Bilaminar Mechanics of the Human Optic Nerve Sheath

Tensile properties of glaucomatous human sclera, optic nerve, and optic nerve sheath

We characterized the tensile behavior of sclera, optic nerve (ON), and ON sheath in eyes from donors with glaucoma, for comparison with published data without glaucoma. Twelve freshly harvested eyes were obtained from donors with history of glaucoma, of average age 86 ± 7 (standard deviation) years. Rectangular samples were taken from anterior, equatorial, posterior, and peripapillary sclera, and ON sheath, while ON was in native form and measured using calipers. Under physiological temperature and humidity, tissues were preconditioned at 5% strain before loading at 0.1 mm/s. Force-displacement data were converted into engineering stress-strain curves fit by reduced polynomial hyperelastic models and analyzed by tangent moduli at 3% and 7% strain. Data were compared with an age-matched sample of 7 published control eyes. Optic atrophy was supported by significant reduction in ON cross section to 73% of normal in glaucomatous eyes. Glaucomatous was significantly stiffer than control in equatorial and peripapillary regions (P < 0.001). However, glaucomatous ON and sheath were significantly less stiff than control, particularly at low strain (P < 0.001). Hyperelastic models were well fit to stress-strain data (R2 > 0.997). Tangent moduli had variability similar to control in most regions, but was abnormally large in peripapillary sclera. Tensile properties were varied independently among various regions of the same eyes. Glaucomatous sclera is abnormally stiff, but the ON and sheath are abnormally compliant. These abnormalities correspond to properties predicted by finite element analysis to transfer potentially pathologic stress to the vulnerable disk and lamina cribrosa region during adduction eye movement.

Postmortem Digital Image Correlation and Finite Element Modeling Demonstrate Posterior Scleral Deformations during Optic Nerve Adduction Tethering

Postmortem human eyes were subjected to optic nerve (ON) traction in adduction and elevated intraocular pressure (IOP) to investigate scleral surface deformations. We incrementally adducted 11 eyes (age 74.1 ± 9.3 years, standard deviation) from 26° to 32° under normal IOP, during imaging of the posterior globe, for analysis by three-dimensional digital image correlation (3D-DIC). In the same eyes, we performed uniaxial tensile testing in multiple regions of the sclera, ON, and ON sheath. Based on individual measurements, we analyzed eye-specific finite element models (FEMs) simulating adduction and IOP loading. Analysis of 3D-DIC showed that the nasal sclera up to 1 mm from the sheath border was significantly compressed during adduction. IOP elevation from 15 to 30 mmHg induced strains less than did adduction. Tensile testing demonstrated ON sheath stiffening above 3.4% strain, which was incorporated in FEMs of adduction tethering that was quantitatively consistent with changes in scleral deformation from 3D-DIC. Simulated IOP elevation to 30 mmHg did not induce scleral surface strains outside the ON sheath. ON tethering in incremental adduction from 26° to 32° compressed the nasal and stretched the temporal sclera adjacent to the ON sheath, more so than IOP elevation. The effect of ON tethering is influenced by strain stiffening of the ON sheath.

Quantitative ultrasound image assessment of the optic nerve subarachnoid space during 90-day head-down tilt bed rest

The elevation in the optic nerve sheath (ONS) pressure (ONSP) due to microgravity-induced headward fluid shift is the primary hypothesized contributor to SANS. This longitudinal study aims to quantify the axial plane of the optic nerve subarachnoid space area (ONSSA), which is filled with cerebrospinal fluid (CSF) and expands with elevated ONSP during and after head-down tilt (HDT) bed rest (BR). 36 healthy male volunteers (72 eyes) underwent a 90-day strict 6° HDT BR. Without obtaining the pre-HDT data, measurements were performed on days 30, 60, and 90 during HDT and at 6 recovery time points extended to 180-days (R + 180) in a supine position. Portable B-scan ultrasound was performed using the 12 MHz linear array probe binocularly. The measurements of the ONS and the calculation of the ONSSA were performed with ImageJ 1.51 analysis software by two experienced observers in a masked manner. Compared to R + 180, the ONSSA on HDT30, HDT60, and HDT90 exhibited a consistently significant distention of 0.44 mm2 (95% CI: 0.13 to 0.76 mm2, P = 0.001), 0.45 mm2 (95% CI: 0.15 to 0.75 mm2, P = 0.001), and 0.46 mm2 (95% CI: 0.15 to 0.76 mm2, P < 0.001), respectively, and recovered immediately after HDT on R + 2. Such small changes in the ONSSA were below the lateral resolution limit of ultrasound (0.4 mm) and may not be clinically relevant, possibly due to ONS hysteresis causing persistent ONS distension. Future research can explore advanced quantitative portable ultrasound-based techniques and establish comparisons containing the pre-HDT measurements to deepen our understanding of SANS.

