An evaluation of ocular elasticity using real-time ultrasound elastography in primary open-angle glaucoma

New forays into measurement of ocular biomechanics

PURPOSE OF REVIEW

The field of corneal biomechanics has rapidly progressed in recent years, reflecting technological advances and an increased understanding of the clinical significance of measuring these properties. This review will evaluate in-vivo biomechanical properties obtained by current technologies and compare them regarding their relevance to established biomechanical properties obtained by gold-standard ex-vivo techniques normally conducted on elastic materials.

RECENT FINDINGS

Several new technologies have appeared in recent years, including vibrational optical coherence tomography (VOCT) and the corneal indentation device (CID). These techniques provide promising new opportunities for minimally invasive and accurate measurements of corneal viscoelastic properties.

SUMMARY

Alterations in corneal biomechanics are known to occur in several corneal degenerative diseases and after refractive surgical procedures. The measurement of corneal biomechanical properties has the capability to diagnose early disease and monitor corneal disease progression. Several new technologies have emerged in recent years, allowing for more accurate and less invasive measurements of corneal biomechanical properties, most notably the elastic modulus.

Real-time ultrasound elastographic features and color doppler imaging of mitral valve prolapse

Background: The aim of this study was to investigate the elasticity of ocular structures in patients with mitral valve prolapse (MVP). Material and Methods: This prospective study included a total of 35 patients with MVP (study group) and 35 healthy volunteers (control group). The elastography value of the ratio of orbital fat- sclera (ROF/S) was measured with real-time US elastography. For each eye, central retinal artery (CRA), posterior ciliary artery (PCA), and ophthalmic artery (OA) were evaluated, respectively. Results: The mean ages of the patients in the study and the control groups were 31.77 ± 11.40 years, and 30.65 ± 7.45 years, respectively (P =0.511). Mean ROF/S were 1.95 ± 0.81 and 1.37 ± 1.06 (P=0.001) in the study groups and control, respectively. The mean RI of the OA was 0.67 ± 0.05 in the control group, 0.67 ±0.05 (0.55; 0.87) in study group. The mean RI of the PCA was 0.66 ± 0.05 in the control group, 0.68 ±0.06 in study group. . The mean RI of the CRA was 0.66 ± 0.05 in the control group, 0.66 ±0.06 in study group. The RI value was not a significant difference between control and study group (p > 0.05). Conclusion: Scleral elasticity was significantly increased in MVP patients. These could be related to ocular pathologies such as glacouma, kerataconus in MVP.

Ocular Ultrasound: Review of Bioeffects and Safety, Including Fetal and Point of Care Perspective: Review of Bioeffects and Safety, Including Fetal and Point-of-Care Perspective

Ocular ultrasound is an invaluable tool for the evaluation of the eye and orbit. However, the eye and orbit are potentially sensitive to the thermal and mechanical effects of ultrasound. When performing B-mode imaging, dedicated ocular settings should be used. If these settings are not available, limiting the acoustic output to Food and Drug Administration (FDA) recommended maximum levels is strongly advised. Especially important is the acoustic output in spectral (pulsed) and color Doppler modes, which can exceed the FDA's maximum recommended levels for the eye. Adjusting settings to decrease acoustic output and limiting the time of the examination should be done when performing a Doppler examination. The acoustic output of shear wave elastography is significantly higher than FDA guidelines for the eye and should be considered experimental.

Ultrasound Elastography in Ocular and Periocular Tissues: A Review

Ultrasound elastography has become available in everyday practice, allowing direct measurement of tissue elasticity with important and expanding clinical applications. Several studies that have evaluated pathological and non-pathological tissues have demonstrated that ultrasound elastography can actually improve the diagnostic accuracy of the underlying disease process by detecting differences in their elasticity. Ocular and periocular tissues can also be characterized by their elastic properties. In this context, a comprehensive review of literature on ultrasound elastography as well as its current applications in Ophthalmology is presented.

Ultrasound Shear Wave Elastography and Transient Optical Coherence Elastography: Side-by-Side Comparison of Repeatability and Accuracy

Objective: We compare the repeatability and accuracy of ultrasound shear wave elastography (USE) and transient optical coherence elastography (OCE). Methods: Elastic wave speed in gelatin phantoms and chicken breast was measured with USE and OCE and compared with uniaxial mechanical compression testing. Intra- and Inter-repeatability were analyzed using Bland-Altman plots and intraclass correlation coefficients (ICC). Results: OCE and USE differed from uniaxial testing by a mean absolute percent error of 8.92% and 16.9%, respectively, across eight phantoms of varying stiffness. Upper and lower limits of agreement for intrasample repeatability for USE and OCE were ±0.075 m/s and −0.14 m/s and 0.13 m/s, respectively. OCE and USE both had ICCs of 0.9991. In chicken breast, ICC for USE was 0.9385 and for OCE was 0.9924. Conclusion: OCE and USE can detect small speed changes and give comparable measurements. These measurements correspond well with uniaxial testing.

