INFECTIONS OF THE SINUSES AND FACE
The nasal and oral cavities are constantly subjected to a wide variety
of foreign organisms and materials. Allergic sinusitis and acute and chronic
infectious sinusitis are the more common inflammatory diseases of the paranasal
sinuses. In large part they represent a nonspecific inflammatory reaction
of the lining mucous membrane to foreign materials such as proteins, viruses,
and bacteria. In most cases, sinusitis is confined to the sinus cavities.
If a bacterial sinusitis is not treated promptly or if the organism is resistant
to therapy, extension into adjacent compartments can occur. Fungal infections
and granulomatous diseases are more aggressive and often behave like neoplasms.
Traditionally, plain films have been used for screening the paranasal
sinuses. At many centers, CT has replaced plain films for evaluating inflammatory
sinus disease. Contiguous 5 mm sections in the coronal plane are probably
sufficient for screening, but we also routinely include 10 mm axial sections.
MR is reserved for the evaluation of any complications of local sinus infections,
particularly suspected intracranial extension. T2-weighted images with fat
suppression and T1-weighted images effectively evaluate the paranasal sinuses
and adjacent facial compartments. Gadolinium-enhanced T1-weighted images
with fat suppression are helpful for assessing extension to the the skull
base and intracranial cavity.
Allergic rhinitis and sinusitis are general reactions of the upper respiratory tract to a variety of foreign proteins. The inflammatory response results in edema and thickening of the mucous membrane. Usually the margins of the edematous mucosa have a scalloped appearance, but in severe cases mucus may completely fill a sinus, making it difficult to distinguish an allergic process from infectious sinusitis. Characteristically, all of the paranasal sinuses are affected and the adjacent nasal turbinates are swollen. Air-fluid levels and bone erosion are not features of uncomplicated allergic sinusitis, however, swollen mucosa in a poorly draining sinus is more susceptible to secondary bacterial infection.
Anosmia is a common symptom in patients with chronic allergic rhinitis. Membrane thickening and excess secretions in the olfactory recesses (superior nasal cavities) prevents air from reaching the olfactory mucosa.
The osteomeatal complex (OMC) is an important functional unit for drainage of the maxillary sinuses, and the OMC must be carefully evaluated in patients with chronic sinusitis. A number of developmental and acquired conditions can obstruct the OMC. Persistant membrane thickening from allergic rhinitis interferes with normal sinus drainage. Bullous enlargement of an adjacent ethmoid air cell or a Haller cell extending laterally below the orbit can encroach on the infundibulum. Functional endoscopic sinus surgery (FESS) selectively removes some of the bone and soft tissues to open up the OMC.
Chronic allergic sinusitis is often associated with nasal polyposis. Frequently, the polyps arise from the turbinates and bulge into the nasal cavities, where they may completely obstruct the nasal airways. A choanal polyp is a large inflammatory polyp that protrudes posteriorly into the nasopharynx. In more severe cases, involvement of the sinuses can occur, particularly the ethmoid and maxillary sinuses. The ethmoid sinuses may be filled with polypoid tissue, causing destruction of the delicate ethmoid septa and expansion of the ethmoid sinuses into the adjacent orbits.
CT relies on the high contrast between air, soft tissue and bone to evaluate
the paranasal sinuses. Membrane, polyps, and mucous have similar density,
but the polypoid appearance helps distinguish the inflammatory polyps. On
T2-weighted MR images, the edematous membrane and mucous are distinctly
hyperintense, whereas nasal polyps are of more intermediate signal intensity.
Acute infectious sinusitis usually results from extension of organisms from the nasal cavity as a complication of an upper respiratory tract infection. Occasionally, infectious agents enter the maxillary antrum from infected dental roots. In the majority of these infections, pathogens are gram-positive bacteria. The maxillary sinus is most commonly involved, followed in frequency by the frontal, ethmoid, and sphenoid sinuses. Sinus infection results in swelling of the mucous membranes and copious mucoid secretions. In a poorly draining sinus, these secretions are good culture media and can rapidly become purulent.
