Orbital compartment syndrome

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Orbital Compartment Syndrome

Disease Entity

Orbital compartment syndrome (OCS) is an ophthalmic emergency. OCS is a vision threatening elevation of intra-orbital pressure which exceeds the vascular perfusion pressure of the ophthalmic artery. It can result in ischemia and irreversible vision loss if not corrected emergently. Although it is most commonly observed after trauma, there are several other less common etiologies which can produce an elevated orbital pressure capable of causing permanent ischemic damage.

Etiology

OCS most commonly occurs during the acute sequalae of orbital or facial trauma. In this scenario, trauma (including iatrogenic surgical trauma) may result in diffuse soft tissue swelling, as well as retrobulbar and orbital hemorrhage, all of which can increase the intra-orbital pressure because of the inflexibility of the surrounding bony orbital walls and orbital septum. However, any process that results in an increase in the volume within the orbit can lead to an OCS. Less common etiologies include local injections (peribulbar or retrobulbar), orbital infection (with or without abscess), orbital emphysema, foreign bodies, anatomical anomalies, positional edema, spontaneous hemorrhage (e.g. valsalva-related), or fluid resuscitation (especially after burn injuries).

Risk Factors

The main risk factors include orbital or periorbital surgery, orbital trauma, anticoagulation, or coagulopathy. Less common risk factors include orbital cellulitis and/or abscess.

Pathophysiology

The orbit is an anatomical structure bounded by 4 bony walls, and anteriorly by the eyelids and orbital septa. Because of these rigid boundaries, there is little room to accommodate any increase in pressure associated with bleeding, edema, or the introduction of foreign material into the orbit. The normal orbital volume is 30 mL or less. The volume normally occupied by the globe and extraocular soft tissue is 26.5 mL. Any increase in the orbital volume leaves little room to accommodate tissue expansion in this compartment. As a result, the orbital compartment pressure increases. OCS occurs when the orbital compartment pressure exceeds the perfusion pressure of the blood supply to the optic nerve via the optic nerve vasa nervorum or to the retina via the central retinal artery.

Any increase in pressure posterior to the globe results in proptosis of the globe. The optic nerve is naturally redundant within the orbit, allowing for approximately 8 mm of proptosis before damage occurs to the optic nerve axons. The anterior orbital boundary, the eyelids and orbital septa, limits globe proptosis in the acute setting, necessitating the need for lateral canthotomy/cantholysis to allow for full globe proptosis.

Primary prevention

In patients with a history of recent trauma, orbital fracture, or orbital surgery, preventive measures include avoidance of valsalva or nose blowing, and administration of antiemetics when indicated. Anticoagulation should be pursued with caution and withheld unless medically necessary. These medications are usually discontinued pre-operatively if possible and restarted judiciously after orbital or periocular surgery. In a patient with an orbital hemorrhage without an OCS they should be observed for at least 6 hours to assure there is not continued bleeding leading to an OCS. Eye protection and other trauma preventive measures should always be encouraged.

Diagnosis

OCS is a clinical diagnosis. While more information may be added with radiographic evidence, treatment of OCS should never be delayed for radiologic evaluation. This is because of the irreversible nature of OCS sequelae if not corrected emergently. The most important initial clinical findings can include a markedly elevated intraocular pressure (IOP) with or without a decrease in the visual acuity of the affected eye, proptosis, a tight orbit, an afferent pupillary defect, intraocular vascular changes, and decreased extraocular movements. In cases of trauma there may be significant external findings as well, such as ecchymosis.

History

History is useful in establishing the possibility of OCS. History of recent trauma, reported subjective vision decrease or diplopia are important points to elucidate. Additional pertinent history includes a history of coagulopathy or pharmacologic anticoagulation, history of recent orbital or periocular surgery, history of orbital vascular malformations, recent infection including preseptal or orbital cellulitis, and acute or chronic sinusitis.

Physical examination

Important physical exam findings include decreased visual acuity in the affected eye, afferent pupillary defect, and elevated IOP. Additional clinical findings are increased difficulty in opening the eyelids for examination, limited motility, proptosis, and resistance to retropulsion. Color vision may be affected, particularly with red color desaturation testing. Dilated exam may show vascular congestion or even disc edema. In cases of trauma a full eye exam should always be performed of both eyes to rule out other injuries, particularly ruptured globe, and facial sensation should be evaluated. In cases of orbital emphysema crepitus may be noted on the exam.

Diagnostic procedures

There are radiographic signs that can be helpful to confirm the diagnosis of OCS, however OCS is first and foremost a clinical diagnosis. No radiographic test should ever preempt treatment if clinical suspicion is high, especially if there is a decrease or loss of vision. The presence of an orbital fracture does not remove the possibility of an OCS.

