Closed globe macular injuries after blasts in combat

Blast injury: Impact to the cornea

Visual disorders are common even after mild traumatic brain injury (mTBI) or blast exposure. The cost of blast-induced vision loss in civilians, military personnel, and veterans is significant. The visual consequences of blasts associated with TBI are elusive. Active military personnel and veterans report various ocular pathologies including corneal disorders post-combat blasts. The wars and conflicts in Afghanistan, Iraq, Syria, and Ukraine have significantly increased the number of corneal and other ocular disorders among military personnel and veterans. Binocular vision, visual fields, and other visual functions could be impaired following blast-mediated TBI. Blast-associated injuries can cause visual disturbances, binocular system problems, and visual loss. About 25% of veterans exposed to blasts report corneal injury. Blast exposure induces corneal edema, corneal opacity, increased corneal thickness, damage of corneal epithelium, corneal abrasions, and stromal and endothelial abnormality including altered endothelial density, immune cell infiltration, corneal neovascularization, Descemet membrane rupture, and increased pain mediators in animal models and the blast-exposed military personnel including veterans. Immune response exacerbates blast-induced ocular injury. TBI is associated with dry eyes and pain in veterans. Subjects exposed to blasts that cause TBI should undergo immediate clinical visual and ocular examinations. Delayed visual care may lead to progressive vision loss, lengthening/impairing rehabilitation and ultimately may lead to permanent vision problems and blindness. Open-field blast exposure could induce corneal injuries and immune responses in the cornea. Further studies are warranted to understand corneal pathology after blast exposure. A review of current advancements in blast-induced corneal injury will help elucidate novel targets for potential therapeutic options. This review discusses the impact of blast exposure-associated corneal disorders.

Increasing the number and intensity of shock tube generated blast waves leads to earlier retinal ganglion cell dysfunction and regional cell death

The purpose of this study was to examine the effect of a blast exposure generated from a shock tube on retinal ganglion cell (RGC) function and structure. Mice were exposed to one of three blast conditions using a shock tube; a single blast wave of 20 PSI, a single blast wave of 30 PSI, or three blast waves of 30 PSI given on three consecutive days with a one-day inter-blast interval. The structure and function of the retina were analyzed using the pattern electroretinogram (PERG), the optomotor reflex (OMR), and optical coherence tomography (OCT). The in vivo parameters were examined at baseline, and then again 1-week, 4-weeks, and 16-weeks following blast exposure. The number of surviving RGCs was quantified at the end of the study. Analysis of mice receiving a 20 PSI injury showed decreased PERG and OMR responses 16-weeks post blast, without evidence of changed retinal thickness or RGC death. Mice subjected to a 30 PSI injury showed decreased PERG responses 4 weeks and 16 weeks after injury, without changes in the retinal thickness or RGC density. Mice subjected to 30 PSI X 3 blast exposures had PERG deficits 1-week and 4-weeks post exposure. There was also significant change in retinal thickness 1-week and 16-weeks post blast exposure. Mice receiving 30 PSI X 3 blast injuries had regional loss of RGCs in the central retina, but not in the mid-peripheral or peripheral retina. Overall, this study has shown that increasing the number of blast exposures and the intensity leads to earlier functional loss of RGCs. We have also shown regional RGC loss only when using the highest blast intensity and number of blast injuries.

Ocular Trauma and Traumatic Brain Injury on the Battlefield: A Systematic Review After 20 Years of Fighting the Global War on Terror

INTRODUCTION

Operations Iraqi Freedom and Enduring Freedom saw higher rates of combat ocular trauma (COT) than any past U.S. conflict. The improvised explosive device, the signature weapon of the conflicts, as well as improved personal protective equipment and combat medical care all attributed to COT being the fourth most common injury sustained by wounded U.S. service members. This review describes the epidemiology, mechanisms, and treatment patterns and discusses the relationship of traumatic brain injuries (TBIs) to ocular injuries sustained by U.S. service members during the War on Terror.