Acute sinusitis has radiographic features that distinguish it from allergic sinusitis. In acute sinusitis, the thickened mucosal margin is smooth rather than scalloped and follows the contour of the sinus walls. Air-fluid levels are the hallmark of infectious sinusitis. Infections are often confined to a single sinus but may also involve the ipsilateral maxillary, frontal, and ethmoid sinuses owing to some obstructive process in the middle meatus. A pansinusitis usually represents an infectious complication of an allergic sinusitis. Polyp formation is not a feature of infectious sinusitis. In general, bone changes are not seen in acute sinusitis. The delicate ethmoid septa, however, may become indistinct owing to demineralization caused by hyperemia.
Chronic sinusitis is usually of infectious origin and is due either to multiple recurrent infections or to a single prolonged low-grade infection. Long-standing allergic sinusitis can also result in identical radiographic changes. On CT scan the mucosa becomes thickened and appears very dense; the bony walls become sclerotic and thickened and eventually may encroach upon the cavity of the sinus. Chronic sinusitis during childhood may result in hypoplasia of the involved sinus.
Retention cysts are associated with sinusitis, usually becoming apparent after the sinusitis has resolved. These lesions are lined with epithelium and result from obstruction of a mucous gland. Retention cysts occur most commonly in the maxillary sinus and are usually asymptomatic. On both CT and plain films, retention cysts are smooth and broad-based and appear as dome-shaped densities, most often arising from the floor of the maxillary sinus antrum.
When acute infectious sinusitis is properly diagnosed and treated, it
resolves without complication. When infection persists, it may extend to
adjacent compartments of the face. Ethmoid sinusitis is a frequent cause
of orbital cellulitis. An infection may enter the orbit directly through
either foramina, osseous dehiscences, or thin portions of the lamina papyracea.
It may also push the periorbita laterally and thereby bulge into the orbit,
forming a subperiosteal abscess. Infectious involvement of orbital structures
results in lid swelling, proptosis, and scleral thickening with enhancement
on CT scans. Occasionally, orbital emphysema is also present. Edema and
thickening of the extraocular muscles are common. The intraconal extension
of an infectious process can result in retrobulbar neuritis and swelling
of the optic nerve. The superior orbital fissure syndrome sometimes occurs
when such lesions involve the orbital apex. Usually the sixth cranial nerve
is the first to be affected, followed by the third and fourth cranial nerves.
Mucoceles result from obstruction of a sinus ostium. Because of continued secretion of mucus and the resulting increased pressure in the sinus cavity, the walls of the sinus eventually are displaced and encroach upon adjacent facial compartments. Mucoceles are most often associated with chronic infection and allergic sinusitis but may also occur as the result of trauma, previous surgery, or a bone dysplasia. Proptosis is the most common presenting sign of a mucocele. Other clinical features include a mass in the upper medial quadrant of the orbit, pain, vertical diplopia, limited upward gaze, bifrontal headache, and increased tearing. Mucoceles are most commonly found in the frontal and ethmoid sinuses, are infrequent in the sphenoid sinus, and occur rarely in the maxillary sinuses. If a mucocele becomes infected, it is called a mucopyocele.
Mucoceles are recognized on plain films by the presence of an opacified sinus with expansion and thinning of its bony margins. On CT the majority of mucopyoceles are of average density and are homogeneous. Because they are slow growing, these lesions displace and thin the bone around a sinus cavity, but they usually do not cause bone destruction. Occasionally, an aggressive mucocele will cause bone destruction, especially of the orbital wall, and may then simulate a malignant neoplasm. In these cases, CT is particularly helpful to distinguish benign from malignant disease, because an expanding mucocele often preserves a thin fat plane between the margins of the mucocele and the muscle cone.
Mucoceles of the frontal and ethmoid sinuses commonly involve the orbit. In particular, the thin lamina papyracea forming the medial orbital wall offers little resistance to an expanding mucocele. Progressive compression of the orbital contents may lead to visual impairment and, if of sufficient degree, to optic atrophy because of excessive stretching of the optic nerve. As they expand, frontoethmoid mucoceles can produce dehiscence of the posterior wall of the frontal sinus or roof of the ethmoid sinuses and expose the adjacent dura. In these cases, contrast-enhanced CT scans are helpful in defining the position of the dura. Not infrequently, ethmoid mucoceles extend medially into the superior nasal cavity and erode the nasal septum or cribriform plate. Coronal imaging is essential for precise evaluation of these structures. It is also useful for optimal visualization of the roof of the ethmoid sinuses and the ethmoidomaxillary plate and for judging the degree of orbital extension of frontal mucoceles.
