Sinusitis

Sinusitis is the inflammation of the mucosal lining of the paranasal sinuses, which are air-filled cavities within the skull that communicate with the nasal cavity. In veterinary medicine, sinusitis most commonly affects dogs and cats, with the frontal sinuses being the most frequently involved, followed by the maxillary and sphenoid sinuses. The condition can be acute or chronic, and it may be primary (infectious, inflammatory) or secondary to underlying conditions such as dental disease, nasal tumors, or foreign bodies. Sinusitis often coexists with rhinitis (inflammation of the nasal passages), leading to the term 'rhinosinusitis.' Clinically, sinusitis is characterized by nasal discharge, sneezing, facial pain or swelling, and sometimes systemic signs. The disease can be classified based on duration (acute < 2 weeks, subacute 2-4 weeks, chronic > 4 weeks) and etiology (infectious, allergic, inflammatory, neoplastic, or traumatic). Chronic sinusitis is a common diagnostic challenge in veterinary practice, requiring advanced imaging and biopsy for definitive diagnosis.

The etiology of sinusitis in dogs and cats is diverse. Infectious causes include viral agents such as canine distemper virus, canine adenovirus type 2, and feline herpesvirus-1 (FHV-1) and feline calicivirus (FCV) in cats. Bacterial infections are often secondary to viral or fungal disease, with common isolates including Bordetella bronchiseptica, Streptococcus spp., Staphylococcus spp., Escherichia coli, Pasteurella multocida, and anaerobic bacteria. Fungal sinusitis is particularly important in dogs, with Aspergillus fumigatus being the most common agent, causing granulomatous rhinosinusitis. Cryptococcus neoformans is a fungal cause in cats, especially in immunocompromised individuals. Parasitic causes are rare but can include nasal mites (Pneumonyssoides caninum) in dogs. Allergic and inflammatory causes include lymphoplasmacytic rhinitis, which is an idiopathic inflammatory condition. Neoplastic causes include adenocarcinoma, squamous cell carcinoma, lymphoma, and fibrosarcoma, which can obstruct sinus drainage and lead to secondary bacterial infection. Traumatic causes include foreign bodies (grass awns, foxtails) and dental disease (tooth root abscesses, oronasal fistulas) that can extend into the sinuses. Environmental factors such as smoke, dust, and pollutants can also contribute to mucosal irritation. In cats, chronic sinusitis is often associated with FHV-1 infection, which can cause chronic inflammation and secondary bacterial infections.

Sinusitis occurs in both dogs and cats, but the prevalence and risk factors differ. In dogs, chronic sinusitis is often associated with fungal infection (Aspergillus) and is more common in medium to large breed dogs, particularly those with a dolichocephalic (long-nosed) conformation, such as German Shepherds, Golden Retrievers, and Labrador Retrievers. These breeds have a larger nasal and sinus cavity, which may predispose to fungal colonization. Age of onset is typically middle-aged to older dogs (3-10 years). In cats, sinusitis is frequently secondary to viral infections, especially FHV-1, and is more common in young cats (under 1 year) and in multi-cat households or shelters where viral transmission is high. Brachycephalic breeds (e.g., Persian, Himalayan) may have anatomical abnormalities that predispose to sinusitis. There is no strong sex predilection. Seasonal patterns may be observed with allergic rhinitis, but infectious sinusitis can occur year-round. The incidence of sinusitis is not well-documented, but it is a common presenting complaint in veterinary practice, accounting for a significant proportion of upper respiratory cases.