MATERIALS AND METHODS

A mixed-methods review of the literature was conducted by extracting data from PubMed, Embase, and Cochrane research databases between December 15, 2020, and January 25, 2021, using the COVIDENCE review management software.

RESULTS

Of 827 articles for review, 50 were deemed relevant. Articles were separated using the Birmingham Eye Trauma Terminology into open globe, closed globe, mixed/injury management only, and TBI. Seventeen articles were found to discuss data pertaining to particular databases. Overall, six articles discussed open-globe injuries in the setting of overall COT with a reported rate of 38-64%. Three articles discussed closed-globe injuries in the context of overall COT with a rate of 39-47%. Numerous articles discussed the relationship between COT and TBI. Within the Walter Reed Ocular Trauma Database, 40% of patients with ocular trauma had concomitant TBI. Additionally, the visual sequelae of ocular trauma ranged from 9% to 50% among reporting studies. Other ocular injury patterns receiving attention include neuro-ophthalmic and oculoplastic injuries. By far the most common mechanism of COT was blast injury (64-84%), with improvised explosive devices (IEDs) accounting for 51-69% of ocular injuries. Among the large reporting databases, 41-45% of COT required surgical treatment with an overall enucleation rate of 12-17%.

CONCLUSIONS

The Global War on Terrorism saw an evolution in the types of ocular injuries sustained by U.S. service members compared to previous conflicts. The widespread use of IEDs led to injury patterns not encountered in previous conflicts. Weapons of today utilize blast and shrapnel as the mechanism for destruction. Sequelae such as TBIs and complicated head and neck trauma have pushed innovation in the field of ophthalmology. Improvements in medical technology and personal protective equipment have resulted in not only survival of previously life-threatening injuries, but also a greater chance of severe loss of vision. By analyzing ocular injury data from the trauma literature, improvements in education and training can lead to improvements in point-of-injury care and eye protection for the next generation of warfighters.

Assessment of necroptosis in the retina in a repeated primary ocular blast injury mouse model

Primary blast injury (caused by the initial rapid increase in pressure following an explosive blast) to the retina and optic nerve (ON) causes progressive visual loss and neurodegeneration. Military personnel are exposed to multiple low-overpressure blast waves, which may be in quick succession, such as during breacher training or in combat. We investigated the necroptotic cell death pathway in the retina in a mouse repeated primary ocular blast injury (rPBI) model using immunohistochemistry. We further evaluated whether intravitreal injections of a potent necroptosis inhibitor, Necrostatin-1s (Nec-1s), protects the retina and ON axons by retinal ganglion cells (RGC) counts, ON axonal counting and optical coherence tomography (OCT) analysis of vitreous haze. Receptor interacting protein kinase (RIPK) 3, increased in the inner plexiform layer 2 days post injury (dpi) and persisted until 14 dpi, whilst RIPK1 protein expression did not change after injury. The number of degenerating ON axons was increased at 28 dpi but there was no evidence of a reduction in the number of intact ON axons or RNA-binding protein with multiple splicing (RBPMS)⁺ RGC in the retina by 28 dpi in animals not receiving any intravitreal injections. But, when intravitreal injections (vehicle or Nec-1s) were given there was a significant reduction in RBPMS⁺ RGC numbers, suggesting that rPBI with intraocular injections is damaging to RGC. There were fewer RGC lost after Nec-1s than vehicle injection, but there was no effect of Nec-1s or vehicle treatment on the number of degenerating axons. OCT analysis demonstrated no effect of rPBI on vitreous haze, but intravitreal injection combined with rPBI increased vitreous haze (P = 0.004). Whilst necroptosis may be an active cell death signalling pathway after rPBI, its inhibition did not prevent cell death, and intravitreal injections in combination with rPBI increased vitreous inflammation and reduced RBPMS⁺ RGC numbers, implying intravitreal injection is not an ideal method for drug delivery after rPBI.

