This study applied transorbital ultrasound to quantify PGF in patients with IIH using the posterior globe angle (PGA). Here, we were able to show for the first time, that PGA measurements differ significantly between IIH patients and healthy controls. We compared PGA measurements at different positions of the angle vertex using ROC analysis, which found the highest AUC for measurements at 3.0 mm distance from the lamina cribrosa (AUC = 0.74). A cutoff of ≥ 118.5° PGA would identify IIH patients with 100% specificity while retaining a sensitivity of 37.5%. The results of this preliminary case-control study indicate that transorbital ultrasound may be applied to identify and quantify PGF in IIH patients.

In 1993, Gibby et al. described a “flattening of [the] normal outward convexity” of the posterior sclera on computed tomography images of IIH patients [20]. Since then, multiple studies have investigated PGF in IIH using magnetic resonance imaging [10, 11, 13, 21,22,23,24,25,26]. The overwhelming majority of these studies based the radiological assessment on a qualitative, subjective evaluation of axial T2-weighted images. Consequently, reports on diagnostic accuracy have produced inconsistent results, with sensitivity and specificity of PGF in IIH varying between 28.8 and 78.6% and 59.5–99.1%, respectively [27]. Similarly, reports by Delen et al. and Alperin et al. only show moderate interrater-reliability of quantitative PGF evaluation (k = 0.46–0.64) [22, 28]. Still, the available studies suggest that PGF may show diagnostic utility as a rather specific imaging finding.

In 2013, Alperin et al. reported the first quantitative assessment of PGF and optic nerve protrusion in IIH using MRI in a cohort of 7 IIH patients and 6 healthy controls [28]. The authors developed software to convert the three-dimensional structure of the globe into a color-coded two-dimensional distance map, visualizing the distances between the center of the globe to different points on the posterior sclera. While Alperin et al. argued automated, quantitative measurements would likely improve the detection of globe distortions in IIH, their findings have yet to be confirmed in subsequent studies using larger cohorts.

Compared to MRI, transorbital ultrasound is a cost-effective, widely available, and easy-to-use examination method that is particularly suitable for follow-up examinations. Since it is already established to measure OND and papilledema in routine clinical practice, it seems reasonable to extend its scope of application to assess deformations of the posterior ocular globe in IIH. The diagnosis of IIH remains a challenge, as reports on diagnostic error are prominent in the medical literature [6, 29]. The improvement of available diagnostic modalities, as well as the establishment of new diagnostic approaches could improve detection and disease monitoring of IIH.

This preliminary study is limited by its small case size due to the rarity of the disease. Although the current diagnostic criteria were applied, the IIH cohort included patients with active IIH and patients in the stage of ocular remission, which may have affected our results. In addition, the examiners who performed the ultrasound were not blinded to the patients’ medical history, including IIH diagnosis, which introduces a risk of bias. Consequently, further studies are necessary to confirm our findings using extended cohorts, a blinded study design and concomitant MR-imaging of the ocular globes as reference.

It is currently unclear to what extent PGA measurements are influenced by morphological variations of the ocular globe, e.g. in myopia or hyperopia. Theoretically, transorbital ultrasound examination could be extended to include further measurements to adjust for differences in ocular globe morphology. For PGA measurements, a transverse image plane at the insertion of the optic nerve into the eyeball is set. However, bulb movements may impede the assessment and lead to oblique imaging planes, possibly distorting measurements. We recommend placing the examiner’s proximal wrist on the patient’s forehead to avoid unnecessary pressure on the eye during the assessment and to ensure sufficient application of ultrasound gel. These precautions, in addition to direct instruction of the patient, help to reduce the occurrence of bulbar movements.