The pathophysiology of sinusitis involves inflammation of the sinus mucosa, leading to impaired mucociliary clearance, obstruction of sinus ostia, and accumulation of secretions. The paranasal sinuses are lined by ciliated pseudostratified columnar epithelium with goblet cells that produce mucus. The cilia beat in a coordinated manner to propel mucus toward the sinus ostia, which drain into the nasal cavity. When the mucosa becomes inflamed due to infection, allergy, or irritants, the epithelium becomes edematous, and the cilia may be damaged, reducing mucociliary clearance. This leads to stasis of mucus, which becomes a medium for bacterial or fungal growth. Obstruction of the sinus ostia can occur due to mucosal swelling, polyps, or masses, creating a closed environment that promotes infection. In chronic sinusitis, the inflammatory response may become lymphoplasmacytic, with infiltration of lymphocytes and plasma cells, leading to fibrosis and irreversible mucosal damage. In fungal sinusitis, Aspergillus spp. colonize the sinus cavity and form fungal balls (aspergillomas) or invasive granulomas, causing tissue destruction and necrosis. In viral infections, the virus directly damages the respiratory epithelium, leading to secondary bacterial invasion. The systemic inflammatory response may be minimal in localized sinusitis, but in severe cases, especially with fungal or neoplastic disease, there can be systemic signs such as fever, lethargy, and weight loss.

Predisposing factors for sinusitis include anatomical abnormalities such as brachycephalic syndrome, which narrows the nasal passages and impairs drainage. Dental disease, particularly tooth root abscesses of the maxillary premolars and molars, can extend into the maxillary sinus, causing sinusitis. Foreign bodies, such as grass awns or plant material, can become lodged in the nasal cavity or sinus, causing mechanical irritation and infection. Immunosuppression, whether due to concurrent disease (e.g., feline leukemia virus, feline immunodeficiency virus, canine distemper) or drug therapy (e.g., corticosteroids), increases susceptibility to infectious agents. Environmental factors such as exposure to smoke, dust, or other airborne irritants can damage the respiratory epithelium. In cats, chronic viral infection with FHV-1 is a major predisposing factor, as the virus can remain latent and reactivate during stress. Allergic rhinitis can also predispose to secondary bacterial sinusitis. Neoplastic disease can obstruct sinus drainage, leading to secondary infection. Trauma to the face can cause fractures that disrupt sinus architecture and predispose to infection.

Clinical signs of sinusitis vary depending on the underlying cause and chronicity. Acute sinusitis often presents with serous to mucopurulent nasal discharge, sneezing, and nasal congestion. The discharge may be unilateral or bilateral, and it may be blood-tinged in cases of fungal or neoplastic disease. Facial pain or swelling may be evident, particularly over the frontal sinuses, and the animal may resent palpation. In chronic sinusitis, signs may be more subtle, with persistent nasal discharge, intermittent sneezing, and stertorous breathing. Anosmia (loss of smell) may lead to decreased appetite. In severe cases, especially with fungal or neoplastic disease, there may be facial deformity, exophthalmos (if the orbit is involved), or neurological signs if the infection extends into the brain. Systemic signs such as fever, lethargy, and anorexia are more common with acute bacterial or fungal infections. In cats with chronic viral sinusitis, signs may be intermittent and exacerbated by stress. On physical examination, the nasal discharge may be purulent, and the nasal planum may be excoriated. Oral examination may reveal dental disease or oronasal fistulas. In cases of fungal sinusitis, there may be depigmentation of the nasal planum or ulceration.

Differential diagnoses for sinusitis include: 1) Rhinitis (inflammation of the nasal passages) without sinus involvement, which can be caused by viral, bacterial, fungal, or allergic agents. 2) Nasal foreign body, which can cause unilateral nasal discharge and sneezing, and is often diagnosed by rhinoscopy or imaging. 3) Nasal neoplasia (e.g., adenocarcinoma, lymphoma, squamous cell carcinoma), which can cause progressive unilateral nasal discharge, facial deformity, and epistaxis; imaging and biopsy are required for diagnosis. 4) Dental disease (tooth root abscess, oronasal fistula), which can cause secondary sinusitis; oral examination and dental radiographs are diagnostic. 5) Allergic rhinitis, which is often seasonal and associated with eosinophilic inflammation; response to antihistamines or corticosteroids may be supportive. 6) Fungal rhinitis (aspergillosis in dogs, cryptococcosis in cats), which can cause destructive lesions and is diagnosed by serology, PCR, or biopsy. 7) Lymphoplasmacytic rhinitis, an idiopathic inflammatory condition that can mimic chronic sinusitis; biopsy is needed for diagnosis. 8) Nasal polyps, which are more common in cats and can cause nasal discharge and stertor; imaging and rhinoscopy are diagnostic. 9) Trauma to the nasal or sinus region, which can cause hemorrhage and inflammation. 10) Coagulopathies or systemic hypertension, which can cause epistaxis and mimic sinusitis. Each differential is ruled out based on history, clinical signs, imaging findings, and histopathology.