Rapid Repeat Exposure to Subthreshold Trauma Causes Synergistic Axonal Damage and Functional Deficits in the Visual Pathway in a Mouse Model

We examined the effect of repeat exposure to a non-damaging insult on central nervous system axons using the optic projection as a model. The optic projection is attractive because its axons are spatially separated from the cell bodies, it is easily accessible, it is composed of long axons, and its function can be measured. We performed closed-system ocular neurotrauma in C57Bl/6 mice using bursts of 15 or 26-psi (pounds per square inch) overpressure air that caused no gross damage. We quantified the visual evoked potential (VEP) and total and degenerative axons in the optic nerve. Repeat exposure to a 15-psi air blast caused more axon damage and vision loss than a single exposure to a 26-psi air blast. However, an increased VEP latency was detected in both groups. Exposure to three 15-psi air blasts separated by 0.5 sec caused 15% axon degeneration at 2 weeks. In contrast, no axon degeneration above sham levels was detected when the interinjury interval was increased to 10 min. Exposure to 15-psi air blasts once a day for 6 consecutive days caused 3% axon degeneration. Therefore, repeat mild trauma within an interinjury interval of 1 min or less causes synergistic axon damage, whereas mild trauma repeated at a longer interinjury interval causes additive, cumulative damage. The synergistic damage may underlie the high incidence of traumatic brain injury and traumatic optic neuropathy in blast-injured service members given that explosive blasts are multiple injury events that occur in a very short time span. This study also supports the use of the VEP as a biomarker for traumatic optic neuropathy.

Hypothermia Prevents Retinal Damage Generated by Optic Nerve Trauma in the Rat

Ocular and periocular traumatisms may result in loss of vision. Hypothermia provides a beneficial intervention for brain and heart conditions and, here, we study whether hypothermia can prevent retinal damage caused by traumatic neuropathy. Intraorbital optic nerve crush (IONC) or sham manipulation was applied to male rats. Some animals were subjected to hypothermia (8 °C) for 3 h following surgery. Thirty days later, animals were subjected to electroretinography and behavioral tests. IONC treatment resulted in amplitude reduction of the b-wave and oscillatory potentials of the electroretinogram, whereas the hypothermic treatment significantly (p < 0.05) reversed this process. Using a descending method of limits in a two-choice visual task apparatus, we demonstrated that hypothermia significantly (p < 0.001) preserved visual acuity. Furthermore, IONC-treated rats had a lower (p < 0.0001) number of retinal ganglion cells and a higher (p < 0.0001) number of TUNEL-positive cells than sham-operated controls. These numbers were significantly (p < 0.0001) corrected by hypothermic treatment. There was a significant (p < 0.001) increase of RNA-binding motif protein 3 (RBM3) and of BCL2 (p < 0.01) mRNA expression in the eyes exposed to hypothermia. In conclusion, hypothermia constitutes an efficacious treatment for traumatic vision-impairing conditions, and the cold-shock protein pathway may be involved in mediating the beneficial effects shown in the retina.

Firecracker eye exposure: experimental study and simulation

Understanding the mechanisms of traumatic ocular injury is helpful to make accurate diagnoses before the symptoms emerge and to develop specific eye protection. The comprehension of the dynamics of primary blast injury mechanisms is a challenging issue. The question is whether the pressure wave propagation and reflection alone could cause ocular damage. To date, there are dissenting opinions and no conclusive evidence thereupon. A previous numerical investigation of blast trauma highlighted the dynamic effect of pressure propagation and its amplification by the geometry of the bony orbit, inducing a resonance cavity effect and a standing wave hazardous for eye tissues. The objective of the current work is to find experimental evidence of the numerically identified phenomenon. Therefore, tests aimed at evaluating the response of porcine eyes to blast overpressure generated by firecrackers explosion were performed. The orbital cavity effect was considered mounting the enucleated eyes inside a dummy orbit. The experimental measurements obtained during the explosion tests presented in this paper corroborate the numerical evidence of a high-frequency pressure amplification, enhancing the loading on the ocular tissues, attributable to the orbital bony walls surrounding the eye.

