MR IMAGING AND CT OF ORBITAL INFECTIONS AND COMPLICATIONS IN ACUTE RHINOSINUSITIS

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The acute onset of periorbital inflammation presents the clinician with a diagnostic and therapeutic dilemma, the consequence of which may be both vision- and life-threatening. The progression from the initial stage of cellulitis to orbital abscess, cavernous sinus thrombosis, intracranial abscess, or meningitis may at times be quite rapid.1, 2 In the preantibiotic era, morbidity and mortality from orbital cellulitis were significant, with 17% of cases resulting in death and 20% resulting in blindness.10 Fortunately, our ability to treat patients with orbital cellulitis has improved. In a study of 22 patients with serious orbital cellulitis there were no deaths, but 4.5% incidence of permanent ocular sequelae.14 The advent of broad-spectrum antibiotics is recognized as the single most important factor responsible for the improvement in the management of orbital cellulitis.

Another important factor is the improvement in orbital imaging. With use of CT, orbital ultrasonography (US), and MR imaging, we are now able to “look into” the orbit, so that accurate characterization and localization of the infectious process are possible. Indeed, it is the information obtained through orbital imaging that directs the clinician to use the correct therapeutic modality. In keeping with the purpose of this issue of the Radiologic Clinics of North America, this article reviews the authors' experience with a series of patients with orbital infections to delineate the clinical and diagnostic imaging features of acute sinogenic inflammatory diseases of the orbit and their complications. It is our hope to improve the understanding of the pathogenesis, diagnostic imaging, correlations of clinical features, and imaging findings of orbital and paranasal sinus infections so that we might further improve our ability to handle orbital infections. Early diagnosis, adequate treatment, and close cooperation between the clinician, ophthalmologist, otolaryngologist, neurosurgeon, radiologist, and other specialists are paramount in order to improve the morbidity and mortality rate related to infections of sino-orbital regions.

Section snippets

IMAGING STRATEGIES

Orbital imaging should be considered in most patients with the acute onset of periorbital inflammation. If the inflammatory process is obviously preseptal and localized to the skin and subcutaneous areas of the eyelid, orbital imaging is unnecessary. Patients with clinical signs suggesting postseptal involvement, however, including visual acuity less than 20/40, proptosis, chemosis, or limitation of ocular motility should be studied. In addition, any patient in whom the diagnosis is uncertain,

PARAMAGNETIC CONTRAST MATERIAL AND THE USE OF FAT-SUPPRESSION TECHNIQUES

Gadolinium enhancement is extremely useful in the diagnosis of certain orbital pathology including orbital infections.29, 31, 35 With fat-suppression techniques, the T1-weighted MR images demonstrate marked expansion of gray scale with apparent increased intensity of the extraocular muscles and lacrimal glands. Therefore, pregadolinium (with or without fat suppression) and postgadolinium fat suppression T1-weighted MR images are very useful to evaluate the presence of the degree of enhancement.

ANATOMY OF PARAORBITAL SINUSES

Between 60% and 80% of inflammatory diseases of the orbit originate in the sinonasal cavities.25, 39 For this reason, a thorough understanding of the embryology and anatomy of the paranasal (paraorbital) sinuses is essential in understanding the pathogenesis of orbital infections.

VENOUS DRAINAGE OF THE PARAORBITAL SINUSES

Venous drainage of the paranasal (paraorbital) sinuses consists of a system of valveless veins that interconnect the sinuses with the orbits, the cavernous sinus, and the facial tissues (see Fig. 4, Fig. 7).15, 19, 22 This freely anastomosing system allows thrombophlebitic phlebitis spread of inflammation and infection between these adjacent structures.19

The superior ophthalmic vein receives venous drainage from the frontal and ethmoid sinuses via the nasofrontal vein, the ethmoidal veins, and

PERIORBITA

The periosteum of the orbit (periorbita) is an important anatomic structure. It represents the only soft tissue barrier between the sinus and orbital contents.8 It is a tough, fibrous membrane that can be easily stripped from the bone except at the suture lines, where it passes through to fuse with the periosteum on the other side.8 The orbital septum or palpebral fascia is a reflection of the periorbita at the anterior margins of the orbit to become continuous with the tarsal plates. The

IMAGING CHARACTERISTICS OF SINUSITIS AND RELATED DISORDERS

Because the majority of cases of bacterial orbital cellulitis are associated with paranasal sinusitis, one should have a basic understanding of the imaging appearance of these disorders. CT findings in acute sinusitis include mucosal thickening; air–fluid level; enhancing pocket(s) with nonenhanced central zone (pus); or complete opacification of the involved sinus. If the disease extends beyond the mucosa, there may be bone rarefaction or erosion suggestive of osteomyelitis. In chronic

