Anatomy of the Equine Paranasal Sinuses

Caudal Aspect

You need Sinusitis like you need a hole in the head; Nomenclature of the paranasal sinuses

The paranasal sinuses of mammals form complex air-filled structures, and probably exist for a variety of functions including, but not exhaustively: protection of internal cranial structures, maintaining structural strength without a significant increase in weight, augmenting vocalisation, and to increase intranasal cross-sectional area to aid particulate filtration and humidification of inhaled air.

Unfortunately their complexity also leads to disagreement amongst authors regarding the nomenclature as well as the degree of intercommunication present; however, the more common pathological processes are largely confined to the more easily defined (and larger) of the sinuses so in the majority of cases semantics are moot. It does, however, lead to discrepancies in the number of sinuses which are recognised, a fact compounded by some anatomic features unique to equids.

Typically most texts recognise seven paired paranasal sinuses though this is partly coincidental as a result of variations in whether the maxillary sinus is considered as a single structure, and whether the sphenopalatine sinus should be considered as two separate chambers. The former discrepancy becomes more understandable once one considers that the septum between the caudal maxillary sinus and rostral maxillary sinus is peculiar to equids although it is incomplete in donkeys. 1, 2

My interpretation of the anatomy and literature leads me to recognise seven paired paranasal sinuses: the frontal sinus, the rostral maxillary sinus, the caudal maxillary sinus, the sphenopalatine sinus, the dorsal conchal sinus, the ventral conchal sinus and the ethmoidal sinus (also known as the middle conchal sinus).

The “caudal” sinuses

The horse’s nasal cavity is divided longitudinally into three separate meati by two thin scrolls of bone called conchae which extend from the lateral wall of the nasal passage to create the airspaces of the dorsal and ventral conchal sinuses (Figure 1). Whilst conventional sites of access to the paranasal sinuses are generally performed via ostectomy of the maxillary or frontal bones, it is possible to perform direct sinoscopy via the nasal passages through the well vascularised mucosal walls of the nasal conchae.3

Caudal Aspect_ Paranasal Sinuses
Figure 1: Coronal section of a horse’s head at the level of the caudal aspect of PM3

Note the small osseous cups marking the rostral extent of the dorsal (black arrow) and ventral (white arrow) conchal sinuses.

The dorsal conchal sinus (DCS) is formed by the endoturbinate extension of the ethmoid bone, and effectively forms the first ethmoturbinatei.4 It is the largest of the conchal sinuses, and it is continuous with the frontal sinus. Collectively they are often referred to as the conchofrontal sinus.

i Etymologically ethmoturbinate is derived from ethmos, Greek for sieve pertaining to the multiply fenestrated cribriform plate through which the branches of the olfactory nerve pass, and turbinum, Latin for “like a spinning-top”, conical or spiralled (hence turbine). The Latin for sieve is cribrum, hence cribriform.

The paired frontal sinuses are separated in the sagittal plane by a bony septum which extends ventrally as a plate of hyaline cartilage to divide the right and left nasal passages (Figure 2).

       Hyaline Cartilage                         Cartilaginous Strip
Figure 2: Sagittal section of a horse’s head. Upper image: the hyaline cartilage forming the nasal septum remains in place. This section clearly shows part of the ethmoid sinuses. Lower image: the cartilaginous strip has been removed exposing the ventral, middle and dorsal meati. The second endoturbinate is now visible.

The frontal sinuses are roughly triangular in shape and in adults extend caudally to almost the level of the temporomandibular joint. The conchal extension ends rostrally past the suture line conjoining the nasal and incisive bones. The underlying ethmoidal labyrinth forms a spherical upward projection into the caudomedial aspect of the airspace. Just rostral to the medial canthus of the eye, the frontal sinus connects with the caudal maxillary sinus (CMS) through the large fronto-maxillary aperture. The hypothetical junction between these two sinuses is effectively the level of the nasolacrimal duct, which courses from the medial canthus towards the nasoincisive notch. Thus any trephination ventral to this plane will be considered as accessing the caudal maxillary sinus. The presence of the fronto-maxillary aperture also means that, unique amongst the domestic species, the frontal sinus has indirect drainage via the CMS.5

In younger horses the maxillary sinuses are largely filled by the reserve crowns of the caudal four cheek teeth which are embedded in alveolar bone ventral to the infraorbital canal, and therefore the sinus become progressively larger as the horse ages. The CMS extends rostral and ventral to the bony orbit of the eye, and in the horse is separated from the rostral maxillary sinus (RMS) by an oblique septum which is variable in position but usually lies across the periapices of the 1st and 2nd molar teeth (109/209 and 110/210).6-7 The septum projects dorsally and rostrally into the conchofrontal space as a delicate, ossified structure known as the ventral conchal bulla.8-10 Surgical removal of this structure provides direct access into the RMS and ventral conchal sinus (VCS).