Aging Effects on Optic Nerve Neurodegeneration

Common risk factors for many ocular pathologies involve non-pathologic, age-related damage to the optic nerve. Understanding the mechanisms of age-related changes can facilitate targeted treatments for ocular pathologies that arise at any point in life. In this review, we examine these age-related, neurodegenerative changes in the optic nerve, contextualize these changes from the anatomic to the molecular level, and appreciate their relationship with ocular pathophysiology. From simple structural and mechanical changes at the optic nerve head (ONH), to epigenetic and biochemical alterations of tissue and the environment, multiple age-dependent mechanisms drive extracellular matrix (ECM) remodeling, retinal ganglion cell (RGC) loss, and lowered regenerative ability of respective axons. In conjunction, aging decreases the ability of myelin to preserve maximal conductivity, even with "successfully" regenerated axons. Glial cells, however, regeneratively overcompensate and result in a microenvironment that promotes RGC axonal death. Better elucidating optic nerve neurodegeneration remains of interest, specifically investigating human ECM, RGCs, axons, oligodendrocytes, and astrocytes; clarifying the exact processes of aged ocular connective tissue alterations and their ultrastructural impacts; and developing novel technologies and pharmacotherapies that target known genetic, biochemical, matrisome, and neuroinflammatory markers. Management models should account for age-related changes when addressing glaucoma, diabetic retinopathy, and other blinding diseases.

Analysis of Peripapillary Intrachoroidal Cavitation and Myopic Peripapillary Distortions in Polar Regions by Optical Coherence Tomography

Purpose

To compare the peripapillary polar characteristics in eyes combining peripapillary staphyloma and gamma peripapillary atrophy according to whether peripapillary intrachoroidal cavitation (PICC) was present or absent (combination-group).

Patients and methods

This prospective non-interventional cross-sectional study included 667 eyes of 334 subjects. From the polar peripapillary regions to the opening of Bruch’s membrane, the following elements and their topographic relationships were analyzed using optical coherence tomography sections: configuration of the posterior curvature of the choroid, visibility of the subarachnoid space (SAS), and suprachoroidal detachment (SCD). Chi-squared and Fisher exact tests were used for statistical analysis.

Results

The protrusion of the posterior choroidal wall, with anterior elevation on either side, observed in both groups progressed and transformed into a wedge-shaped deformity on the side of gamma peripapillary atrophy. This wedge configuration was significantly more frequent in PICC-group than in combination-group (p = 0.004 and p < 0.001) for the upper and lower poles, respectively. SAS was more frequently observed in PICC-group than in combination-group (p = 0.002 and p < 0.001) for the upper and lower poles, respectively. SCD was detected exclusively in PICC-group (p < 0.001, both poles). The wedge-shaped configuration and the SCD were aligned antero-posteriorly with the SAS.

Conclusion

We confirmed that PICC is an SCD. We observed its constant alignment with the SAS. We suggest that the tensile forces of the optic nerve sheaths during adduction promote the collapse of the scleral flange onto the SAS, leading to PICC. Further studies are warranted to confirm this hypothesis.

Optic nerve-associated connective tissue structures revisited: a histological study using human fetuses and adult cadavers

Unlike the usual peripheral nerve, the optic nerve accompanies a thick "dural sheath," a thin "sheath of pia mater" (SPM), and multiple "septa," which divides the nerve fibers into fascicles. We collected specimens from 25 adult cadavers and 15 fetuses and revisited the histological architecture of the optic and oculomotor nerves. In the optic chiasma, the meningeal layer of the dura joins the pia to form a thick SPM, and the periosteum of the sphenoid is continuous with the dural sheath at the orbital exit of the bony optic canal. The septa appeared as a cluster of irregularly arrayed fibrous plates in the intracranial course near the chiasma. Thus, the septa were not derived from either the SPM or the dural sheath. In the orbit, the central artery of the retina accompanies collagenous fibers from the dural sheath and the SPM to provide the vascular sheath in the optic nerve. These connective tissue configurations were the same between adult and fetal specimens. At the optic disk, the dural sheath and SPM merged with the sclera, whereas the septa appeared to end at the lamina cribrosa. However, in fetuses without lamina cribrosa, the septa extend into the nerve fiber layer of the retina. The SPM and septa showed strong elastin immunoreactivity, in contrast to the absence of reactivity in the sheaths of the oculomotor nerve. Each S100 protein-positive Schwann sheath of the oculomotor nerve was surrounded by collagenous endoneurium. Glial fibrillary acidic protein-positive astrocytes showed a linear arrangement along the septa. This article is protected by copyright. All rights reserved.

