[Pharmacological vitreolysis]. / Pharmakologische Vitreolyse.

BACKGROUND: The vitreous plays an important role in the development and progression of vitreoretinal diseases. Vitrectomy is the treatment modality of choice in these cases. However, mechanical vitrectomy is incomplete. Therefore, alternative strategies have been pursued including pharmacological means such as enzymes. The goal of pharmacological vitreolysis is to make the surgical intervention easier and less traumatic. METHODS: Different substances have been investigated, including chondroitinase, dispase, hyaluronidase, plasmin, and microplasmin. Besides preclinical investigations, hyaluronidase and microplasmin (ThromboGenics Ltd., Dublin, Leuven) have been tested clinically. Results from the literature are reported herein. RESULTS: Plasmin and microplasmin are both capable of inducing posterior vitreous detachment (PVD) in a dose- and time-dependent manner. There are no morphological or functional changes of the retina at therapeutic doses. Two phase II studies published to date demonstrate both efficacy and safety. Phase III studies are ongoing, and results are expected during 2010. Other enzymes tested show limitations in that retinal damage may occur (dispase) or liquefaction (hyaluronidase) occurs without cleavage of the vitreous cortex from the retina. CONCLUSIONS: Microplasmin induces PVD. Results from clinical trials show that microplasmin helps to detach the vitreous cortex from the retina. This may be advantageous in terms of complete vitreous removal and less traumatic intervention compared to mechanical techniques, such as vitrectomy and peeling of the internal limiting membrane.

[Retinal detachment in children and adolescents. Specific clinical features]. / Ablatio retinae bei Kindern und Jugendlichen. Spezifische Befundmerkmale.

BACKGROUND: Due to the long life expectancy, retinal detachment is a special threat to visual acuity in children and adolescents. This study presents the clinical features of retinal detachment in childhood and adolescence up to the age of 20 years. PATIENTS AND METHODS: A cohort was selected comprising 259 patients who suffered from unilateral or bilateral retinal detachment, were not older than 20 years of age at the first diagnosis of the first or only affected eye and had undergone surgery at least once at the Department of Ophthalmology of the University Medical Center of Munich during a period of 18 years (1980-1998). This patient collective was retrospectively analyzed with respect to the clinical features of the first retinal detachment. The group consisting of only one affected eye or the first affected eye (259 eyes) was included. The fellow eyes affected later were excluded (19 eyes). RESULTS: The time period between the first visual symptoms and the diagnosis of retinal detachment was on average 9.6 weeks and the most commonly manifested symptom was loss of vision (36.3% of patients). In 40.2% of the patients the detachment was discovered fortuitously. The most frequent presentation (34.0%) was a 2-quadrant retinal detachment and was (sub)total in 27.0% of eyes. Macular detachment was found in 154 eyes (59.5%). The commonest type of retinal break was a tear near the ora serrata (36.1% of all breaks). Giant tears (12.8% of all breaks) occurred preferentially in the area of the ora serrata, round atrophic holes were identified especially in the area of the equator, often in the form of a chain of holes. Breaks most frequently occurred in the inferior temporal quadrant. In 22.4% of retinal detachments no break was found even intraoperatively. A primary proliferative vitreoretinopathy (PVR) of at least stage C was involved in 25.5% of detachments. CONCLUSION: In childhood and adolescence a characteristic delay of diagnosis enables a large sized expansion of the retinal detachment with frequent macular involvement and a high proportion with (sub)total detachment and severe primary PVR. Tears in the ora serrata area, giant tears, multiple round atrophic holes in the area of the equator and a high rate of undetectable breaks are the intrinsic characteristics of juvenile retinal detachment.

[Anti-VEGF treatment for retinal angiomatous proliferation]. / Intravitreale Anti-VEGF-Behandlung retinaler angiomatöser Proliferationen.