Biomechanical properties of retina and choroid: a comprehensive review of techniques and translational relevance

Studying the biomechanical properties of biological tissue is crucial to improve our understanding of disease pathogenesis. The biomechanical characteristics of the cornea, sclera and the optic nerve head have been well addressed with an extensive literature and an in-depth understanding of their significance whilst, in comparison, knowledge of the retina and choroid is relatively limited. Knowledge of these tissues is important not only to clarify the underlying pathogenesis of a wide variety of retinal and vitreoretinal diseases, including age-related macular degeneration, hereditary retinal dystrophies and vitreoretinal interface diseases but also to optimise the surgical handling of retinal tissues and, potentially, the design and properties of implantable retinal prostheses and subretinal therapies. Our aim with this article is to comprehensively review existing knowledge of the biomechanical properties of retina, internal limiting membrane (ILM) and the Bruch’s membrane–choroidal complex (BMCC), highlighting the potential implications for clinical and surgical practice. Prior to this we review the testing methodologies that have been used both in vitro, and those starting to be used in vivo to aid understanding of their results and significance.

Effects of glaucoma and central corneal thickness on optic nerve head biomechanics

PURPOSE

To evaluate effects of glaucoma and central corneal thickness (CCT) on optic nerve head biomechanics.

METHODS

Four groups were formed according to CCT measurements and the presence of glaucoma. Glaucomatous patients with thin (< 510 µ) and thick (> 570 µ) corneas composed groups 1 and 3, respectively. Nonglaucomatous patients with thin and thick corneas composed groups 2 and 4, respectively. Real-time elastography (RTE) was performed on all groups, and optic nerve strain rate (ONSR), orbital fat strain rate (OFSR), and strain ratio of orbital fat to the optic nerve and medial and lateral parts of the optic nerve (SROFON, SROFMON, SROFLON, respectively) were obtained and compared between groups. The correlations between CCT and these parameters were also investigated.

RESULTS

Statistically significant difference was not found between groups in terms of strain rate of optic nerve and orbital fat, SRFON, SROFMON and SROFLON. There was a positive correlation between ONSR and OFSR and mean CCT in patients with CCT thinner than 510 µ (p: 0.03 r: 0.26, p: 0.01 r: 0.32 respectively).

CONCLUSION

SROFON, SROFLON and SROFMON values did not differ between glaucomatous and nonglaucomatous patients with thin or thick CCTs. The correlations between CCT and OFSR and ONSR were found to be statistically significant in patients with thin CCT.

Biomechanical properties of optic nerve and retrobulbar structures with 2D and 3D shear wave elastography in patients affected by glaucoma

This study consists of an elastosonography evaluation of both eyes of 40 glaucomatous patients and 40 healthy subjects. We used shear wave elastography with a one-dimensional-array probe to capture two-dimensional images in order to study the optic nerve near the papilla, chorioretinal complex, lateral rectus muscle, and periorbital fat tissue. Furthermore, we used a two-dimensional array probe to capture three-dimensional images to study the optic nerve in toto with multilevel sampling. We obtained qualitative and quantitative data ("absolute" stiffness values). Then, we have investigated these tissue also measuring the "stiffness ratio" values. Statistically significant differences (p < 0.05), between glaucomatous patients and healthy patients, were noted in "absolute" stiffness values in the two-dimensional analysis at the emergence of the optic nerve and chorioretinal complex, and in the three-dimensional analysis at the emergence of the optic nerve (level I°). This result was supported by the statistically significant differences in the "stiffness ratio" values between the optic nerve and the adjacent adipose tissue, obtained both in two- and three-dimensional analyses. Data were subsequently compared with diagnostic tests currently used for glaucoma, which showed a sensitivity of 83% and specificity of 80% relative to those of elastosonography. We propose the use of elastosonography to verify the existence of pathological changes in the mechanical and elastic properties of peri-ocular structures and their variations as a complementary tool in the diagnosis of glaucoma and for follow-up during treatment.

Regional Deformation of the Optic Nerve Head and Peripapillary Sclera During IOP Elevation

Purpose

To measure the deformation of the porcine optic nerve head (ONH) and peripapillary sclera (PPS) in response to intraocular pressure (IOP) elevation.