Eye-Directed Overpressure Airwave-Induced Trauma Causes Lasting Damage to the Anterior and Posterior Globe: A Model for Testing Cell-Based Therapies

PURPOSE

Characterization of the response of the Balb/c mouse to an eye-directed overpressure airwave, with the hypothesis that this mouse strain and model is useful for testing potential therapeutics for the treatment of traumatic eye injury.

METHODS

The left eyes of adult Balb/c mice were exposed to an eye-directed overpressure airwave. Intraocular pressure (IOP) was measured and eyes were inspected for gross pathology changes. Optical coherence tomography and histology were used to examine the structural integrity of the retina and optic nerve. Immunohistochemistry, in vivo molecular fluorophores, and a multiplex enzyme-linked immunosorbent assay were utilized to identify changes in cell death, neuroinflammation, and oxidative stress.

RESULTS

This model induced a transient increase in IOP, corneal injuries, infrequent large retinal detachments, retinal pigment epithelium (RPE) vacuolization, glial reactivity, and retinal cell death. Both the corneal damage and RPE vacuolization persisted with time. Optic nerve degeneration occurred as early as 7 days postinjury and persisted out to 60 days. Retinal cell death, increased levels of reactive oxygen species, and neuroinflammation were detected at 7 days postinjury.

CONCLUSIONS

The injury profile of the Balb/c mouse is consistent with commonly observed pathologies in blast-exposed patients. The damage is throughout the eye and persistent, making this mouse model useful for testing cell-based therapies.

Outer retinal structure after closed-globe blunt ocular trauma

PURPOSE

To evaluate outer retinal structural abnormalities in patients with visual deficits after closed-globe blunt ocular trauma.

METHODS

Nine subjects with visual complaints after closed-globe blunt ocular trauma were examined between 1 month after trauma and 6 years after trauma. Spectral domain optical coherence tomography was used to assess the outer retinal architecture, whereas adaptive optics scanning light ophthalmoscopy was used to analyze the photoreceptor mosaic integrity.

RESULTS

Visual deficits ranged from central scotomas to decreased visual acuity. Spectral domain optical coherence tomography defects included focal foveal photoreceptor lesions, variable attenuation of the interdigitation zone, and mottling of the outer segment band, with one subject having normal outer retinal structure. Adaptive optics scanning light ophthalmoscopy revealed disruption of the photoreceptor mosaic in all subjects, variably manifesting as foveal focal discontinuities, perifoveal hyporeflective cones, and paracentral regions of selective cone loss.

CONCLUSION

We observe persistent outer retinal disruption in subjects with visual complaints after closed-globe blunt ocular trauma, albeit to a variable degree. Adaptive optics scanning light ophthalmoscopy imaging allows the assessment of photoreceptor structure at a level of detail not resolvable using spectral domain optical coherence tomography or other current clinical imaging tools. Multimodal imaging seems to be useful in revealing the cause of visual complaints in patients after closed-globe blunt ocular trauma. Future studies are needed to better understand how photoreceptor structure changes longitudinally in response to various traumas.

Exacerbation of blast-induced ocular trauma by an immune response

Background

Visual prognosis after an open globe injury is typically worse than after a closed globe injury due, in part, to the immune response that ensues following open globe trauma. There is a need for an animal model of open globe injury in order to investigate mechanisms of vision loss and test potential therapeutics.

Methods

The left eyes of DBA/2 J mice were exposed to an overpressure airwave blast. This strain lacks a fully functional ocular immune privilege, so even though the blast wave does not rupture the globe, immune infiltrate and neuroinflammation occurs as it would in an open globe injury. For the first month after blast wave exposure, the gross pathology, intraocular pressure, visual function, and retinal integrity of the blast-exposed eyes were monitored. Eyes were collected at three, seven, and 28 days to study the histology of the cornea, retina, and optic nerve, and perform immunohistochemical labeling with markers of cell death, oxidative stress, and inflammation.

