Ultrasound elastography is a noninvasive imaging method that evaluates the mechanical and elastic properties of soft tissues by estimating the strain modules from radiofrequency signals during externally applied compression–relaxation cycles . This method has been used to measure elasticity and stiffness of the tissues to differentiate for instance the tumor tissue from inflammation and normal tissue. Evaluation of some solid organs such as thyroid , prostate , liver , breast  and cervix  have been performed with this method for many years it was first found in 2010 to be useful in ocular and periocular tissues  as well.
With the introduction of USE into ophthalmology, a limited number of preclinical and clinical studies have been conducted to assess its usefulness. In 2014, Detorakis et al. revealed that the USE was useful in detecting in vivo rigidity changes in the anterior segment in an experimental study, which was conducted in a rabbit eye model to evaluate lens, ciliary body, and total ocular rigidity changes . In a clinical study, Unal et al. investigated the effects of ocular rigidity changes on optic nerve in patients with POAG using USE method and found a stiffer ONH in eyes with POAG when compared with non-glaucomatous control eyes . Then, the evaluation of the ONH by using USE became more common and has been applied in some other optic nerve related diseases such as multiple sclerosis , Behçet's disease , non-arterial anterior ischemic optic neuropathy , and optic neuritis . In all these diseases, the elastography findings of the ONH were significantly different from the healthy volunteers. With the light of this significant information, we aimed to utilize USE in the present study in the evaluation of ODD and in the differential diagnosis of ODD with ODE, which has been still considered as a challenging condition in clinical practice.
Since ODD is known as calcified hyaline body located within the ONH, it can be expected that its’ stiffness is higher than other tissues around . With this hypothesis, in this study, we chose the orbital fat tissue as a reference, which is softer than optic nerve, and evaluated ONH-elasticity in proportion to this tissue. According to our results, the ONH- elasticity was significantly lower in patients with ODD compared to patients with ODE and healthy controls. There was no statistically significant difference with respect ONH-elasticity between patients with ODE and healthy controls. In addition, we performed a ROC analysis to evaluate how effective the ONH-elasticity could be in the differential diagnosis of ODD and ODE. When the cut-off point for IF/ONH ratio was set at 2.29, 80% sensitivity and 60% specificity were obtained. The area under curve (AUC) value of ONH-elasticity was 0.728 with a significant difference. With this result, we could state that ONH-elasticity measurement seems to be a good predictor in the differential diagnosis of ODD and ODE and it can be used in daily clinic, since the differentiation of these two diagnoses is quite challenging and the diagnostic tools are still limited.
On the other hand, a recent study investigated the relationship between the cornea and the optic nerve in terms of biomechanical properties using USE, found a positive correlation between corneal rigidity and optic nerve rigidity in patients with NAION . In the light of these results, we wanted to investigate the corneal biomechanics (by evaluating CH, CRF, IOPcc and IOPG values) in our study groups and their relationship with optic nerve elasticity. According to our results, there was no significant difference between groups with respect to corneal biomechanics, however there was a significant correlation between corneal and ONH biomechanics only in drusen group (negative correlation between CH and ONH biomechanics and positive correlation between IOPcc, IOPg and ONH biomechanics). There was no correlation between CRF and ONH biomechanics in any group, however CRF was previously shown to be relatively independent of IOP, therefore our results are consistent with the literature.
Our study has some limitations. We used strain elastography in our study since it was the only tool we had in our hospital, however it has some limitations for application in ocular and periocular tissues (the compression effects in deeper orbital layers may not be adequate). Besides, the examination area was small and tightly organized and obtaining a high-resolution image was not easy, therefore, we had to repeat tests to get the best image. Despite these limitations, we believe that our study presents promising findings in support of the use of USE in differentiating ODE from ODD. As far as we know, this is the first study in the literature using USE in the evaluation of ODD and in the differential diagnosis of ODD with ODE.
In conclusion, USE seems to provide useful data in the evaluation of the ONH and in the differentiating ODD from ODE. Revealed significant correlation between corneal and ONH biomechanics also seems interesting, however further studies with larger population are needed to confirm these promising results.