The equine paranasal sinuses (PNS) are an intricate area of the head. There are 6 paired sinuses (frontal, maxillary, dorsal
conchal, ventral conchal, middle conchal, and sphenopalatine) and all of these spaces communicate with each other and the
nasal passage either directly or indirectly. A thorough understanding of the spatial structural relationships will improve
one's ability to interpret imaging studies and plan the best approach to the PNS. Trephination of the PNS is a basic procedure
that can be readily performed in the field, and familiarity with anatomy is the most important aspect of the procedure. It
is imperative to know what, and where, normal is for the PNS, as often the visual field is one of accumulated fluid (e.g.
blood, purulent material), and abnormal architecture (e.g. presence of a mass).
Frontal (Conchofrontal or Nasofrontal Sinus)
The frontal sinus (FS) and dorsal conchal sinus (DCS) freely communicate with each other and as such are referred to as the
combined conchofrontal sinus (CFS) or nasofrontal sinus (NFS). The FS extends caudally to a transverse plane through the middle
of the zygomatic arch however the caudal surgical limit is at the level of a transverse plane through the zygomatic process
of the frontal bone (or a perpendicular line from the dorsal midline to a point midway between the supraorbital foramen and
the medial canthus of the eye) (Figure 1). The DCS serves as a rostromedial extension of the FS. The rostral boundary of the
DCS, represented by a bony septum, is located about midway between the rostral rim of the orbit and the infraorbital foramen.
Figure 1. Skull showing outline of surgical boundaries of CFS and MS.
In the rostrolateral floor of the FS is the large, 3-4 cm diameter, oval-shaped frontomaxillary opening which provides direct
communication with the caudal maxillary sinus (CMS). Through this opening the tooth roots of the last two molars (10s and
11s) can be identified. The ventral conchal bulla wall and maxillary sinus septum can be penetrated to access the ventral
conchal sinus (VCS) and rostral maxillary sinus (RMS).
The maxillary sinus is partitioned into rostral and caudal regions by a variably located bony septum that extends obliquely
mediocaudodorsad from the lateral wall of the maxilla (Figure 1). Commonly, the septum lies about 5 cm caudal to the rostral
end of the facial crest and passes over the roots of the 1st and/or 2nd molars (maxillary 09, 10) (see later). The septum blends dorsally with the bulla of the VCS. Anatomically the RMS ends as
a small cavity at the level of the rostral end of the facial crest, extending just beyond it in some horses. The CMS extends
caudally, ventromedial to the bony orbit, to a vertical plane through the midpoint of the orbit. In addition to the CFS, the
CMS has direct communication with the sphenopalatine sinus and the middle conchal sinus. The surgical boundaries of the MS
are ventrally, the facial crest, dorsally a line between the medial canthus and the infraorbital foramen, rostrally a line
connecting the infraorbital foramen and the rostral end of the facial crest and caudally a tangential line drawn at the rostral
rim of the orbit between the ventral and dorsal limits of the MS. The lacrimal canal, which protects the initial part of the
nasolacrimal duct (Figure 1), is a bony channel on the internal side of the lacrimal and maxilla bones. The path of the canal
and duct is a line from the medial canthus of the eye toward the nasoincisive notch, running slightly dorsal to the infraorbital
foramen. This structure is preferably avoided when performing trephination or sinusotomy.