The MR signal intensity of the mucous contents of an obstructed sinus
or a mucocele depends on the protein content. Typically, fluid is hypointense
on T1 and hyperintense on T2-weighted images. As the protein content increases,
the T1 relaxation time decreases, increasing the signal intensity on T1-weighted
images. When the protein content approaches 25%, T2 shortening results in
progressively lower signal intensity on T2-weighted images.
The nose is the portal of entry for rhinocerebral mucormycosis, a treacherous and often fatal disease. Hope for a cure rests in early recognition and aggressive therapy. Ninety per cent of such mycotic infections occur in the clinical setting of poorly controlled diabetes mellitus with ketoacidosis. They may also occur in association with other debilitating diseases. These organisms have a special affinity for vascular walls and produce thrombosis that results in ischemic necrosis of involved tissues.
Dark gangrenous nasal mucosa is the classic clinical sign of mucormycosis. This finding is associated with a purulosanguinous exudate and an unpleasant odor. Nasal septal perforation and necrosis of the turbinates are commonly seen. Orbital involvement is indicated by the presence of proptosis, ophthalmoplegia, and even blindness due to thrombosis of the ophthalmic artery. Extension of the infection posteriorly through the superior orbital fissure and optic canal leads to intracranial involvement. A more common pathway for intracranial spread of these infections is through the cribriform plates along the path of small penetrating arteries and veins. Complications of intracranial involvement are many and include increased intracranial pressure, arterial thrombosis and infarction, cerebral abscesses, and occasionally mycotic aneurysm formation.
On plain films mucormycosis is radiographically featured by focal destruction
of the bony walls of the nasal cavity, associated with soft-tissue densities
in the nasal cavity and adjacent ethmoid and maxillary sinuses. On CT and
MRI the inflammatory mass has a homogeneous appearance. Fungal infections
are often hypointense on both T1 and T2-weighted images due to the solid
components of the fungal growth, high protein concentration, and the presence
of manganese, which is paramagnetic. Early orbital involvement may be indicated
by lateral displacement and thickening of the medial rectus muscle. Further
infiltration of the retrobulbar space results in increased density in the
orbital apex and thickening of the optic nerve. Coronal imaging is essential
for evaluating the cribriform plate region. The radiographic findings of
mucormycosis are not always specific, and the process may simulate closely
an aggressive tumor. Whenever a nasal mass is found in a patient who has
poorly controlled diabetes or other chronic debilitating disease, mucormycosis
must be seriously considered.
Aspergillosis is another mycotic infection that may produce clinical
and radiographic findings similar to those of mucormycosis. The Aspergillus
organism is a ubiquitous mold in agricultural dusts; in the immunosuppressed
patient it can become a vicious pathogen. Characteristically, this infection
involves several sinuses but is often unilateral. The pattern is unusual
in that normal aerated cells may be seen among completely opacified cells.
Smooth thickening of the sinus mucous membranes without associated air-fluid
levels is another feature of aspergillosis. Cerebral involvement is more
often due to hematogenous spread, primarily from the lungs, than to direct
extension from the nose and sinuses. The CT and MRI appearance is similar
Granulomatous diseases of the head and neck frequently start in the nasal cavity. The etiology of Wegener's granulomatosis and midline lethal granuloma is unknown, but it seems likely that both represent an immune vasculitis related to polyarteritis nodosa or some other type of autoimmune process. Fortunately, they are rare diseases.
Wegener's granulomatosis consists of a triad of (1) necrotizing granulomas in the respiratory tract, (2) vasculitis of small vessels, and (3) glomerulitis. The clinical course is usually short and ends with renal failure. Early findings include mucosal thickening, ulceration, and sinus opacification. Later, destruction of the bony walls and nasal septum occurs. Perforation of the hard palate can occur and results in an oral-nasal fistula. In more advanced cases, bone destruction can extend into the skull base and may result in cranial nerve palsies.