BACKGROUND: Retinal angiomatous proliferation (RAP) is a subform of neovascular age-related macular degeneration (AMD), which is characterized by a particularly poor prognosis. The aim of this study is to describe the loading phase and maintenance phase for RAP during intravitreal anti-VEGF treatment. MATERIAL AND METHODS: A total of 82 eyes in 82 patients with RAP stages 1-3 were treated during upload therapy with repeated intravitreal injections of 1.25 mg bevacizumab at intervals of 4 weeks until the retinal edema resolved. Baseline examination included measurement of the best corrected distance visual acuity (ETDRS chart), central retinal thickness using optical coherence tomography (OCT), and fluorescein angiography (FLA). During maintenance therapy, the patients' distance visual acuity was monitored at 4- to 12-week intervals and OCT or FLA performed if needed. The average follow-up was 7.4 months (SD 4.2). Treatment with intravitreal anti-VEGF therapy was repeated if there was evidence of sub- or intraretinal fluid with a decrease in visual acuity of 5 points or more, increase of the central retinal thickness of 100 microm or more on OCT, or subjective deterioration with verifiable sub- or intraretinal fluid. RESULTS: During upload therapy an improvement in visual acuity of an average of +5.1 letters (mean, n=82 eyes) was observed. During maintenance therapy it was initially possible to sustain this treatment effect. However, 5 months after loading therapy was concluded, a deterioration of -5.5 letters (mean, n=31) was evident in comparison with the end of loading therapy. During the further course deterioration continued (12-month follow-up: -8.6 letters, n=7). Recurrence occurred in 60% of the cases, on average 8 weeks after termination of loading therapy. During an observation period of 6 months (n=66) a total of 3.6 injections were necessary. CONCLUSIONS: Therapy with intravitreal anti-VEGF medications represents a treatment option for RAP, but in the long term the disease continues to progress accompanied by functional deterioration. We thus recommend that patients with RAP be monitored at 4-week intervals to permit early treatment of recurrence.

[The primary objective in macular hole surgery. Ultrastructural features of the vitreomacular interface]. / Operative Zielvorgabe in der Makulaforamenchirurgie. Neue Aspekte zur Feinstruktur der vitreomakulären Grenzfläche.

We compared the ultrastructure of the inner limiting membrane (ILM) and epiretinal tissue in closed and non-closed, idiopathic macular holes (MH). Peeling of ILM and epimacular tissue during vitrectomy was successfully performed on 77 eyes with stage III MH and on 19 eyes with stage IV MH. In 16 additional eyes with non-closed MH, we performed a second vitrectomy with extended ILM removal. The specimens were processed for transmission electron microscopy. Fibrocellular proliferation at the vitreal side of the ILM was found in 57% of MH that were closed by one operation, and in 100% of non-closed MH. Mono- and multilayered cellular membranes as well as native vitreous collagen at the ILM were significantly more frequent in eyes with stage IV MH than in eyes with stage III MH. In non-closed MH, cellular proliferation was mostly seen as irregular cell accumulation, and masses of newly formed collagen were found. Since ILM remnants and collagen represent a stimulus for the early formation of tangential traction preventing successful MH closure, it appears mandatory to create a complete vitreoretinal separation and to remove the ILM and collagen thoroughly during MH surgery.

[Possible role of alkylphosphocholines in retinal reattachment surgery]. / Mögliche rolle von alkylphosphocholinen bei der ablatio-chirurgie.

BACKGROUND: Proliferative vitreoretinopathy (PVR) is a major complication after retinal detachment surgery, but there is no established pharmacotherapy available to control the cell biology of the disease. The aim of this study was to investigate the role of alkylphosphocholines [APCs; erucylphosphocholine (ErPC) was used in this study], novel pharmacologic substances with antiproliferative properties, on intraretinal proliferation initiated by experimental retinal detachment in a well-established in vivo model. METHODS: Retinal detachments were created in adult pigmented rabbits. ErPC was injected intravitreally on either day 1 or day 2 after detachment. Bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) was injected on day 3. Following fixation, retinas were triple-labelled with anti-BrdU (proliferation marker), Isolectin B4 (retinal microglia marker), and anti-vimentin (retinal Mueller glia cell marker). The number of anti-BrdU-labelled cells per millimeter of retina was determined from sections imaged by laser scanning confocal microscopy. Toxicity was assessed by light and electron microscopy. RESULTS: A single intravitreal injection of ErPC had a significant effect on reducing the number of proliferating non-neural retinal cells on day 3 after experimental retinal detachment in the rabbit. Injection of ErPC on day 1 was more effective than when given on day 2. No evidence of toxicity was observed in the retina on day 3 for any of the conditions. CONCLUSIONS: APCs are novel pharmacologic substances that significantly inhibited intraretinal proliferation after experimental retinal detachment in this in vivo model. They could be considered as an adjunct therapy at the time of retinal reattachment surgery to potentially prevent proliferative vitreoretinal diseases such as PVR. However, long-term toxicity studies must be performed before APCs can be considered for clinical application.

