Young adult with left eyelid swelling . Axial (a) and coronal (b) noncontrast CT scan demonstrating soft tissue of the left preseptal region and left medial canthus
Orbital cellulitis on CT demonstrates inflammation of the intraconal fat (Fig. 4.2) with possible enlargement of the extraocular muscles or osseous erosion. A phlegmon is seen as a homogenously enhancing lesion. A phlegmon can be indistinct from other pathologies such as orbital inflammatory syndrome, lymphoid lesion, and malignancy on CT, and, therefore, MRI plays an important, complimentary role. An orbital abscess has similar imaging features to a subperiosteal abscess, though location differs (Fig. 4.2). The amount of tension produced in the orbital cone by a space-occupying lesion such as an abscess can result in proptosis and tenting of the posterior globe .
Contrast-enhanced axial orbital CT (a) shows left intraconal and retrobulbar fat infiltration by a soft tissue density lesion representing orbital cellulitis and phlegmon in a patient, who has preseptal cellulitis. Axial postcontrast soft tissue images (b) of the orbit of a different patient exhibits a retrobulbar abscess and in bone window (c) reveals the cause of the abscess as left maxillary sinusitis
In the assessment of periorbital or orbital cellulitis, the paranasal sinuses, nasal cavities, skin, and dentition are a part of the search process. This also includes searching for a foreign body.
It is also important to image pediatric and adult patients with forehead swelling, fever, and headache as this could be a Pott’s puffy tumor, also known as osteomyelitis of the frontal bone, where the bone of the frontal sinus dehisces due to the underlying sinusitis and extends to the periorbital and orbital soft tissues (Fig. 4.3). This process can also have intracranial complications like extra-axial abscess or meningitis .
Young child with headaches and forehead swelling . Axial (a) and sagittal (b) CT bone window images show a wide area of dehiscence of the outer table of the right frontal sinus with a rounded soft tissue in the right medial extraconal compartment. Contrast-enhanced MRI shows a rim-enhancing fluid collection (c), which has increased signal on diffusion-weighted images (d)consistent with an abscess (e), with corresponding dark signal on the ADC map
CT is best at depicting the complications of cellulitis including superior ophthalmic vein thrombosis, intracranial abscess, cavernous sinus thrombosis, cavernous internal carotid artery aneurysm, and meningitis, though in certain cases MRI can be complimentary . These complications occur primarily from the extensive anastomoses among the valveless venous network that drains the orbit, skin of the periorbital tissues, and the maxillary and ethmoid sinuses .
The superior ophthalmic vein (SOV) begins in the medial aspect of the orbit as a confluence of the angular, supratrochlear, and supraorbital veins, for short distance follows the same course as the ophthalmic artery, passes between the two heads of the lateral rectus muscle, and exits via the superior orbital fissure to end in the cavernous sinus. When a thrombus forms in the SOV, there is enlargement and hypoenhancement of the vein, and on occasion, there can be perivenular stranding from inflammation (Fig. 4.4). Extension of the thrombus to the cavernous sinus can also be seen on contrast-enhanced CT as a filling defect in the cavernous sinus or lateral dural enhancemen (Fig. 4.4). There are instances where a CT venogram or contrast-enhanced MRI would better depict and/or compliment the contrast-enhanced CT findings.
A patient with right preseptal cellulitis (a) and postseptal cellulitis has increased size of the hypodense right superior ophthalmic vein with perivenular inflammatory stranding (arrow) on contrast-enhanced axial CT scan of the orbits (b). Intracranial CTA, which was performed on a different patient with multiple cranial neuropathies and left orbital cellulitis, shows left lateral dural enhancement (arrows) on the (c) and coronal (d) planes, consistent with cavernous sinus thrombosis
Magnetic Resonance Imaging
MRI is a more sensitive examination with better anatomic detail and contrast resolution compared to CT and is ostensibly true for the orbit. In cases of suspected orbital cellulitis, MR can be performed as a complimentary modality to CT when there is concern for intracranial complication or as a stand-alone modality to assist in diagnosis of orbital cellulitis and its complications.
In order to assist in patient management, the best possible images with patient cooperation are necessary. The MRI examination is lengthy, and therefore, to obtain quality, diagnostic images particularly in the pediatric or claustrophobic adult patient, oral or intravenous sedation under the supervision of a radiologist and radiology nurse is unavoidable.
Similar to CT, contrast material is necessary for MRI as images are performed before and after contrast administration to better elucidate pathology. Gadolinium-based contrast agents (GBCA) are used in MRI, but administration is contraindicated in certain situations, such as in patients with stage IV or stage V renal disease due to the possibility of a rare but grave nephrogenic systemic fibrosis or in those patients with anaphylaxis [15, 30–34].
The multiplanar capability and variety of pulse sequences performed allow for better visualization of pathology. There are a few sequences that are most helpful to look at the orbit and in particular, for orbital cellulitis. For better discrimination of orbital pathology localized to the lacrimal gland, extraocular muscles, and intraconal compartment including the optic nerve sheath complexes, orbital fat suppression is paramount on T2-weighted and pre- and postcontrast T1-weighted images [15, 34]. The pre- and postcontrast T1 fat suppression is also necessary for distinguishing true enhancement. Fat suppression techniques are also susceptible to metal and air-bone interface causing artifacts to be seen on images .