Midline lethal granuloma is similar to Wegener's granulomatosis but, as its name implies, is a more aggressive process. It is an ulcerative lesion that begins in the nasal cavity and then tends to spread to involve the facial structures symmetrically. It is a slowly progressive gangrenous inflammatory process that ultimately leads to severe mutilation of the face, nose, paranasal sinuses, palate, and pharynx. The margins of this lesion are usually well-demarcated. Usually death results from either sepsis or hemorrhage.
Foreign bodies in the nasal cavities can induce a chronic inflammatory
reaction, with deposition of mineral salts such as calcium and magnesium.
The rock-hard mass that results is called a rhinolith. Usually these are
found in the inferior meatus and, when present over many years, can reach
a very large size.
Infections of Other Facial Compartments
Hypertrophy of the adenoidal tissue along the walls of the nasopharynx is common, and its differentiation from occult nasopharyngeal carcinoma is a continual problem in interpretation of both CT scans and plain films. Lymphoid hyperplasia is confined to the surface of the nasopharynx by the pharyngobasilar fascia, and, as it does not invade other facial planes, the motion of the nasopharynx is normal. Another differential feature is that lymphoid hyperplasia tends to occur along the midline, whereas carcinomas usually arise asymmetrically along one of the lateral walls of the nasopharynx.
The pharyngeal bursa lies at the lower end of the pharyngeal tonsil. The orifice of this structure opens into the pharyngeal mucosa. If the orifice becomes occluded, a cyst may develop (Thornwaldt's cyst). This appears as a sharply defined mass projecting from the midline into the nasopharynx. Such a cyst may become secondarily infected.
A retropharyngeal abscess can occur either anterior or posterior to the prevertebral fascia. An anterior location is much more common and usually results either from a penetrating injury to the posterior pharyngeal wall or from suppuration of retropharyngeal lymph nodes after an acute upper respiratory infection. This type of abscess lies between the prevertebral and pharyngobasilar fascia. The classic symptoms are painful swallowing, dyspnea, and nasal obstruction. The radiographic findings consist of marked widening of the retropharyngeal soft tissue which can compromise the adjacent airway. Small amounts of air are sometimes seen in the area of a foreign body that caused the initial injury. If not properly drained and treated with antibiotics, the infection may extend laterally into the parapharyngeal space. This is a dangerous complication because of the proximity of the carotid artery as well as other vital neural structures.
Abscesses located posterior to the prevertebral fascia are usually the result of a vertebral osteomyelitis. This location should be suspected if a soft-tissue mass is seen in association with erosion of the adjacent vertebral body. Such infections are rare, but when they occur, tuberculosis or a fungal infection such as coccidioidomycosis should be considered.
The vast majority of infections of the oral cavity are superficial and respond readily to appropriate therapy. Occasionally, however, a peritonsillar abscess will occur as a complication of either acute or chronic tonsillitis. Peritonsillar abscesses bulge into the oropharynx and are very painful. If not properly drained and treated with antibiotics, they, like retropharyngeal abscesses, may spread into the adjacent parapharyngeal space and cause damage to either vascular or neural structures. Focal abscesses may also result from a foreign body that has penetrated the oral mucosa.
Inflammatory conditions of the salivary glands usually present with pain
and swelling. The more acute inflammatory conditions are usually associated
with obstruction of a major salivary duct. The more chronic conditions include
recurrent sialadenitis, Sjogren's syndrome, and Mikulicz's disease. Plain
films may be helpful in identifying any radiopaque calculi, but sialography
is usually necessary to identify the specific location of an obstructing
stone or stricture. CT is helpful for evaluation of cases complicated by
either an abscess formation or involvement of adjacent soft-tissue compartments
of the face.
Lanzieri, CF, Paranasal Sinuses and Nasal Cavity, in Edelman
RR, Hesselink JR, Zlatkin MB, Crues JV, eds., Clinical Magnetic Resonance Imaging,
Saunders-Elsevier, Philadelphia, 2006, Chapter 64, pp.2030-47.
Hudgins PA: Sinonasal imaging, Neuroimag Clin North Am, 6:319-332, 1996.