Assessment of Orbital Compartment Pressure: History
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Contributor:
Tim J. Enz
,
Markus Tschopp

The orbit is a closed compartment defined by the orbital bones and the orbital septum. Some diseases of the orbit and the optic nerve are associated with an increased orbital compartment pressure (OCP), e.g., retrobulbar hemorrhage or thyroid eye disease. Assessment of the indirect clinical markers of elevated OCP is relatively easy, fast, inexpensive, and hence widely available. Furthermore, these surrogates appear to relatively reliably indicate elevated OCP in orbital compartment syndrome. Thus, assessing these clinical findings will continue to be part of the management of orbital diseases. In many cases, these indirect clinical findings allow for diagnosis and therapeutic decision making sufficiently reliably without the need for further testing. In cases of suspected orbital compartment syndrome with potential vision loss, the indication for surgical intervention should be made at a low threshold. 

  • orbital compartment pressure
  • minimally invasive measurement
  • orbital compartment syndrome

1. Introduction

The orbit is a closed compartment defined by the orbital bones and the orbital septum, containing a number of structures essential for the visual process. As such, the orbit is subject to a certain compartment pressure. Some diseases of the orbit are associated with an increased orbital compartment pressure (OCP), a key clinical finding leading to compression and the subsequent damage of the intraorbital structures, e.g., retrobulbar hemorrhage or thyroid eye disease [1][2][3]. Furthermore, it has been postulated that the OCP is a determining factor of intraocular pressure and optic nerve subarachnoid space pressure, and hence of the lamina cribrosa pressure gradient [4][5]. Thus, the OCP may be important in the pathogenesis of glaucoma or in estimating intracranial pressure based on optic disc signs. Accurate measurements of the pressure dynamics within the orbital compartment may hence be helpful for enhancing the understanding of glaucoma and other optic neuropathies possibly associated with altered OCP.
Historically, an assessment of the tissue resistance provoked by the retropulsion of the eye bulb was the method of choice for estimating OCP, either by digital palpation or with specifically designed devices [6][7]. Today, with widely experimental methods [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], the direct measurement of OCP is considered technically feasible yet impractical in a clinical setting due to limited availability, difficult arrangement, and invasiveness. Thus, in suspected orbital compartment syndrome, for example, OCP is generally not measured directly. Rather, it is estimat0ed based on clinical findings that are considered surrogates, e.g., proptosis or the intraocular pressure [1][9][29][30].

2. Assessment of Orbital Compartment Pressure

2.1. Indirect Orbital Compartment Pressure Assessments: Historical Perspectives

Historically, an assessment of the tissue resistance provoked by the retropulsion of the eye bulb was considered the method of choice for estimating the pressure within the orbital compartment. In 1868, von Graefe recommended digital retropulsion to evaluate the ease with which the globe can be retropulsed in patients with suspected thyroid eye disease [6]. In 1910, Langenhan introduced the first device to measure and quantify orbital tissue resistance during ocular retropulsion [31], followed by Gutmann [32], Plegge [33], and Georg [34], who assessed the weight necessary to retropulse the globe in patients in a supine position. However, only Copper’s instrument described in 1948 was used more extensively to indirectly measure the OCP [7]. With Copper’s orbitotonometer, the OCP was measured by putting weights in 100 g increments up to 400 g on a rod resting on the orbital rims of a patient lying in a supine position, exerting force on the eye bulb that had previously been anesthetized and covered with a contact lens. Baseline exophthalmometer values, the globe position relative to the orbital rim at various applied forces, and the graph generated when plotting the ocular position against the amount of force applied were the parameters assessed by Copper’s method. In this setting, the amount of force was pre-given while the extent of retropulsion was the independent variable. In the following years, a number of reports on the clinical application of Copper’s orbitotonometer for monitoring thyroid eye disease and orbital tumors were published by various investigators [35][36][37][38]. In 1968, Elsby described a device for measuring orbital compliance, which appeared to be, however, very similar to Copper’s orbitotonometer [39]. In 1975, Doege described an orbitotonometer with electronic recordings of the obtained data. Although this led to many advantages, the working principle remained the same [40].
A different approach to quantify OCP was described only in 1984 by McGowan et al., who developed a technique involving swim goggles. Fluid was added to the space between the goggles and the eye alongside continuous electronic measurements of the pressure inside this space [41][42]. With the pressure increase plotted against the volume added to the space over time, conclusions on the OCP were drawn. Frueh and associates revived the principle of using a contact lens for transmitting force onto the eye bulb and measuring OCP. However, in their setting, the patients were seated in an upright position, eliminating gravity-associated effects. The device retropulsed the eye bulb at a certain distance and measured the forces generated using pressure transducers, making the extent of retropulsion the pre-given variable and the amount of applied force the independent variable. Thus, the method allowed for much more accurate OCP measurements and a better comparison between different patients and eyes. Using this technique, Frueh et al. indirectly assessed OCP in healthy subjects and patients with orbital diseases. However, given the complicated arrangement and the lack of convenience for the patients, the technique could not establish itself as routine clinical practice [43].

