Equine Sinusitis Study Guide

Overview and Clinical Importance

Equine sinusitis is the most common disease affecting the paranasal sinuses in horses. It represents an inflammatory condition of the sinus mucosa that can be either primary (resulting from upper respiratory tract infection) or secondary (caused by an underlying condition such as dental disease). Understanding the complex anatomy of the equine paranasal sinuses, recognizing clinical presentations, and selecting appropriate diagnostic and treatment modalities are essential skills for the NAVLE examination.

The equine paranasal sinus system consists of seven paired compartments: frontal, dorsal conchal, ventral conchal, rostral maxillary, caudal maxillary, sphenopalatine, and middle conchal (ethmoidal) sinuses. The prevalence of sinusitis is approximately 0.4% based on large population studies, but it remains a clinically significant condition due to its chronic nature and potential treatment challenges.

Anatomy of the Paranasal Sinuses

The equine paranasal sinuses are air-filled cavities within the skull that develop by evagination into the spongy bone between the external and internal plates of cranial and facial bones. Each sinus is lined by respiratory epithelium (pseudostratified ciliated columnar epithelium with goblet cells) and has direct or indirect communication to the nasal cavity.

Sinus Compartments and Communications

The sinus system is divided into two functional groups based on drainage patterns:

Caudal Paranasal Sinus System: This group includes the conchofrontal sinus (frontal + dorsal conchal), caudal maxillary sinus, sphenopalatine sinus, and middle conchal (ethmoidal) sinus. These compartments communicate with each other and drain through the caudal nasomaxillary aperture into the middle nasal meatus.

Rostral Paranasal Sinus System: This group includes the rostral maxillary sinus and ventral conchal sinus. These communicate with each other via the conchomaxillary aperture and drain separately through the rostral nasomaxillary aperture. Importantly, these two systems do NOT communicate with each other, which has significant implications for treatment.

Paranasal Sinus Compartments

Dental-Sinus Anatomical Relationship

The roots of the maxillary cheek teeth (Triadan 08-11) project into the paranasal sinuses. This relationship varies with age as teeth erupt and reserve crowns shorten:

Etiology and Classification

Primary Sinusitis

Primary sinusitis results from bacterial infection of the paranasal sinuses, typically following an upper respiratory tract infection. The infection causes mucosal inflammation, impaired mucociliary clearance, and accumulation of mucopurulent exudate. Can occur in horses of any age.

Common bacterial isolates: Streptococcus equi subspecies zooepidemicus (most common), Streptococcus equi subspecies equi, mixed anaerobes, and occasionally Staphylococcus aureus.

Key feature: Primary sinusitis typically involves ALL sinus compartments and usually responds well to antimicrobial therapy when diagnosed early.

Secondary Sinusitis

Secondary sinusitis is more common than primary sinusitis and results from an underlying condition. Treatment requires addressing the primary cause in addition to treating the sinus infection.

Clinical Signs and Physical Examination

Cardinal Signs of Sinusitis

Unilateral nasal discharge is the hallmark clinical sign of paranasal sinus disease. The character of discharge provides diagnostic clues:

Physical Examination Findings

• Facial asymmetry/swelling: Suggests expansile mass (cyst, neoplasia) or chronic infection with bony remodeling
• Percussion: Dull sound over affected sinus (normally resonant) indicates fluid or mass
• Ipsilateral submandibular lymphadenopathy: Common with active infections (primary, dental, mycotic sinusitis)
• Reduced nasal airflow: Suggests space-occupying lesion (cyst, neoplasia, large ethmoid hematoma)
• Epiphora (tear overflow): May occur with sinus expansion affecting nasolacrimal drainage
• Exophthalmos: Rare; indicates significant mass effect or mycotic/neoplastic invasion

Diagnostic Approach

A systematic diagnostic approach is essential to differentiate primary from secondary sinusitis and identify treatable underlying conditions.

Diagnostic Modalities

Radiographic Technique Tips

• Head positioning: Nose pointed toward ground to visualize fluid lines clearly
• Multiple projections: Lateral, dorsoventral, and oblique views recommended
• Contralateral comparison: Radiograph opposite side to distinguish pathology from normal variation
• Radiopaque markers: Place on facial swelling or in draining tracts to localize pathology

Treatment Options

Medical Management

Primary sinusitis usually responds well to medical therapy when diagnosed early. Treatment goals are to eliminate infection and restore normal mucociliary function.

Antimicrobial Therapy

NSAIDs: Flunixin meglumine (1.1 mg/kg IV/PO q24h) or phenylbutazone (2.2-4.4 mg/kg PO q12h) reduce mucosal swelling and may facilitate sinus drainage. Typically administered for 5-7 days.

Surgical Treatment

Surgical intervention is indicated when medical therapy fails, when inspissated exudate is present, or when secondary causes (dental disease, cyst, hematoma) require treatment.

Trephination and Sinus Lavage

Trephination creates a portal into the affected sinus for lavage, sample collection, sinoscopy, or catheter placement. This procedure can be performed standing under sedation with local anesthesia.

Lavage protocol: Instill 4-5 liters of sterile saline or dilute povidone-iodine solution 2-3 times daily. Continue until outflow is clear. A Foley catheter (24-28 Fr) can be secured in the trephine hole for repeated lavage.

Sinusotomy (Bone Flap)

Indicated when trephination provides inadequate access for mass removal, debridement, or visualization. Can be performed standing or under general anesthesia. Two main approaches:

• Frontonasal bone flap: Access to frontal, dorsal conchal, and caudal maxillary sinuses
• Maxillary bone flap: Access to rostral maxillary, caudal maxillary, and ventral conchal sinuses

Treatment by Specific Condition

Complications and Prognosis

Potential Complications

• Biofilm formation: Bacteria colonizing sinuses form biofilms that are resistant to antibiotics and lavage
• Inspissated exudate: Thick, cheese-like material (chondroids) in ventral conchal sinus; requires surgical removal
• Bone sequestrum: May develop following sinusotomy; prolongs healing
• Sinonasal fistula: Abnormal communication between sinus and nasal cavity
• Bacterial meningitis: Rare but life-threatening complication of sinus/dental surgery
• Recurrence: Particularly with ethmoid hematoma (up to 40%) and if underlying cause not addressed

Prognosis

Horses with primary or secondary sinusitis typically have a good to excellent prognosis for return to athletic performance when the underlying cause is appropriately treated. Cosmetic results of sinus surgery are usually good, with only a small palpable depression at the surgical site. The exception is sinonasal neoplasia, which carries a poor prognosis due to its invasive nature and typically advanced stage at diagnosis.