Optic Nerve Sheath Viscoelastic Properties: Re-Examination of Biomechanical Behavior and Clinical Implications

BACKGROUND

Meta-analyses show a variable relationship between optic nerve sheath diameter (ONSD) and the presence of raised intracranial pressure (ICP). Because optic nerve sheath (ONS) tissue can be deformed, it is possible that ONSD reflects not only the current ICP but also prior deforming biomechanical exposures. In this post hoc analysis of two published data sets, we characterize ONS Young's modulus (E, mechanical stress per unit of strain) and calculate threshold pressure for plastic deformation.

METHODS

The authors of two previously published articles contributed primary data for these unique post hoc analyses. Human cadaveric ex vivo measurements of ONSD (n = 10) and luminal distending pressure (range 5 to 65 mm Hg) were used to calculate E and the threshold pressure for plastic deformation. Clinical in vivo measurements of ONSD and ICP during endotracheal tube suction from patients with traumatic brain injury (n = 15) were used to validate the ex vivo cadaveric findings.

RESULTS

Ex vivo ONS estimate of E was 140 ± 1.3 mm Hg (mean ± standard error), with evidence of plastic deformation occurring with distending pressure at 45 mm Hg. Similar E (71 ± 10 mm Hg) was estimated in vivo with an average ICP of 34 ± 2 mm Hg.

CONCLUSIONS

Ex vivo, ONS plastic deformation occurs at levels of pressure commonly seen in patients with raised ICP, leading to distortion of the ICP-ONSD relationship. This evidence of plastic deformation may illustrate why meta-analyses fail to identify a single threshold in ONSD associated with the presence of raised ICP. Future studies characterizing time-dependent viscous characteristics of the ONS will help determine the time course of ONS tissue biomechanical behavior.

Glaucoma and biomechanics

PURPOSE OF REVIEW

Biomechanics is an important aspect of the complex family of diseases known as the glaucomas. Here, we review recent studies of biomechanics in glaucoma.

RECENT FINDINGS

Several tissues have direct and/or indirect biomechanical roles in various forms of glaucoma, including the trabecular meshwork, cornea, peripapillary sclera, optic nerve head/sheath, and iris. Multiple mechanosensory mechanisms and signaling pathways continue to be identified in both the trabecular meshwork and optic nerve head. Further, the recent literature describes a variety of approaches for investigating the role of tissue biomechanics as a risk factor for glaucoma, including pathological stiffening of the trabecular meshwork, peripapillary scleral structural changes, and remodeling of the optic nerve head. Finally, there have been advances in incorporating biomechanical information in glaucoma prognoses, including corneal biomechanical parameters and iridial mechanical properties in angle-closure glaucoma.

SUMMARY

Biomechanics remains an active aspect of glaucoma research, with activity in both basic science and clinical translation. However, the role of biomechanics in glaucoma remains incompletely understood. Therefore, further studies are indicated to identify novel therapeutic approaches that leverage biomechanics. Importantly, clinical translation of appropriate assays of tissue biomechanical properties in glaucoma is also needed.

Material properties and effect of preconditioning of human sclera, optic nerve, and optic nerve sheath

The optic nerve (ON) is a recently recognized tractional load on the eye during larger horizontal eye rotations. In order to understand the mechanical behavior of the eye during adduction, it is necessary to characterize material properties of the sclera, ON, and in particular its sheath. We performed tensile loading of specimens taken from fresh postmortem human eyes to characterize the range of variation in their biomechanical properties and determine the effect of preconditioning. We fitted reduced polynomial hyperelastic models to represent the nonlinear tensile behavior of the anterior, equatorial, posterior, and peripapillary sclera, as well as the ON and its sheath. For comparison, we analyzed tangent moduli in low and high strain regions to represent stiffness. Scleral stiffness generally decreased from anterior to posterior ocular regions. The ON had the lowest tangent modulus, but was surrounded by a much stiffer sheath. The low-strain hyperelastic behaviors of adjacent anatomical regions of the ON, ON sheath, and posterior sclera were similar as appropriate to avoid discontinuities at their boundaries. Regional stiffnesses within individual eyes were moderately correlated, implying that mechanical properties in one region of an eye do not reliably reflect properties of another region of that eye, and that potentially pathological combinations could occur in an eye if regional properties are discrepant. Preconditioning modestly stiffened ocular tissues, except peripapillary sclera that softened. The nonlinear mechanical behavior of posterior ocular tissues permits their stresses to match closely at low strains, although progressively increasing strain causes particularly great stress in the peripapillary region.

Adduction-Induced Strain on the Optic Nerve in Primary Open Angle Glaucoma at Normal Intraocular Pressure

PURPOSE/AIM

The optic nerve (ON) becomes taut during adduction beyond ~26° in healthy people and patients with primary open angle glaucoma (POAG), but only retracts the globe in POAG. We used magnetic resonance imaging (MRI) to investigate this difference.