[Diagnostic and therapeutic options in diabetic retinopathy]. / Diagnostik und Therapie der diabetischen Retinopathie Schicken Sie lhre Diabetiker regelmässig zur Fundoskopie.

Diabetes mellitus is the systemic disease that most often leads to blindness. Since the diminishment of visual acuity is a late symptom of the disease, screening examinations are of particular importance, as only in this way can the optimal time point for treatment be determined. Stage-oriented laser therapy prevents blindness due to macular edema or proliferative diabetic retinopathy. For a number of years, vitreoretinal surgery has enabled the treatment of late ocular manifestations such as bleeding into the vitreous body and traction retinal detachment. With appropriate stage-oriented treatment, hopeless cases of diabetic retinopathy ending in blindness should become the exception. The only useful and confirmed effective medical treatment capable of delaying this late complication continues to be careful blood glucose and blood pressure control.

[Surgical treatment of cataract]. / Raten Sie zur OP, damit die Welt wieder bunt wird.

Today, the most frequently performed of all operative interventions is considered to be surgery for cataract. Modern surgical techniques applied under local anesthesia, tiny incisions that close spontaneously, reliable biometric methods, and the availability of artificial lenses, all combine to produce excellent results. In view of the low complication rate, this procedure can be recommended even in very old patients.

[Circulatory disorders of the retina]. / Probleme mit den Augen - Fehler im System?

Cardiovascular diseases lead to systemic vascular regression, which, among other consequences, may result in an impairment of retinal perfusion. In view of the high level of comorbidity observed in patients with circulatory problems affecting the retina, the early recognition of mild forms of vascular retinopathy is of considerable importance. A major prerequisite for an improvement in outcome is the recognition of the fact that most circulatory disorders of the retina are due to systemic vascular disease, and the eye is an important prognostic indicator of cardiovascular morbidity and mortality. As a result, effective communication between family physician/internist and ophthalmologist is basic to the successful treatment of these diseases of the eye.

[Age-related macular degeneration]. / Zur Prophylaxe: Spinat, ein Vitaminpräparat und Zink

In the western world, macular degeneration is the most common cause of severe loss of vision and blindness in persons older than 50. The underlying cause of the condition is a disturbance in the interaction between the retina and choroid of the macula. Apart from age itself, genetic disposition and smoking are confirmed risk factors. In the initial stages, the patient experiences merely a mild blurring of vision. The wet form, which is usually progressive, is experienced as an acute loss of vision or distortion of the objects viewed. Underlying this wet macular degeneration is of new vessel growth from the choroid, known as choroidal neovascularization, which as a result of exudation of fluid and bleeding into the macula, destroys central vision. Apart from the administration of vitamins to slow down progression, laser coagulation, photodynamic treatment or vitreoretinal surgery may be helpful in some cases. A specific causal therapy is, however, not available.

[Clinicopathological correlations at the vitreoretinal interface]. / Klinisch-pathologische Korrelationen an der vitreoretinalen Grenzfläche.

BACKGROUND: Clinicopathological studies of the vitreoretinal interface (VRI) improve our understanding of the pathogenesis of vitreal maculopathy, facilitate differential diagnoses and help to develop new treatment strategies. OBJECTIVE: The aim of the study was to provide a comprehensive overview on clinicopathological correlations of the VRI. METHODS: A semi-structured literature search was performed in the Medline and Embase databases for relevant original studies on clinicopathological correlations of vitreal maculopathy, in addition to the latest books and review articles. RESULTS: Age-related vitreous changes with persistent vitreomacular adhesions on the retinal surface promote cellular migration and proliferation onto the vitreal side of the internal limiting membrane (ILM), thereby cementing the vitreomacular adhesions and strengthening the traction forces on retinal layers. Cellular or fibrocellular proliferation at the vitreomacular interface can be seen in all vitreal maculopathies. Furthermore, vitreoschisis in the context of anomalous posterior vitreous detachment causes the presence of vitreous cortex collagen fibrils on the vitreal side of the ILM which is associated with epiretinal membrane formation. Glial cells, hyalocytes and myofibroblasts represent the major cell types in the epiretinal cell proliferation. Glial cells and hyalocytes are capable of transdifferentiation into myofibroblasts which possess strong contractive properties and are well known for the production of extracellular matrix components. CONCLUSION: Removing vitreomacular adhesions and vitreous cortex collagen fibrils from the retinal surface is most important for successful treatment. In cases with epiretinal cell proliferation, however, removal of the ILM during macular surgery is mandatory to avoid reproliferation and recurrence. Improving the detection of epiretinal cell proliferation and cell distribution in patient eyes by optical coherence tomography or by introduction of new technologies should be addressed in the future.