2.2. Direct Orbital Compartment Pressure Measurements

Despite the paramount clinical importance of OCP in many respects, only few attempts have been made to directly measure it. Consequently, there is a paucity of data on ranges of pressure values in health and disease, and no established threshold pressure values for surgical intervention have been determined yet either. So far, no equipment has been designed and manufactured specifically for direct OCP measurements.

Kratky and colleagues were among the first to directly measure the OCP in 1990. They assembled a custom pressure measuring device consisting of a slit catheter and a pressure transducer to measure changes in OCP in patients with different orbital diseases, as well as in patients with healthy orbits during major orbital surgeries (enucleation, exenteration, or orbital decompression). Their study aimed to establish a normal range of OCP (3–6 mmHg). Since normal subcutaneous tissue pressure levels are sub-atmospheric, the scholars concluded that their findings support the concept that the orbit indeed functions as an enclosed compartment. Furthermore, their study showed that the OCP can be elevated in thyroid eye disease (7–15 mmHg) [16]. Similarly, in 1996, Otto and associates aimed to investigate OCP dynamics in patients with thyroid eye disease during surgical decompression. To that purpose, they used a self-assembled custom device consisting of a micro-pressure transducer placed into the orbit and attached to a research amplifier and a chart recorder [19]. In 2010, Berthout et al. used a Codman© Pressure Monitor commonly used for measuring the intracranial pressure, and they investigated changes in OCP in patients with thyroid eye disease during orbital decompression procedures. Their findings confirm that OCP is significantly increased in severe thyroid eye disease and that it can be reduced or normalized with surgical decompression of the orbit [8].

Only four groups of investigators attempted to measure the OCP minimally invasively [9][10][12][22][23]. Egbert and colleagues, as well as Riemann et al., assembled custom devices to that purpose [10][22][23]. Riemann et al. used a 23-gauge blunt-tipped retrobulbar needle to which a pressure transducer kit commonly used for measuring hemodynamic parameters was attached. This transducer was connected to a patient monitoring system which yielded alphanumeric and graphic representations of the pressure. Using this custom device, the investigators assessed OCP dynamics during intraorbital anesthetic injection for ocular surgery. Their studies aimed to determine a physiologic range of OCP in patients with healthy orbits (6.3 ± 1.7 mmHg) as well as with thyroid eye disease (9.7 ± 4.8 mmHg). In addition, the scholars concluded that intraorbital anesthetic injections consistently cause changes in OCP and that directly assessing OCP dynamics in vivo may prove useful both as an adjunct in the clinical evaluation of patients with disorders resulting in orbital compartment syndrome, as well as in assessing the risk of retrobulbar injection in orbits at greater risk of complications from this procedure [22][23]. Egbert’s device consisted of a specially designed cannula which was made with side ports for piezometric tappings. At these side ports, pressure values were measured and the OCP was calculated from these values during corticosteroid injections into the orbital capillary hemangiomas. Their findings suggest that intralesional pressure may rise tremendously during injections (range 18.65–842.2 mmHg) and may routinely exceed systemic arterial pressures. Since a sufficient volume of corticosteroid injected at a high injection pressure would account for the embolization of corticosteroid particles into the ocular circulation from the retrograde arterial flow, the scholars concluded that the volume of corticosteroid to be injected should be limited and indirect ophthalmoscopy should be performed on all patients receiving injections of long-acting corticosteroids into the orbit and periorbital soft tissue [10]. On the other hand, Czyz and Strand, as well as Enz and associates, used commercially available devices commonly used for minimally invasively monitoring of the compartment pressure of the limbs. Both groups used a Compass Compartment Monitor© device for measuring OCP in real and simulated orbital compartment syndrome [9][12]. Czyz and Strand assessed a single patient with clinical signs of orbital compartment syndrome and recorded an OCP value of 14 mmHg [9]. Enz et al. used patients scheduled for ocular surgery under peribulbar anesthesia as a human in vivo model of orbital congestion. They assessed OCP changes before and during intraorbital anesthetic injections and reported a mean baseline pressure value of 2.5 ± 1.5 mmHg and an increase to 12.8 ± 9.2 mmHg following injection [12]. Both groups came to the conclusion that the direct measurement of OCP using this device appears feasible and useful in diagnosing and monitoring patients with suspected orbital compartment syndrome.