Results

The overpressure airwave caused anterior injuries including corneal edema, neovascularization, and hyphema. Immune infiltrate was detected throughout the eyes after blast wave exposure. Posterior injuries included occasional retinal detachments and epiretinal membranes, large retinal pigment epithelium vacuoles, regional photoreceptor cell death, and glial reactivity. Optic nerve degeneration was evident at 28 days post-blast wave exposure. The electroretinogram (ERG) showed an early deficit in the a wave that recovered over time. Both visual acuity and the ERG b wave showed an early decrease, then a transient improvement that was followed by further decline at 28 days post-blast wave exposure.

Conclusions

Ocular blast injury in the DBA/2 J mouse recapitulates damage that is characteristic of open globe injuries with the advantage of a physically intact globe that prevents complications from infection. The injury was more severe in DBA/2 J mice than in C57Bl/6 J mice, which have an intact ocular immune privilege. Early injury to the outer retina mostly recovers over time. In contrast, inner retinal dysfunction seems to drive later vision loss.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0192-5) contains supplementary material, which is available to authorized users.

Pathophysiology of blast-induced ocular trauma in rats after repeated exposure to low-level blast overpressure

BACKGROUND

The incidence of blast-induced ocular injury has dramatically increased due to advances in weaponry and military tactics. A single exposure to blast overpressure (BOP) has been shown to cause damage to the eye in animal models; however, on the battlefield, military personnel are exposed to BOP multiple times. The effects of repeated exposures to BOP on ocular tissues have not been investigated. The purpose of this study is to characterize the effects of single or repeated exposure on ocular tissues.

METHODS

A compressed air shock tube was used to deliver 70 ± 7 KPa BOP to rats, once (single blast overpressure [SBOP]) or once daily for 5 days (repeated blast overpressure [RBOP]). Immunohistochemistry was performed to characterize the pathophysiology of ocular injuries induced by SBOP and RBOP. Apoptosis was determined by quantification activated caspase 3. Gliosis was examined by detection of glial fibrillary acidic protein (GFAP). Inflammation was examined by detection of CD68.

RESULTS

Activated caspase 3 was detected in ocular tissues from all animals subjected to BOP, while those exposed to RBOP had more activated caspase 3 in the optic nerve than those exposed to SBOP. GFAP was detected in the retinas from all animals subjected to BOP. CD68 was detected in optic nerves from all animals exposed to BOP.

CONCLUSION

SBOP and RBOP induced retinal damage. RBOP caused more apoptosis in the optic nerve than SBOP, suggesting that RBOP causes more severe optic neuropathy than SBOP. SBOP and RBOP caused gliosis in the retina and increased inflammation in the optic nerve.

Molecular changes and vision loss in a mouse model of closed-globe blast trauma

PURPOSE

To characterize retinal changes and assess vision after an eye-directed air blast.

METHODS

Adult C57Bl/6 mice were exposed to a blast directed at one eye. Optical coherence tomography and histology were performed to assess retina and optic nerve integrity. Cell death, oxidative stress, and glial reactivity were examined by immunohistochemistry. Visual changes were measured by ERG recordings and the optokinetic reflex.

RESULTS

In the outer retina, eye blast caused retinal pigment epithelium vacuoles and rare retinal detachments followed by regional cell death. Labeling for nitrotyrosine and markers of pyroptosis (caspase-1) and necroptosis (receptor-interacting protein kinases-1, -3) increased, primarily in the inner retina, after blast. Caspase-1 labeling was restricted primarily to the starburst amacrine cells. A few degenerating axons were detected at 28 days post blast. Despite a lack of substantial cell death or decreased ERG, there was a deficit in visual acuity after blast.

CONCLUSIONS

Oxidative stress, neuroinflammation, and cell death became increasingly prevalent, over time post blast suggestive of an ongoing neurodegenerative response. Outer retinal changes either resolved or remained focal. In contrast, inner retinal changes were more robust and spread from focal regions to the entire retina over time post blast. Our model of eye blast trauma causes molecular changes and a decrease in visual acuity within the first month post blast despite a lack of overt eye injury. This subtle response matches the delayed presentation of visual deficits in some blast-exposed Veterans.