At times, the diagnosis of orbital cellulitis can be difficult as the clinical presentation and imaging appearance can be indistinguishable with considerable overlap on conventional MR imaging from orbital inflammatory syndrome and lymphoid lesion. A collaborative approach among specialties, in particular ophthalmology and radiology, would be important as a history of cutaneous infection, sinusitis, trauma with or without orbital fracture, dental procedure, strabismus surgery, or scleral banding are associated with orbital cellulitis . A trial with treatment for orbital inflammatory syndrome can also be performed with follow-up imaging to assess response. Kapur et al. looked at diffusion-weighted imaging in all three pathologies and found that their small sample size produced increased signal on DWI, which is typical for lymphoid lesions due to increased cellularity as seen with lymphoid tissue elsewhere in the body.
Orbital Cellulitis Complications
The incidence of subperiosteal abscess in children as a known complication of rhinosinusitis is 9% [6, 35]. It is an infected collection that occurs between in the periorbita and the orbital bone.
On ultrasonographic examination, a subperiosteal abscess is seen as an anechoic lesion with a hyperechoic wall emanating from the lamina papyracea and resultant lateral displacement of the medial rectus muscle and optic nerve sheath complex. On the other hand, subperiosteal inflammation medial to the medial rectus muscle is seen as a hyperechoic lesion in a similar location.
Cross-sectional imaging with CT shows an extraconal hypodense fluid collection along the lamina papyracea that either homogenously or rim enhances depending on the size and cavitation of the lesion and may have perilesional fat stranding from surrounding inflammation. MR imaging demonstrates a T1 hypointense and T2 hyperintense peripherally enhancing lesion and bright signal on diffusion with reciprocal dark signal on the apparent diffusion coefficient map due to lack of water movement from dense cellular packing and increased viscosity [36, 37] (Fig. 4.5). Osteomyelitis can also be seen of the orbital wall in advanced cases.
Young child with contrast-enhanced CT in axial (a) and coronal planes (b) demonstrates a rim-enhancing fluid collection along the left lamina papyracea and with medial displacement of the left medial rectus muscle
The mainstay of treatment is primarily with a combination of antibiotic medical therapy and surgical drainage and decompression. Surgical abscess drainage is necessary to prevent mass effect on the globe and optic nerve sheath complex [10, 19]. Generally, very few practitioners solely rely on medical management though this has been more successful in young children .
Cavernous Sinus Thrombosis
Rarely, infection of the orbit can produce cavernous sinus thrombosis, a neurologic condition, wherein the thrombus of the cavernous sinus results in third, fourth, and sixth and first and second trigeminal division cranial neuropathies. The mechanism that allows for this stems from the absence of valves in dural venous sinuses and cerebral and emissary veins resulting in variable flow that depends solely upon pressure gradients. These intracranial sinuses and veins have extensive direct and indirect communication with the veins of the face, sinonasal cavity, and orbits .
Though the clinical picture with cranial neuropathies and/or meningismus points toward the diagnosis, imaging would be performed with either CT or MR venogram for validation.There are a variety of ways cavernous sinus thrombosis can manifest on imaging and includes cavernous sinus expansion, filling defect in the cavernous sinus, lateral dural enhancement or superior ophthalmic vein enlargement. Unlike CT, MRI has many more sequences that can confirm the diagnosis, such as precontrast T1, which shows acute thrombus as bright signal, or T2 imaging, which shows thrombus as intermediate to bright signal. In addition, there has been a single case report by Parmar et al. that suggests restricted diffusion of the thrombus .
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Intracranial Infectious Aneurysm
An even more rare complication is infectious aneurysm formation particularly of the cavernous internal carotid artery due to its close proximity to the paranasal sinuses and orbits [42, 43]. The incidence ranges from 0.7 to 6%  with extravascular spread of infection from adjacent orbital cellulitis, meningitis, or cavernous sinus thrombosis spurring on inflammatory-mediated destruction of the vessel wall. As this is an infectious aneurysm, prompt diagnosis is necessary because once the aneurysm occurs, it can progress rapidly and result in intracranial hemorrhage (Fig. 4.6). When compared to other intracranial aneurysms, infectious aneurysms have a higher mortality rate with a 30% rate in unruptured and 80% in ruptured aneurysms [42, 45, 46]. Imaging can be performed via either CT or MR angiogram or digital subtraction angiography. The limitation of CTA is for very small aneurysms particularly at the skull base .
Middle-aged male on dialysis presents with nonspecific inflammatory mucosal thickening of the left sphenoid sinus (a) on noncontrast CT of the face, which progressed rapidly in size on subsequent imaging (images not available). Patient develops left third and fourth cranial neuropathies and noncontrast brain MR (b) due to renal failure, which shows marked narrowing of the left intracranial internal carotid relative to the right (arrow) with biospy proven fungal sinusitis at the orbital apex (not shown). Emergent intracranial CTA was done (c) and showed a new clinoid left internal carotid artery aneurysm and narrowing (arrow) when compared to prior CTA (not shown). Patient became confused, and noncontrast head CT (d) shows subarachnoid hemorrhage from rupture of the infectious clinoid internal carotid artery aneurysm
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