Equine Upper Respiratory Tract - Horse Anatomy

Paranasal Sinuses

Overview

The paranasal sinuses of the horse are extensive, consisting of six pairs:

• Frontal and dorsal conchal sinuses (known as the conchofrontal sinus)
• Ventral conchal sinus
• Sphenopalatine sinus
• Rostral and caudal maxillary sinuses

The most clinically significant sinuses are the frontal and maxillary. The sinuses all communicate with the nasal cavity to allow drainage. The rostral and caudal maxillary sinuses communicate directly with the nasal cavity. The dorsal, middle & ventral conchal, frontal and sphenopalatine sinuses drain indirectly via the maxillary sinuses. The conchal sinuses lie within the fine, scroll-shaped bones known as conchae or turbinates. These conchae are attached to the lateral wall of the nasal passages. The paranasal sinuses are lined with respiratory epithelium (pseudostratified ciliated columnar) and goblet cells.

Frontal Sinus

The frontal sinus occupies the skull from a point midway between the infraorbital foramen and the medial canthus of the eye to a point midway between the caudal edges of the orbit. The frontal sinus is divided into right and left compartments by a midline septum. The conchofrontal sinus is formed by a communication between the rostromedial frontal sinus and the dorsal conchal sinus. The frontomaxillary aperture is a large area of communication between the frontal sinus and the caudal maxillary sinus, this is important to allow drainage. Blood supply to the frontal sinus is provided by the ethmoidal artery.

Conchal Sinuses

The main blood supply is provided by the arterial ethmoid rete, which is an anastamosis between the internal and external ethmoid arteries. A minor suply is provided by the caudal nasal branch of the sphenopalatine artery.

The conchal sinuses include the dorsal, ventral and middle. Each conchus is divided into two compartments, rostral and caudal, by a complete septum.

• Dorsal conchal sinus: This is formed by the caudal compartment of the concha
• Ventral conchal sinus: This is formed by the caudal compartment of the ventral concha
• Middle conchal sinus: Lies within the greater ethmoturbinate, not clinically significant

Maxillary Sinus

The blood supply is provided by branches of the sphenopalatine artery. This is the largest sinus and is divided into rostral and caudal compartments by a bony septum. The position of this septum is variable, but it usually lies obliquely across the roots of the 4th and 5th cheeck teeth (Tridan 109, 110, 209, 210). In horses less than 5 years of age, the reserve crown of the 3rd-6th cheek teeth (Tridan 108, 208, 109-111, 209-211) almost fills the maxillary sinus.

The rostral maxillary sinus opens via the nasomaxillary opening into the middle nasal meatus. There is also a communication between the rostral maxillary sinus and the ventral conchal sinus, via the conchomaxillary opening; located just medial to the infraorbital canal. Dorsally, there is communication with the frontal/conchofrontal sinus through the frontomaxillary opening. Between the rostral margin of the frontomaxillary opening and the conchal bulla, there is a passageway which connects the rostral and caudal compartments. This allows the caudal maxillary sinus to drain via the rostral maxillary sinus via the nasomaxillary opening into the middle nasal meatus.

Guttural Pouches

Equine Guttural Pouch - Copyright David Bainbridge

Also known as: Auditory Tube Diverticulum

Introduction

The guttural pouches are paired ventral diverticulae of the eustachian (auditory) tubes, formed by escape of mucosal lining of the tube through a relatively long ventral slit in the supporting cartilages. The auditory tube connect the nasal cavity and middle ear and the diverticulum dilates to form pouches which can have a capacity of 300-500ml in the domestic horse. The pouches are normally air filled.

Structure

The Guttural Pouch is located below the cranial cavity, towards the caudal end of the skull/wing of atlas. It is covered laterally by the Pterygoid muscles, parotid and mandibular glands. The floor lies mainly on the pharynx and beginning of the Oesophagus. The medial retropharyngeal lymph node lies between the pharynx and ventral wall of the pouches.

Right and left pouches are separated dorsomedially by rectus capitis ventralis and longus capitis muscles. Below this, by fused walls of the two pouches, the median septum is formed.

Each pouch is moulded to the stylohyoid muscle which divides the medial and lateral compartments, the medial compartment being approximately double the size of the lateral one and extends further caudally and ventrally.

The guttural pouch has close association with many major structures including several cranial nerves (glossopharyngeal, vagus, accessory, hypoglossal), the sympathetic trunk and the external and internal carotid arteries. The pouch directly covers the temporohyoid joint. The pouch has an extremely thin wall which is lined by respiratory epithelium which secretes mucus. This normally drains into the pharynx when the horse is grazing.

Several cranial nerves and arteries lie directly against the pouch as they pass to and from foramina in the caudal part of the skull (vessels within mucosal folds that indent the pouches):

Medial Compartment:

Cranial nerves IX, X, XI, XII.

Continuation of the sympathetic trunk beyond the cranial cervical ganglion.

Internal carotid artery.

Lateral Compartment:

Cranial nerve VII - limited contact with the dorsal part of the compartment.

External carotid artery crosses the lateral wall of the lateral compartment in its approach (as maxillary artery) to the atlas canal. The external maxillary vein is also visible.

Drainage:

Natural drainage of the pouch is throught the slit-like (pharyngeal) openings of the eustachian tube in the lateral wall of the nasopharynx. The connection opens when the horse swallows and grazing normally provides drainage. However, most of the pouch is ventral to his slit, and therefore drainage may be rather ineffective. If blocked, secretions accumulate and the pouch distends producing a palpable swelling.

Function

The function of guttural pouches is largely unknown, however hypotheses have been put forward:

1. It may influence internal carotid artery blood pressure. Air pressure varies with phase and forcefulness of respiration and the artery in the mucosal fold is exposed sufficiently to be affected.
2. Cerebral blood cooling mechanism. Operates at times of physical stress/exercise, when core body temperature is raised. Vigorous respiration, cool air in guttural pouches, and the exposed artery all lead to cooling of the blood.

Links

Pathology of guttural pouches

References

Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of Veterinary Anatomy. 3rd ed. Philadelphia: Saunders.