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Neuroradiology Case of the Week

Case 296

Scott Rudzinski, Ashwani Sharma, MD, and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 64-year-old male presented status post assault. He has loss of vision in the right eye. There is clinical suspicion of intracranial bleed with fracture of orbit and facial bones.

Imaging Findings: A coronal CT of orbits reveals a medial blowout fracture of the right orbit with intraocular air. An axial CT scan of the orbits reveals herniation of the right medial rectus muscle in the fracture defect. There is irregularity and thickening of the right optic nerve at its junction with the globe suggesting right optic nerve injury. There is dislocation of the lens of the right globe. There is a right periorbital hematoma. There is no intracranial bleed or fracture.

Diagnosis: Blowout fracture of the medial wall of right orbit

Discussion: Orbital blowout fractures are common sequelae to blunt periorbital trauma. The term orbital blowout fracture describes an injury resulting from a blow to the orbit by an object that is too large to enter the orbit. Intraorbital pressure increases as the globe is displaced posteriorly, causing a “blow out” fracture to occur at the weakest site of the orbit wall. In a pure blowout fracture the orbital rim remains intact.
     Orbital fractures occur more commonly in young adult and adolescent males and constitute 3% to 5% of midface fractures. The most frequent mechanism of injury is due to assaults, motor vehicle accidents and falls. Significant force is required to produce orbital fractures and they are often associated with other facial fractures, extraocular muscle injury and entrapment, intracranial injuries, and intraocular injury.
     Pure orbital blowout fractures occur at the weakest point of the orbital wall. The hydraulic and buckling force theories have been proposed to explain the mechanism of blowout fractures. It has usually been reported that pure orbital blowout fractures most frequently involve the orbital floor. However, one recent study suggests the lamina papyracea of the ethmoid bone comprising the medial wall should be the weakest area and when the study reviewed CT scans of patients with suspected orbital blow outs, the most common blow out fracture was an isolated medial wall fracture (55%), followed by medial and inferior wall fractures (27%), and isolated inferior fractures (18%).
     Thin cut coronal CT of the orbit is the imaging modality of choice to diagnose orbital fractures. Even on CT scans, an ethmoid fracture may be missed if there is no displacement of the bone. Herniation of fat into the ethmoid sinus aids in the diagnosis. Air from the disrupted sinus may present as orbital emphysema, which is often present with blow out fractures. CT may also aid in distinguishing orbital edema from entrapped extraocular muscle. The sensitivity of plain radiographs for detecting orbital fractures has been sited as 50% in literature. On plain films the ethmoid fracture is often poorly visualized, with only some clouding of ethmoid cells being evident. On MR imaging, the high T1-weighted signal intensity of the orbital fat and any hemorrhage can be well seen, however, small, nondisplaced fractures may not be visualized.
     Indications for immediate surgery are definite muscle entrapment and acute enophthalmos. Chronic enophthalmos, which can lead to cosmetic deformity, develops when too much fat has herniated into the sinus. The enophthalmos goes clinically undiscovered because of compensatory intraorbital edema. It is therefore important for the imager to consult the clinician when a high degree of fat herniation is present so the patient may undergo surgical intervention. Diplopia is a common complaint that can resolve after swelling decreases or can persist if there is extraocular muscle involvement.

References:

1. Burm JS, Chung CH, Oh SJ. Pure orbital blowout fracture: new concepts and importance of medial orbital blowout fracture. Plast Reconstr Surg. 1999 Jun;103(7):1839-49. [Medline]
2. Neuman MI, Bacher RG. Orbital fractures. UpToDate. September 14, 2006. http://patients.uptodate.com/topic.asp?file=ped_trau/2166.
3. Nolasco FP, Mathog RH. Medial orbital wall fractures: classification and clinical profile. Otolaryngol Head Neck Surg. 1995 Apr;112(4):549-56. [Medline]
4. Som SM, Hugh CD. Head and Neck Imaging, 4th ed. Mosby, St. Louis, Missouri. 2003; 389-397.