Indocyanine green selectively stains the internal limiting membrane.

PURPOSE: To demonstrate whether indocyanine green stains the inner limiting membrane of the retina or residual vitreous cortex. METHODS: We report on the intraoperative staining patterns of the vitreomacular interface in 10 eyes of 10 consecutive patients who underwent vitrectomy with indocyanine green staining for macular hole formation and diffuse diabetic macular edema. RESULTS: In five eyes of five patients with macular holes, indocyanine green staining of the macula after posterior vitreous detachment resulted in an immediate visibility of a discernable membrane that was not previously seen. In five eyes of five patients with diffuse diabetic macular edema and adherent cortical vitreous, indocyanine green failed to stain the vitreomacular interface. After peeling off the residual vitreous cortex, however, a discernable membrane could be identified using indocyanine green dye again. Light and transmission electron microscopy revealed the inner limiting membrane as the membrane that had been stained and removed in all specimens. CONCLUSION: Indocyanine green selectively stains the inner limiting membrane. Staining of the vitreomacular interface using indocyanine green as a vital dye enables the surgeon to distinguish between the residual vitreous cortex and the inner limiting membrane, and it allows safer and easier removal of the inner limiting membrane.

Macular changes after peeling of the internal limiting membrane in macular hole surgery.

PURPOSE: To report the incidence of macular changes following pars plana vitrectomy with peeling of the internal limiting membrane (ILM) for idiopathic macular hole. DESIGN: Prospective consecutive series. METHODS: In a prospective study 105 eyes of 105 patients underwent vitrectomy for idiopathic macular holes. Surgery consisted of a standard three-port vitrectomy, induction of a posterior hyaloid detachment, removal of epiretinal membranes including the ILM, fluid-air exchange and intraocular gas tamponade (15% hexafluoroethane (C2F6) gas mixture) followed by head-down positioning for at least five days. No adjuvants were used during surgery. In addition to the clinical examination, static microperimetry using a Rodenstock scanning laser ophthalmoscope (SLO-105) was performed pre- and 6 or 12 weeks postoperatively. The stimulus size was 0.2 degrees (Goldmann II), intensities employed were 0 and 12 dB. For all tests, 20-degree fields were used. RESULTS: Anatomic closure of macular holes was achieved in 92 (87.6%) of 105 patients by one operation. Eight patients underwent a successful second procedure. The closure rate after two operations was 95.2%. Best corrected visual acuity increased from a median of 0.2 (range 0.05 to 0.5) preoperatively to a median of 0.5 (range 0.05 to 1.0) postoperatively. Anatomical macular changes were found in 8 (7.6%) patients: There were two cases of macular edema following secondary cataract extraction and six cases of retinal pigment epithelium changes. Formation of postoperative epiretinal membranes or late reopenings were not noted. Small, mostly asymptomatic paracentral scotomata were seen in 59 (56.2%) of 105 patients. CONCLUSION: Anatomical changes of the macula following vitrectomy with removal of the ILM are infrequent. However, paracentral scotomata observed in our series might be caused by a trauma to the nerve fibers during ILM peeling. To achieve reliable results a standardized procedure for microperimetry should be developed.

Indocyanine green-assisted peeling of the internal limiting membrane may cause retinal damage.