Methods

High-frequency ultrasound was used to image the ONH and PPS of 12 porcine eyes during ex vivo inflation testing from 5 to 30 mm Hg. A speckle tracking algorithm was used to compute tissue displacements in the anterior-posterior direction and expansion of the scleral canal. Through-thickness, in-plane, and shear strains were calculated within the ONH. Regional displacements and strains were analyzed and compared.

Results

The ONH and PPS showed overall posterior displacement in response to IOP elevation. Posterior displacement of the ONH was larger than and strongly correlated with the posterior displacement of the PPS throughout inflation testing. Scleral canal expansion was much smaller and leveled off quicker than ONH posterior displacement as IOP increased. Through-thickness compression was concentrated in the anterior ONH, which also experienced larger in-plane and shear strains than the posterior ONH. Within the anterior ONH, all three strains were significantly higher in the periphery compared with the center, with the shear strain exhibiting the greatest difference between the two regions.

Conclusions

High-resolution ultrasound speckle tracking revealed the full-thickness mechanical response of the posterior eye to IOP elevation. A mismatch in posterior displacement was found between the ONH and PPS, and regional analyses showed a concentration of strains within the periphery of the anterior porcine ONH. These deformation patterns may help in understanding IOP-associated optic nerve damage and glaucoma susceptibility.

An assessment of ocular elasticity using real time ultrasound and ocular response analyzer in active or remission rheumatoid arthritis

PURPOSE

To investigate the elasticity of ocular structures in patients with rheumatoid arthritis (RA) without ocular involvement.

METHODS

The study included 56 RA patients (study group) and 24 healthy volunteers as the control group. The rheumatoid arthritis patients were divided into two subgroups as those in active phase (Group 1, n = 25) or in remission phase (Group 2, n = 31) according to the disease activity index (DAS 28) score. The elastography values of the ratio of orbital fat-sclera (ROF/S) were measured with real-time US elastography, and corneal mechanical values were measured with the Reichert Ocular Response Analyzer in each eye.

RESULTS

The mean ROF/S value was 5.2 ± 1.8 in Group 1, 0.7 ± 0.4 Group 2, and 0.6 ± 0.1 in the control group. There was a significant difference between the Group 1 and control group with regard to ROF/S (p < 0.001), but no significant difference was determined between Group 2 and control group (p > 0.05). The mean ROF/S value was a significant difference between the Group 1 and 2 (p < 0.001). ROF/S was significantly correlated with DAS-28 and C-reactive protein (CRP) (r = 0.816, p < 0.001 and r = 0.259, p = 0.006).

CONCLUSIONS

ROF/S was significantly increased in patients in the active phase of RA. Findings revealed that ocular tissue structural changes may occur in the active phase and these could be related to ocular complications as a prognostic factor.

In Vivo Evaluation of the Biomechanical Properties of Optic Nerve and Peripapillary Structures by Ultrasonic Shear Wave Elastography in Glaucoma

Background

Primary open-angle glaucoma is a multifactorial serious disease characterized by progressive retinal ganglion cell death and loss of visual field.

Objectives

The purposes of this study were to investigate shear wave elastography (SWE) use in the evaluation of the optic nerve (ON) and peripapillary structures, and to compare the findings between glaucomatous and control eyes.

Patients and Methods

A case-controlled study, including 21 patients with primary open-angle glaucoma and 21 age-matched control subjects, was carried out. All of the participants had comprehensive ophthalmological exams that included corneal biomechanical measurements with ocular response analyzer. In vivo evaluation of the biomechanical properties of the ON and peripapillary structures were performed with SWE in all participants. The Kolmogorov–Smirnov test was used to analyze the normal distribution of data. Differences of parameters in ophthalmologic data and stiffness values of patients with and without glaucoma were evaluated using the Mann-Whitney U test.

Results

There were no statistically significant differences between the glaucoma and control groups in terms of age (P > 0.05) and gender (P > 0.05). Corneal hysteresis was lower in the glaucoma group (P < 0.05). Corneal compensated intraocular pressure and Goldmann correlated intraocular pressure were higher in the glaucoma group (P < 0.0001 for both). The mean stiffness of the ON and peripapillary structures were significantly higher in glaucoma patients for each measured region (P < 0.05).

Conclusion

The study evaluated the biomechanical properties of the ON and peripapillary structures in vivo with SWE in glaucoma. We observed stiffer ON and peripapillary tissue in glaucomatous eyes, indicating that SWE claims new perspectives in the evaluation of ON and peripapillary structures in glaucoma disease.