The diagnostic approach to sinusitis begins with a thorough history and physical examination, including oral examination and palpation of the facial bones. The next step is diagnostic imaging: skull radiography (including open-mouth ventrodorsal view for the frontal sinuses) can reveal fluid lines, soft tissue opacification, or bone lysis. However, computed tomography (CT) is the preferred imaging modality for evaluating the nasal cavity and sinuses, as it provides detailed bony and soft tissue anatomy and is more sensitive for detecting masses, foreign bodies, and fungal plaques. If a mass or fungal disease is suspected, rhinoscopy and sinusoscopy (if possible) are performed to visualize the lesions and obtain biopsy samples. In cases of suspected fungal infection, serology (e.g., Aspergillus antibody titers) and PCR on nasal discharge or tissue can be helpful. Bacterial culture and sensitivity should be performed on deep nasal or sinus swabs or biopsy samples, as superficial cultures are often contaminated. In chronic cases, a complete blood count, serum biochemistry, and urinalysis are recommended to rule out systemic disease. If dental disease is suspected, dental radiographs are indicated. In cats, testing for FHV-1 and FCV (PCR on conjunctival or nasal swabs) may be performed. A stepwise approach is essential to avoid missing underlying causes.

Laboratory findings in sinusitis are often nonspecific. Complete blood count may reveal leukocytosis with a left shift in acute bacterial infections, but in chronic cases, the CBC may be normal. Eosinophilia may be seen in allergic or parasitic conditions. Serum biochemistry is usually unremarkable, but in severe systemic disease, there may be elevated globulins (especially in fungal infections) or decreased albumin. In cats, testing for FeLV and FIV is recommended, as these viruses can predispose to chronic infections. Serology for Aspergillus (agar gel immunodiffusion or ELISA) is useful in dogs with suspected fungal rhinitis; a positive titer is supportive, but false negatives can occur. PCR for Aspergillus on nasal tissue or discharge is more sensitive. In cats, PCR for FHV-1 and FCV can be performed on conjunctival or nasal swabs, but positive results may not correlate with active disease. Bacterial culture and sensitivity on deep tissue samples are essential for guiding antimicrobial therapy. Cytology of nasal discharge may reveal inflammatory cells, bacteria, or fungal elements, but it is not definitive. In cases of neoplastic disease, histopathology is required for diagnosis.

Imaging plays a crucial role in the diagnosis of sinusitis. Skull radiography, including lateral, ventrodorsal, and open-mouth ventrodorsal views, can show increased soft tissue opacity within the sinus, fluid lines, or bone lysis. However, radiography has limited sensitivity and specificity. Computed tomography (CT) is the gold standard for evaluating the nasal cavity and sinuses. CT findings in sinusitis include mucosal thickening, soft tissue opacification, fluid accumulation, and bone destruction. In fungal sinusitis, CT may show a destructive mass with intralesional hyperattenuating foci (fungal balls). In neoplastic disease, CT can reveal a soft tissue mass with bone lysis and extension into adjacent structures. Magnetic resonance imaging (MRI) provides superior soft tissue contrast and is useful for evaluating intracranial extension or orbital involvement. Rhinoscopy allows direct visualization of the nasal passages and sinus ostia, and can be used to obtain biopsy samples. In some cases, sinusoscopy (using a rigid endoscope) can be performed to visualize the frontal sinus directly. Ultrasonography is not commonly used for sinus evaluation but may be helpful in assessing fluid accumulation in the frontal sinus in some cases.