PURPOSE: To demonstrate possible retinal damage caused by indocyanine green dye for staining of the internal limiting membrane in surgery for idiopathic macular hole. METHODS: Consecutive interventional case series. We report on the ultrastructural findings of the internal limiting membrane in 10 eyes of 10 patients. RESULTS: All specimens revealed not only the internal limiting membrane, but also some small amounts of retinal elements, such as the plasma membrane of Müller cells and other undetermined structures. This indicates a cleavage plane not exactly at the inner undulating aspect of the internal limiting membrane but within the innermost retinal layers. CONCLUSION: Dilutions of indocyanine green as recommended in the literature may alter the structure of the retina to some degree. Possible factors responsible for this inadvertent action may include (1) concentration, (2) osmolarity pH, (3) time of tissue contact, and (4) mechanical factors from more forceful traction during peeling. Although functional consequences of these findings remain unclear as yet, factors that may induce damage to the innermost retina should be elucidated.

Macular hole surgery in an eye with an optic pit.

PURPOSE: To report an eye with a full-thickness macular hole and an associated optic pit and the noteworthy intraoperative findings. METHODS: Case report. A 56-year-old woman presented with visual acuity LE: 20/100, a full thickness macular hole, and an optic pit. Optical coherence tomography and ophthalmic examination were performed preoperatively and postoperatively. RESULTS: Although usually a macular hole associated with an optic pit tends to be a lamellar and characterized by outer layer defects within preexisting macular detachments or schisis-like cavities, this type of macular hole was not presented in this case. Although the macular hole resembled the idiopathic type on clinical examination as well as on optical coherence tomography, it could only be closed in the third surgical attempt after using silicone oil as a long-standing tamponade. Peeling of an epiretinal membrane or the internal limiting membrane was not possible during any of the three surgeries. CONCLUSION: Our observations suggest that in cases of macular hole in association with optic pit, instillation of silicone oil should be considered in the first surgical procedure, especially if no epiretinal membrane or internal limiting membrane peeling is possible intraoperatively.

Epiretinal pathology of vitreomacular traction syndrome.

AIMS: To investigate the ultrastructure of the vitreoretinal interface in patients with vitreomacular traction syndrome. METHODS: 14 patients with vitreomacular traction syndrome underwent standard pars plana vitrectomy. After induction of posterior vitreous detachment, epiretinal tissue and the inner limiting membrane (ILM) of the retina were removed, and processed for transmission electron microscopy. RESULTS: Ultrastructural analysis revealed two basic patterns of vitreoretinal pathology in eyes with vitreomacular traction syndrome. Seven specimens showed mostly single cells or a cellular monolayer covering closely the vitreal side of the ILM, not resulting in a biomicroscopically detectable epiretinal fibrocellular proliferation. The other seven specimens revealed premacular fibrocellular tissue which was separated from the ILM by a layer of native collagen, resembling the clinical features of idiopathic epiretinal membranes. In both groups of eyes, the myofibroblast was the predominant cell type. Fibrous astrocytes and fibrocytes were less frequent. Retinal pigment epithelial cells and macrophages were absent. Deposits of newly formed collagen were present only adjacent to fibrocellular multilayers. CONCLUSIONS: There are two distinct clinicopathological features of vitreomacular traction syndrome which suggest different forms of epiretinal fibrocellular proliferation: (1) epiretinal membranes interposed in native vitreous collagen and (2) single cells or a cellular monolayer proliferating directly on the ILM. The presence of remnants of the cortical vitreous which remain attached to the ILM following posterior vitreous separation may determine the clinicopathological feature of the disease. The predominance of myofibroblasts may help to explain the high prevalence of cystoid macular oedema and progressive vitreomacular traction characteristic for this disorder.

Ultrastructure of the vitreoretinal interface following plasmin assisted vitrectomy.

AIMS: To investigate the ultrastructure of the vitreoretinal interface following plasmin induced posterior vitreous detachment. METHODS: Plasmin (1 or 2 U/0.1 ml) was injected into the vitreous cavity of 24 eyes of freshly slaughtered pigs. The 24 fellow eyes received calcium-free and magnesium-free PBS and served as a control. After incubation at 37 degrees C for 30 and 60 minutes, the globes were placed in fixative and hemisected. Specimens for light, scanning, and transmission electron microscopy were obtained from the posterior pole, the equator, and the vitreous base using a corneal trephine. RESULTS: All plasmin treated eyes showed posterior vitreous detachment. However, the inner limiting membrane (ILM) was covered by remnants of cortical vitreous at the posterior pole and at the equator. There was a direct correlation between the concentration and exposure times of plasmin and the degree of vitreoretinal separation. Eyes exposed to 1 U plasmin for 30 minutes had a dense network of residual collagen fibrils while those exposed to 1 U plasmin for 60 minutes had only sparse collagen fibrils covering the ILM. Eyes treated with 2 U plasmin for 60 minutes had a smooth retinal surface, consistent with a bare ILM. At the vitreous base there was no vitreoretinal separation. In all control eyes the vitreous cortex was completely attached to the retina. There was no evidence of retinal damage in any plasmin treated eye. CONCLUSION: Plasmin induces a cleavage between the vitreous cortex and the ILM without morphological changes to the retina. In contrast with previous reports, plasmin produces a smooth retinal surface and additional surgery is not required in this experimental setting. The degree of vitreoretinal separation depends on the concentration and length of exposure to plasmin.

