracranial hypertension was ONSD more than 5.6 mm with a sensitivity of 93.75% and a specificity of 86.67%. Robba et al. [15] also showed that the ONSD more than 5.85 is the best cut-off value for diagnosing intracranial hypertension in post-traumatic brain injury patients. A lower cut-off value for diagnosing high ICP was concluded in other studies measuring the ONSD by ultrasound technique in patients suffering from various brain pathology including intracerebral hemorrhage or subarachnoid hemorrhage; Rajajee et al. [16] showed that best ONSD cut-off value was 4.8 mm or more with 96% sensitivity and 94% specificity and with higher cut-off value of 5.2 mm the sensitivity was reduced down to 67% with minimal increase of specificity up to 98%. Moretti and Pizzi [17] stated that the best ONSD cut-off value was 5.2 mm with a sensitivity of 94% and specificity of 76%. Kimberly et al. [18] showed that a cut-off value of 5 mm or more was the best predictor of increased intracerebral pressure with a sensitivity of 88% and specificity of 93%. The difference in the cut-off value among these studies was attributed to different techniques used for measuring the ONSD and the different pathology involved in the elevated ICP.
To our knowledge, there was only one previous study which was done by del Saz-Saucedo et al. [19] who correlated the ONSD with the elevated ICP in adult patients suffering from IIH, yet their study showed a higher cut-off value by 1 mm more than our study, where their optimal cut-off value was 6.3 mm, with 94.7% sensitivity and 90.9% specificity; the difference in our cut-off values may be attributed to different modality used in ONSD measurement as the ONSD was measured by ultrasound technique in his study, it may be also related to the difference of the medical status of patients suffering from IIH as both of us did not correlate the ONSD with the clinical status of patient, e.g., the patients received medication for lowering the ICP or not.
The limitation of this study was related to the limited number of patients and we did not study different age groups nor different values of increased ICP as we used only the cut-off value of increased ICP more than 20 mmHg. Furthermore, a limitation related to the cost, availability, and the contraindication of MRI examination which may be of little value as the brain MRI/MRV is usually done as a routine investigation in patients with suspicious or sure diagnosis of IIH to exclude underlying neurological abnormalities of venous thrombosis. Hence, the ONSD measurement can add important clinical data on the presence of intracranial hypertension, and it may help to identify those patients who require more invasive monitoring.