MATERIALS AND METHODS

MRI was obtained in 2-mm quasi-coronal planes in central gaze, and smaller (~23-25°) and larger (~30-31°) adduction and abduction in 21 controls and 12 POAG subjects whose intraocular pressure never exceeded 21 mmHg. ON cross-sections were analyzed from the globe to 10 mm posteriorly. Area centroids were used to calculate ON path lengths and changes in cross-sections to calculate elongation assuming volume conservation.

RESULTS

For both groups, ON path was nearly straight (<102.5% of minimum path) in smaller adduction, with minimal further straightening in larger adduction. ON length was redundant in abduction, exceeding 103% of minimum path for both groups. For normals, the ON elongated 0.4 ± 0.5 mm from central gaze to smaller adduction, and 0.4 ± 0.5 mm further from smaller to larger adduction. For POAG subjects, the ON did not elongate on average from central gaze to smaller adduction and only 0.2 ± 0.4 mm from smaller to larger adduction (P = .045 vs normals). Both groups demonstrated minimal ON elongation not exceeding 0.25 mm from central gaze to smaller and larger abduction. The globe retracted significantly more during large adduction in POAG subjects than normals (0.6 ± 0.7 mm vs 0.2 ± 0.5 mm, P = .027), without appreciable retraction in abduction. For each mm increase in globe axial length, ON elongation in large adduction similarly increased by 0.2 mm in each group.

CONCLUSIONS

The normal ON stretches to absorb force and avert globe retraction in adduction. In POAG with mild to severe visual field loss, the relatively inelastic ON tethers and retracts the globe during adduction beyond ~26°, transfering stress to the optic disc that could contribute to progressive neuropathy during repeated eye movements.

In vivo estimation of optic nerve sheath stiffness using noninvasive MRI measurements and finite element modeling

The optic nerve sheath (ONS) is biomechanically important. It is acted on by tension due to ocular movements, and by fluid shifts and/or alterations in intracranial pressure (ICP) in human disease, specifically in pathologies leading to intracranial hypertension. It has also been hypothesized that the ONS is acted on by altered ICP in astronauts exposed chronically to microgravity. However, a non-invasive method to quantify ONS biomechanical properties is not presently available; knowledge of such properties is desirable to allow characterization of the biomechanical forces exerted on the optic nerve head and other ocular structures due to the ONS. Thus, the primary objective of this study was to characterize the biomechanical properties (stiffness) of the human ONS in vivo as a necessary step towards investigating the role of ICP in various conditions, including Spaceflight Associated Neuro-ocular Syndrome (SANS). We acquired non-invasive magnetic resonance imaging (MRI) scans of ostensibly healthy subjects (n = 18, age = 30.4 ± 11.6 [mean ± SD] years) during supine and 15-degree head-down-tilt (HDT) postures, and extracted ONS contours from these scans. We then used finite element modeling to quantify ONS expansion due to postural changes and an inverse approach to estimate ONS stiffness. Using this non-invasive procedure, we estimated an in vivo ONS stiffness of 39.2 ± 21.9 kPa (mean ± SD), although a small subset of individuals had very stiff ONS that precluded accurate estimates of their stiffness values. ONS stiffness was not correlated with age and was higher in males than females.

Bilaminar Structure of the Human Optic Nerve Sheath

PURPOSE/AIM

We aimed to characterize the connective tissue microanatomy, elastin abundance, and fiber orientation in the human optic nerve sheath, also known as the optic nerve dura mater, for correlation with its biomechanical properties.

MATERIALS AND METHODS

Seven whole human orbits aged 4-93 years, and five isolated human optic nerve sheaths aged 26-75 years were formalin fixed, paraffin embedded, coronally sectioned, stained by Masson trichrome and van Gieson's elastin methods, and analyzed quantitatively for elastin fiber abundance and orientation. Elastin area fraction was defined as area stained for elastin divided by total area.

RESULTS

While unilaminar in children, the adult ON sheath exhibited distinct inner and outer layers. Collagen was denser and more compact in the inner layer. Elastin area fraction was significantly greater at 6.0 ± 0.4% (standard error of mean) in the inner than outer layer at 3.6 ± 0.4% (P < 10^-5). Elastin fibers had three predominant orientations: longitudinal, diagonal, and circumferential. Of circumferential fibers, 63 ± 4.7% were in the inner and 37 ± 4.7% in the outer layer (P < 10^-4). Longitudinal and diagonal fibers were uniformly distributed in both layers. Elastin density and sheath thickness increased significantly with age (P < .01).

CONCLUSIONS

The adult human optic nerve sheath is bilaminar, with each layer containing elastin fibers oriented in multiple directions consistent with isotropic properties. Differences in laminar elastin density and orientation may reflect greater tensile loading in the inner than in the outer layer.