The sphenopalatine sinus is extremely inconsistent anatomically, with variations in communication between the component parts and into the other paranasal sinuses which has led to the suggestion that they should be considered as separate sphenoidal and palatine sinuses.11 Typically drainage of the sphenoidal sinus is via the palatine sinus into the caudal maxillary sinus via an aperture caudomedial to the infraorbital canal; however, McCann et al. (2004) also identified drainage may also be via the ethmoidal sinus in 5/16 cases when the two sinuses are distinct and not directly connected. They are positioned ventral to the ethmoturbinate labyrinth and cranium positioning them adjacent to vital structures including the optic chiasma, pituitary gland and other important vascular and neurological tissues. Consequently diseases of this sinus system may lead to severe secondary neurological disease.12

Whilst the first endoturbinate forms the dorsal nasal concha, the second nasal endoturbinate encapsulates the middle nasal concha which is easily visible on nasal endoscopy situated caudal to the sinus drainage angle. In the horse there are six endoturbinates which extend into the nasal cavity together with 25 shorter, more superficial ectoturbinates.13 Together the cavities of the second to sixth endoturbinates form the ethmoidal sinuses. These drain either directly into the CMS, or via the (spheno)palatine sinus.1 Similar to the sphenopalatine sinus, the relationship of the ethmoid sinus and their proximity to the cribriform plate mean that disease processes can lead directly to severe neurological disease and death or as a result of treatment of lesions originating within the ethmoid sinuses.14

In a study of 200 cases of sinus disease recognised pathology of the sphenopalatine and ethmoid sinuses only occasionally (4/52 cases); however, they cautioned that inflammation was not easily recognised radiographically.15 They advised that the effort should be made to assess the sphenopalatine sinus endoscopically, and that ethmoid sinusitis was only identified in cases where gross changes of the second ethmoturbinate were visible on rhinoscopy.

Ultimately these five sinuses share a common drainage from the CMS into the nasal cavity via a tortuous passageway between the rostral edge of the frontomaxillary opening and the ventral conchal bulla through the nasomaxillary aperture.16 Both the RMS and CMS communicate with the middle meatus, although authors disagree regarding whether there is a common ostia, or whether they are separated by a vertical plate continuous with the maxillary septum.5, 17-18 The effect of impeded drainage via this ostium resulting in chronic sequestration of fluids within the sinus has led to balloon catheter sinoplasty aimed to increase the diameter of this aperture with some degree of success.19


The “rostral” sinuses

As already stated the RMS is separated from the CMS by the maxillary septum.

The VCS is formed by a scroll of the maxillary bone, although both conchae attach rostrally to the nasal bone at the level of the second premolar tooth by folds of vascularised membrane.1, 13 The rostral extent of the VCS is continuous with the alar fold.

The RMS is significantly smaller than the CMS from which it is separated by the maxillary septum, and from the conchofrontal sinus by the ventral conchal bulla. The rostral border is usually at the level of the fourth premolar tooth such that the rostral aspect of the 08 is usually embedded within the maxillary bone, with the caudal aspect situated within the sinus in younger horses.20 It communicates directly with the VCS via the conchomaxillary opening situated dorsal to the infraorbital canal which bisects the two compartments. This means the VCS effectively forms the medial compartment of the RMS; however, the VCS extends much further caudally to occupy a position axial to the CMS. Furthermore, this opening may be narrow especially in young horses (1-8mm) and may isolate each compartment compounded by inflammation of the sinus mucosa. 21-22

These two sinuses drain collectively via the nasomaxillary aperture of the RMS, itself positioned rostrally to the drainage path of the CMS. This is often particularly narrow which in addition to the numerous diverticulae and degree of rostral extension allowing sequestration and pooling of secretions means that the VCS is extremely prone to inflammation and infection.23


Standard sites for trephination

Frontal sinus: 60% of the distance from the midline to the medial canthus, and 5mm caudal to the medial canthus 4

Caudal maxillary sinus: 2cm rostral and 2cm ventral to the medial canthus 4

Rostral maxillary sinus: 40% of the distance from the rostral limit of the facial crest to the medial canthus, and 1cm ventral to a line between the infra-orbital foramen and the medial canthus 24


Standard landmarks for bone flaps

Frontonasal flap

Caudal osteotomy: a line perpendicular to the midline extending to a point midway between the supra-orbital foramen and the medial canthus, approximately 2cm medial to the medial canthus

Ventral osteotomy: from the limit of the caudal osteotomy cranially to a point 2/3 of the distance between the medial canthus and the infra-orbital foremen (no alteration was required in those skulls in which a flap was performed to avoid the position of the nasolacrimal duct)

Rostral osteotomy: from the cranial limit of the ventral osteotomy to the midline

Dorsal osteotomy: extending between the rostral and caudal osteotomy approximately 5mm ipsilateral to the median plane.


Maxillary flap

Caudal osteotomy: a line perpendicular to the facial crest extending dorsally to the level of the medial canthus

Dorsal osteotomy: from the caudal osteotomy directly towards the infraorbital canal ending at the rostral limit of the facial crest

Rostral osteotomy: from the dorsal osteotomy to the rostral limit of the facial crest

Ventral osteotomy: just dorsal the facial crest extending between the caudal and rostral osteotomies



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Lizzie Barnard

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