2.3. Surrogates for Elevated Orbital Compartment Pressure

Today, orbital compartment pressure is usually not measured directly. Instead, in clinical situations, it is mainly estimated using clinical findings that are considered surrogates for elevated OCP. Decisions against or in favor of possible therapeutic interventions are usually based only on these clinical surrogates, occasionally supported by findings of imaging procedures (CT, MRI) showing evidence of retrobulbar fluid accumulation, pronounced proptosis, extraocular muscle enlargement, or optic nerve compression [1][9][29][30][44].
In suspected orbital compartment syndrome, raised intraocular pressure (IOP) is recognized as one of the most important of those clinical signs, besides visual disturbances, tight orbits, proptosis, ocular motility restrictions, choroidal folds, or signs of optic nerve compression [1][9][12][28][45]. The underlying rationale is that the orbit is a closed compartment and, thus, space-occupying intraorbital lesions affect both the orbital compartment and intraocular pressure similarly [28][46]. Indeed, Oester et al. and Zoumalan et al. reported correlating changes in OCP and IOP in their cadaver-based model of orbital compartment syndrome [18][28]. The findings of Zoumalan’s group suggested a steady increase in both OCP and IOP starting from 3 mL intraorbital volume increments [28]. Zhou et al. and Enz et al. assessed the relationship between OCP and IOP changes in living humans and found similar correlations [12][27]. Furthermore, Enz et al., as well as Stanley et al., also found a correlation between elevations in OCP and the extent of proptosis, concluding that proptosis can be considered a valid surrogate as well [12][24].

2.4. Future Developments

While digital palpation of the globe and the periocular tissue continues to be commonly performed as an indirect approach for assessing tight orbits, the devices described above for quantifying the resistance provoked by the retropulsion of the eye bulb did not endure time, given their complicated arrangement and the lack of convenience for the patients, and can thus be considered obsolete. Assessment of the indirect clinical markers of elevated OCP is relatively easy, fast, inexpensive, and hence widely available. Furthermore, these surrogates appear to relatively reliably indicate elevated OCP in orbital compartment syndrome. Thus, assessing these clinical findings will continue to be part of the management of orbital diseases. In many cases, these indirect clinical findings allow for diagnosis and therapeutic decision making sufficiently reliably without the need for further testing. In cases of suspected orbital compartment syndrome with potential vision loss, the indication for surgical intervention should be made at a low threshold. However, all published experimental attempts to directly measure OCP using custom devices or devices commonly used for other purposes were considered successful by the investigators, and provided useful clinical and diagnostic information. Particularly minimally invasive approaches appear promising for directly measuring OCP. Thus, in the future, more sophisticated, custom-made equipment might allow for even safer and more precise direct, minimally invasive OCP measurements and facilitate the diagnosis of orbital compartment syndrome or thyroid eye disease. Furthermore, such measurement modalities may prove helpful in the research and assessment of other optic neuropathies possibly associated with altered orbital compartment pressure.

3. Conclusions

In conclusion, to date, the diagnosis and monitoring of elevated OCP is based on clinical signs considered as surrogates, particularly, elevated IOP and proptosis. These established indirect clinical diagnostic markers appear to be reliable indicators for elevated OCP. However, growing evidence supports direct OCP measurement as a diagnostic adjunct. Particularly minimally invasive approaches show promise for routine use. To date, no device/equipment has been specially designed for this purpose; hence, clinicians are left with the sole option of using either inappropriate or experimental equipment. In the future, more sophisticated, specifically designed equipment might allow for even better and safer direct, minimally invasive OCP measurements, and hence facilitate the diagnosis and monitoring of orbital diseases and optic neuropathies possibly associated with altered orbital compartment pressure.

This entry is adapted from the peer-reviewed paper 10.3390/diagnostics12061481

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