Silicone oil removal strategies.

Although it is widely accepted to use silicone oil in the treatment of very complex retinal detachments, there is no definite agreement on when and why silicone oil should be removed. Frequently found is a statement on the necessity of silicone oil removal after a certain period of time. However, the rate of retinal redetachment after silicone oil removal varies widely and appears to correlate with the underlying disease process and its severity. The literature on strategies of silicone oil removal, the rate of complications, and, thus, the risk-benefit ratio is scant. Therefore, for the discussion of silicone oil removal we rely on personal experience and the rather rare studies on silicone oil removal. Regarding the data given in the literature, the rate of vitreoretinal complications after silicone oil removal, even in cases with a clinically stable-appearing retinal situation, is rather high in severe proliferative vitreoretinopathy (PVR) and lower in most advanced cases of severe proliferative diabetic retinopathy (PDR) requiring silicone oil tamponade. The benefits of silicone oil removal are better in cases of cytomegalovirus retinitis or other situations without a PVR component. Silicone oil removal has to be considered a procedure of ill-defined risks, especially if silicone oil is really used as a last therapeutic resort in most severe cases of complicated retinal detachment. Further, exact criteria for the timing and safe removal of silicone oil in these complex vitreoretinal disorders still needs to be defined. A strategy for the removal of silicone oil is discussed.

[Pars plana vitrectomy in diabetic retinopathy. From pathogenetic principle to surgical strategy]. / Pars-plana-Vitrektomie bei diabetischer Retinopathie. Vom pathogenetischen Prinzip zur operativen Strategie.

Diabetic retinopathy is the most important manifestation of diabetic eye disease. There are retinopathy, maculopathy, pathology of the vitreoretinal interface, and tractional retinal detachment. The development of surgical approaches based on pathogenetic ideas are shown. The beneficial effect of pars plana vitrectomy for persisting and recurrent vitreous hemorrhage, epiretinal fibrovascular proliferation, and retinal detachment is discussed. The surgical skills and experiences in diabetic eyes are stressed.

[Expansion of intraocular gas due to reduced atmospheric pressure. Case report and review of the literature]. / Expansion intraokularer Gase infolge Reduktion des Atmosphärendruckes. Kasuistik und Literaturübersicht.

Since the advent of vitrectomy and the increased use of intraocular gases, there had been concern voiced about the safety of air travel for patients with intraocular gas. Anecdotal reports and experimental models verify the danger of acute glaucoma and central retinal artery occlusion following depressurization and expansion of intraocular gas. However, the amount of gas, that can be compensated for, is a matter of controversy. CASE REPORT I: A 58-year old man underwent vitrectomy with intraocular air tamponade as a primary procedure for retinal detachment. The patient went home by airplane with a residual gas volume of less than 30% of the volume of the eye. Before takeoff at 530 m (1739 ft) above sea level, the intraocular pressure was 12 mmHg. During the ascent, severe ocular pain and loss of vision occurred. CASE REPORT II: A 38-year old woman underwent vitrectomy for primary repair of retinal detachment. The eye was injected with a 15% mixture of perfluoroethan (C2F6). When traveling home to Italy by car, the eye contained a residual gas volume of 50% of the volume of the globe. Intraocular pressure was 17 mmHg. During the ascent to the "Brennerpass", 1375 m (4511 ft) above sea level, the eye became severely painful and vision was lost for approximately three minutes. Descending to a lower altitude relieved the symptoms. CONCLUSION: The expansion of intraocular gases depends on the atmospheric pressure and the mechanisms for compensation. Small volumes of intraocular gas or moderate traveling altitudes can cause a symptomatic rise in intraocular pressure.