Anatomy

The paranasal sinuses occupy a large portion of the equine skull. Seven interconnected sinuses are located on each side of the skull. These are air-filled cavities that are lined by a thin mucosal layer that constantly secretes mucus. In the normal horse, mucus drains via a complex system into the nasopharynx from where it is swallowed.

The upper cheek teeth are intimately associated with the sinus cavity and as the horse ages, the teeth grow downwards and away from the sinus cavity further increasing its size. The functions of the paranasal sinuses are not completely understood, however, they are purported to reduce the weight of the skull, act as resonance chambers during phonation and humidify inhaled air.

Sinusitis

Sinusitis is a non-specific term used to describe inflammation of the paranasal sinuses. Sinusitis is divided into two categories depending on the inciting cause of the inflammation.

1. Primary Sinusitis

In the majority of horses that acquire an upper respiratory tract (URT) infection, sinusitis also develops. Infection of the URT results in increased mucus production by the sinus mucosa which itself becomes inflamed and swollen. As the URT infection resolves so does the sinusitis and the owner notes no ill effects. However, in some cases, the swollen sinus mucosa may impair normal mucus drainage into the nasopharynx. This stagnant mucus is prone to colonization by bacteria and the subsequent production of pus, which builds up in the sinus. This is known as a primary sinusitis

2. Secondary Sinusitis

Secondary sinusitis occurs when sinus inflammation occurs subsequent to a disease process other than that which causes a primary sinusitis.

The most common cause of secondary sinusitis is dental disease. As stated, the roots of the upper cheek teeth are intimately in contact with the floor of the sinus cavity separated by only a thin sheet of bone. Infections of tooth roots may result in erosions through this thin bone into the sinus cavity, which is then seeded with bacteria. Other causes of secondary sinusitis include fungal infections, paranasal sinus cysts, sinus tumors, sinus trauma/fractures and chronic periodontal disease that may erode a tract from the mouth into the sinus.

Clinical Signs

The most common sign noted by owners is a nasal discharge. Generally this is from a single nostril and purulent in nature. Occasionally the discharge may be present in both nostrils and blood tinged or malodorous. Other signs include:

  • Enlargement of the lymph nodes under the jaw on the affected side,

  • Swelling or deformity of the facial bones,

  • Obstructed airflow from the nostril,

  • Abnormal respiratory noises,

  • A discharging wound on the side of the face,

  • Malodorous breath or difficulty eating or

  • Head shaking

    • Diagnosis

    Obtaining a detailed history combined with a thorough physical examination including an intra-oral examination is an essential starting point for the veterinary surgeon to establish a tentative diagnosis of sinusitis. However, further diagnostics are required to confirm any initial suspicions and further classify the type of sinusitis present. Passing a flexible camera up the nose (endoscopy) is frequently performed to confirm that the nasal discharge is truly originating from the paranasal sinuses and rule out other sources such as the guttural pouches or lower respiratory tract. Radiographs of the skull also aid in making a diagnosis of sinusitis and may demonstrate classical signs such as fluid lines created by liquid pus (Fig 1) or distinctive intrasinus masses such as sinus cysts or progressive ethmoid haematomas (Fig 2).

    More recently the use of advanced imaging modalities such as magnetic resonance imaging (MRI) computed tomography (CT), scintigraphy (making the horse radioactive) and sinoscopy (placing a small camera through a hole in the forehead) have been employed. This has allowed surgeons to more specifically diagnose the size, location and type of sinusitis, particularly in cases unresponsive to previous therapy and as a result, apply more appropriate and targeted treatments to specific conditions.

    Treatment

    Uncomplicated cases of primary bacterial sinusitis generally respond well to one or two courses of systemic antibiotics. If this treatment fails to resolve the infection and it becomes a recurrent problem, a small hole may need to be created in the forehead to gain access to the sinus cavity.

    A flexible indwelling catheter is passed into the sinus, which is flushed several times daily (Fig 3). This removes any inflammatory exudate such as pus and encourages normal drainage. Approximately 63% of cases will resolve following a single treatment cycle.

    The remainder require either multiple flushing cycles or creation a bone flap over the sinus cavity to allow for direct visualization and manual evacuation of purulent material (Fig 3). With appropriate treatment 94% of primary sinusitis cases achieve long-term resolution.

    Management of secondary sinusitis cases can be more challenging. An accurate diagnosis before the initiation of treatment is essential if a successful outcome is to be achieved. In the case of dental sinusitis, antibiotic therapy is generally the first line treatment for mild cases.

    More advanced or non-responsive cases require either endodontic treatment or tooth extraction. Sinus cysts and tumors require creation of a bone flap into the sinus and surgical removal of as much abnormal tissue as possible in an attempt to achieve resolution. Facial bone fractures are allowed to heal or can be surgically repaired depending on their individual configuration.

    Summarily, early and accurate diagnosis of sinusitis coupled with the appropriate treatment will allow a full recovery in most horses.

    Turlough McNally MVB Dip ACVS Dip ECVS MRCVS is a member of the Veterinary Ireland Equine Group and is a specialist in equine surgery at Anglesey Lodge Equine Hospital, The Curragh, Co Kildare

    Email: hq@vetireland.ie

    Telephone: 01-4577976