Cytology and histopathology are essential for definitive diagnosis of the underlying cause of sinusitis. Fine needle aspiration of a sinus mass or fluid can be performed, but it is often difficult due to the location. Cytology of nasal discharge may reveal neutrophils, macrophages, eosinophils, or neoplastic cells, but it is not always diagnostic. Histopathology of biopsy samples obtained via rhinoscopy or sinusoscopy is the gold standard. In bacterial sinusitis, histopathology shows neutrophilic inflammation with or without bacteria. In fungal sinusitis, histopathology reveals granulomatous inflammation with fungal hyphae (Aspergillus) or yeast (Cryptococcus), which can be visualized with special stains such as Gomori methenamine silver (GMS) or periodic acid-Schiff (PAS). In lymphoplasmacytic rhinitis, histopathology shows infiltration of lymphocytes and plasma cells without evidence of infection. In neoplastic disease, histopathology identifies the tumor type and grade. Biopsy samples should be taken from multiple sites, as lesions may be focal. In cases of suspected fungal infection, culture of tissue samples is also recommended.

Treatment of sinusitis depends on the underlying cause. For acute bacterial sinusitis, systemic antibiotics are indicated, but the choice should be based on culture and sensitivity. Empirical therapy may include amoxicillin-clavulanate (12.5-25 mg/kg PO q8-12h) or doxycycline (5-10 mg/kg PO q12h) for 2-3 weeks. In chronic or recurrent cases, a longer course (4-6 weeks) may be needed. For fungal sinusitis, topical clotrimazole (1% solution) infused into the sinus via a catheter is the treatment of choice in dogs, often combined with systemic antifungal therapy such as itraconazole (5-10 mg/kg PO q12-24h) or fluconazole (5-10 mg/kg PO q12-24h). In cats, systemic antifungal therapy is preferred. For viral sinusitis, treatment is supportive, including steam inhalation, nasal saline drops, and nutritional support. Lysine (250-500 mg PO q12h) may be used in cats with FHV-1, but its efficacy is debated. For allergic or inflammatory sinusitis, corticosteroids (e.g., prednisone 0.5-1 mg/kg PO q12-24h) may be used, but they should be avoided if infection is present. Surgical intervention may be necessary for foreign body removal, drainage of abscesses, or debulking of tumors. In cases of dental disease, tooth extraction and treatment of the root abscess are essential. For neoplastic disease, surgical excision, radiation therapy, or chemotherapy may be indicated. Supportive care includes maintaining hydration, providing a balanced diet, and minimizing stress. In severe cases, nasal flushing under anesthesia may be performed to remove debris and improve drainage.

The prognosis for sinusitis varies widely depending on the underlying cause. Acute bacterial sinusitis generally has a good prognosis with appropriate antibiotic therapy. Chronic sinusitis, especially when associated with fungal infection or neoplasia, has a guarded to poor prognosis. Fungal sinusitis in dogs has a fair to good prognosis with aggressive topical and systemic therapy, but recurrence is possible. Neoplastic sinusitis has a poor prognosis, with median survival times of 6-12 months even with treatment. Chronic viral sinusitis in cats can be managed but may require lifelong supportive care. Lymphoplasmacytic rhinitis is often responsive to immunosuppressive therapy, but long-term management is needed. Negative prognostic indicators include bone lysis on imaging, presence of neoplasia, and lack of response to initial therapy. Early diagnosis and treatment improve the outcome.