CT imaging may reveal retrobulbar and/or orbital hemorrhage which would correlate with clinical findings. Generally this is not a discrete hemorrhage but more diffuse blood between normal orbital structures. Additional signs include proptosis of the affected eye and loss of the normal redundancy of the optic nerve, normally 8 mm. Tenting of the posterior aspect of the globe at the area of the optic nerve insertion may also be seen. Additional findings include orbital emphysema or space occupying intraorbital foreign body. MRI or MRA, if warranted, may reveal similar findings to CT as well as any vascular malformations which could lead to an acute hemorrhage resulting in OCS. In cases of trauma CT should always be the initial radiographic imaging study.

Laboratory test

CBC, BMP can be useful as part of a basic workup in a patient with a history of trauma or cellulitis, however the most useful test in this setting is PT, aPTT, and INR to diagnose anticoagulation or coagulopathy, which would predispose the patient to retrobulbar and orbital hemorrhage if the etiology is unclear.

Differential diagnosis

Both subacute and chronic etiologies are present within the differential diagnoses. While orbital hemorrhage from trauma is the most common etiology, there are a variety of other potential causes. It is worth considering orbital inflammatory syndromes, including thyroid ophthalmopathy and idiopathic orbital inflammatory syndrome. Other possibilities would include a space occupying lesion of the orbit, infection, and post-surgical edema.

Management

Because the pathogenesis of OCS results in permanent vision loss if not correctly treated in the first two hours, immediate intervention is required to restore blood flow to the retina and the optic nerve. This can be accomplished at the bedside with a lateral canthotomy combined with inferior cantholysis with added septolysis procedures as needed. Superior cantholysis should be considered with caution due to the increased risk of significant bleeding from the lacrimal artery and little evidence of improved decompression if an adequate inferior cantholysis was performed. Special attention must be made in this instance, as the lacrimal gland resides near the lateral aspect of the superior canthal tendon. If an adequate inferior approach is performed, a superior cantholysis is generally not needed. If cantholysis fails to achieve improvement in visual acuity, consultation of an orbital surgeon may be necessary for surgical decompression of the orbit under general anesthesia using a subciliary, or transconjunctival approach. Additionally, medical therapy can be used as an adjuvant treatments as well as treatment for other underlying disorders which could exacerbate bleeding.

Lateral Canthotomy/Inferior Cantholysis

The surgical area should be prepped and draped in the usual sterile fashion, focusing on the lateral aspect of the eyelids. Local anesthesia is then obtained using lidocaine with epinephrine, being sure to keep the syringe aimed parallel to the globe and a stabilizing hand on the face to minimize the risk of globe penetration if the patient should move. Next, a sterile hemostat may be used to compress the tissues of the lateral canthal angle in the area of the intended canthotomy and extending towards the lateral orbital rim. In severely edematous or ecchymotic tissues, this can aid in achieving hemostasis as well as identifying the desired tissues to be incised for the canthotomy. The hemostat is then removed and the canthotomy is performed by advancing blunt tipped scissors along the line from the lateral canthal angle towards the lateral orbital rim. This will expose the lateral canthal tendon closer to its insertion point on inner aspect of the lateral orbital rim. Attention is then turned to the inferior canthal tendon. Inferior cantholysis can be achieved by using blunt tipped scissors to sever the inferior canthal tendon near its lateral insertion point. The incision can be extended downward until cantholysis is assured. A successful inferior cantholysis is usually accompanied by immediate release of the eyelid, proptosis of the globe, and sometimes by immediate improvement in visual acuity. The adequacy of this procedure should be reflected in subsequent reduction of IOP.

Medical therapy

Additional measures which can be employed along with bedside canthotomy and cantholysis include IV acetazolamide or IV mannitol8. Some surgeons may employ the use of IV methylprednisolone in the hope of reducing orbital tissue edema, however there are no conclusive studies demonstrating its effectiveness. Other medical interventions involve addressing the underlying cause of the acute OCS, e.g. IV antibiotic therapy for fulminant orbital cellulitis. Patients may benefit from antiemetics, cough suppressants, and nasal decongestants to reduce fluctuations in orbital pressure and reduce orbital emphysema.

Complications

If uncorrected, OCS can cause permanent blindness via interruption of the blood supply to the vasa nervorum of the optic nerve as well as perfusion of the retinal artery.

Prognosis

The most important prognostic indicator is the time of intervention to orbital decompression, with best results achieved with restoration of blood flow in the orbit within two hours of OCS onset. Some studies have shown poorer prognosis in older patients as well as patients with more tenting of the globe on imaging with a decreased posterior globe angle.

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