BACTERIAL ORBITAL CELLULITIS CLASSIFICATION

Orbital complications of paranasal sinusitis are relatively common in children, but are uncommon in adults. Predisposing factors for the spread of the infection to the orbit include congenital, surgical, or traumatic dehiscence in the common bony walls, the anterior and posterior ethmoid neurovascular foramina, and the valveless sinus and orbital veins and diploic veins of Breschet, which are avenues for septic thrombophlebitic spread of sinus disease to the orbit.29 Based upon the anatomy and

BACTERIAL PRESEPTAL CELLULITIS

Preseptal cellulitis describes infections limited to the skin and subcutaneous tissues of the eyelid anterior to the orbital septum. Clinically, the patient has erythema and swelling of the eyelids. There is no proptosis, chemosis, or limitation of ocular motility. It is unusual for a preseptal infection to traverse the orbital septum and result in postseptal cellulitis (Fig. 9).

Postseptal cellulitis defines an infectious process as one that occurs within the orbit proper, behind the orbital

Inflammatory Edema.

The first manifestation of orbital involvement in patients with paranasal sinusitis is the development of inflammatory edema. Contrast-enhanced study is necessary. The edematous orbital fatty reticulum and adjacent tissues demonstrate some enhancement on postcontrast CT and MR imaging scans (Fig. 10). Generally, the involvement is maximal in the extraconal fat directly adjacent to the most severely affected sinus. Although proptosis is usually present, suggesting more diffuse involvement,

SUBPERIOSTEAL ABSCESS

The next event in the course of the disease is the formation of a subperiosteal abscess (SPA) (Figs. 13 and 14). The process occurs due to spread of the infection through the congenital dehiscenses and foramina of the thin orbital bones as well as thrombophlebitic spread, although the latter is less common. Clinically, the patient has marked swelling and erythema of the eyelids, chemosis, and proptosis. Motility is limited, usually more so when ductions toward SPA are attempted, and visual loss

ORBITAL ABSCESS

The development of a true intraconal orbital abscess secondary to paranasal sinusitis or dental infection is uncommon since the advent of modern antibiotic therapy. More commonly, this occurs secondary to penetrating orbital injury; ocular surgery; or as a metastatic process (septic emboli).26 If orbital abscess is present as a sequela to paranasal sinusitis, progression to this stage can usually be linked to delay in diagnosis and therapy, or an immunocompromised state. Clinically, patients

CAVERNOUS SINUS THROMBOSIS

Cavernous sinus thrombosis results from the spread of infections of sinonasal cavities, orbit, and from infection of the middle third of the face.2, 3 Cavernous sinus thrombosis originates from a septic thrombophlebitis arising in the ophthalmic veins (Fig. 19).2 Proptosis and ophthalmoplegia are common. Offending organisms may be aerobic or anaerobic, with Staphylococcus aureus and anaerobic streptococci being the most common organisms.13 Clinically, patients with cavernous sinus thrombosis

FUNGAL ORBITAL CELLULITIS

Orbital fungal inflammations are often seen in patients with a history of uncontrolled diabetes mellitus and immunocompromised patients, such as those with AIDS. AIDS, caused by infection with HIV, produces profound derangement of the host's immune system. Immunologic abnormalities in AIDS patients include defective cell-mediated immunity with T-cell lymphopenia, functional defect of CD4+ T cells, inadequate antigen-specific antibody response to new antigenic stimuli due to B-cell dysfunction,

DIFFERENTIAL DIAGNOSIS

The differential diagnosis of orbital cellulitis is extensive and includes the following:

  • Inflammatory and allergic

    • Idiopathic orbital inflammation

    • Mucocele

    • Wegener's granulomatosis

    • Angioneurotic edema

  • Congenital

    • Dermoid cyst

    • Encephalocele

    • Infantile cortical hyperostosis

  • Vascular

    • Cavernous sinus thrombosis

    • Cavernous sinus fistula

    • Leukemia

    • Hemophilia (orbital hemorrhage)

  • Traumatic

    • Laceration and hemorrhage of the orbit

    • Orbital wall fracture with orbital hemorrhage

ACKNOWLEDGMENT

The authors express sincere appreciation to Dale Peal and Margie Yates for their secretarial assistance.

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