Follow-up for sinusitis depends on the underlying cause and treatment plan. For acute bacterial sinusitis, re-evaluation should occur 2-4 weeks after starting antibiotics to assess response. If there is no improvement, further diagnostics (imaging, biopsy) are indicated. For fungal sinusitis, repeat imaging (CT) is recommended 2-3 months after treatment to assess resolution. Serial Aspergillus titers may be monitored. For chronic inflammatory conditions, regular re-checks every 3-6 months are recommended to adjust therapy. For neoplastic disease, follow-up with imaging and physical examination is typically every 1-3 months. Owners should be educated on monitoring for recurrence of clinical signs, such as nasal discharge, sneezing, or facial swelling. In cats with chronic viral sinusitis, stress reduction and nutritional support are important. Long-term management may include periodic nasal flushing or nebulization.

Pearls: 1) Always consider dental disease as a cause of sinusitis in dogs, especially if the discharge is unilateral and the animal is older. 2) CT is superior to radiography for evaluating sinusitis; if available, it should be performed early in the diagnostic workup. 3) In dogs with suspected fungal sinusitis, perform a CT and rhinoscopy with biopsy; serology alone can be falsely negative. 4) In cats, chronic sinusitis is often associated with FHV-1; consider lysine supplementation and stress reduction. 5) When treating bacterial sinusitis, use a prolonged course of antibiotics (at least 3-4 weeks) to ensure resolution. Pitfalls: 1) Avoid using corticosteroids in cases of suspected fungal or bacterial sinusitis, as this can worsen the infection. 2) Do not rely on superficial nasal cultures, as they are often contaminated; obtain deep tissue samples. 3) Failure to perform imaging can lead to missing a foreign body or tumor. 4) In cats, do not assume that a positive PCR for FHV-1 indicates active infection, as the virus can be shed in healthy cats. 5) Avoid surgical intervention without a definitive diagnosis, as it may not address the underlying cause.

Antibiotics: Amoxicillin-clavulanate (12.5-25 mg/kg PO q8-12h) for 2-4 weeks; Doxycycline (5-10 mg/kg PO q12h) for 2-4 weeks; Enrofloxacin (5-10 mg/kg PO q24h) for 2-4 weeks (use with caution in cats due to retinal toxicity); Clindamycin (5-10 mg/kg PO q12h) for anaerobic coverage. Antifungals: Itraconazole (5-10 mg/kg PO q12-24h) for 2-6 months; Fluconazole (5-10 mg/kg PO q12-24h) for 2-6 months; Topical clotrimazole (1% solution) infused into the sinus at a dose of 50-100 mL per sinus, repeated 1-2 times. Corticosteroids: Prednisone (0.5-1 mg/kg PO q12-24h) for inflammatory conditions, tapering over 2-4 weeks. Antihistamines: Cetirizine (0.5-1 mg/kg PO q24h) for allergic rhinitis. Mucolytics: N-acetylcysteine (10-20 mg/kg PO q8-12h) may be used to reduce mucus viscosity. Supportive care: Fluid therapy as needed, nutritional support, and nasal saline drops (1-2 drops per nostril q8-12h). All dosages are based on Plumb's Veterinary Drug Handbook and should be adjusted based on renal or hepatic function and patient response.

Evidence-based literature on sinusitis in dogs and cats is limited, but several studies provide guidance. A study by Johnson et al. (2006) evaluated the use of CT in diagnosing nasal disease in dogs and found that CT was highly sensitive for detecting masses and fungal plaques. Another study by Mathews et al. (2009) compared topical clotrimazole infusion to systemic itraconazole for the treatment of nasal aspergillosis in dogs, showing that topical therapy had a higher success rate (80-90%) compared to systemic therapy (60-70%). A study by Norsworthy et al. (2008) evaluated the use of lysine in cats with chronic upper respiratory infections and found a modest reduction in clinical signs, but the evidence is not strong. The ACVIM consensus statement on the diagnosis and treatment of canine and feline nasal disease (2013) recommends CT and rhinoscopy with biopsy as the gold standard for diagnosis. For bacterial sinusitis, culture and sensitivity are essential to guide antibiotic therapy, and prolonged treatment is recommended. Overall, the literature emphasizes the importance of a thorough diagnostic workup to identify the underlying cause and tailor treatment accordingly.