Introduction

The popularity of brachycephalic breeds (French bulldogs, Bulldogs, Pugs, Cavalier King Charles Spaniels, Shih Tzus, etc.) has resulted in the inappropriate selection of hypertypes and extreme modifications of skull morphology. Among these changes, abnormalities in the upper respiratory tract have resulted in an increased frequency of obstructive airway syndromes and severe respiratory failure in these animals.

The clinical examination can be used to assess certain components of the obstructive upper airway syndrome. A CT scan of the head complements this examination by assessing abnormalities in the nasal turbinates, elongation and thickening of the soft palate, enlargement and positioning of the tongue base, nasopharyngeal dimensions, thickening of the soft tissues within the nasopharynx or laryngeal abnormalities. In addition to these specific indications, CT scans are useful to detect many other clinically relevant abnormalities: dental, sinus and mandibular disorders, neurological disorders, trauma, mass investigations or tumour extension assessments, surgical planning, otitis, etc.

A solid understanding of anatomy is essential for interpreting CT images of the head. This module aims to provide students, practitioners, surgeons, and radiologists with detailed information on the normal anatomy of the brachycephalic dog's head.
 

Material and methods

This CT examination of the head was conducted by Dr. Susanne AEB Boroffka, Dipl. ECVDI, PhD (Utrecht, Netherlands), on a brachycephalic dog. The examination extends from the oral region to the second cervical vertebra.

The anatomical structures were labeled by Stephan Mahler, DVM (Veterinary Anatomist, Montpellier, France), following the Nomina Anatomica Veterinaria (NAV).

They have been grouped into different themes:

• General Anatomy
• Regions
• Bone
  • Cranium (General Anatomy)
  • Occipital bone
  • Basisphenoid bone
  • Presphenoid bone
  • Pterygoid bone
  • Temporal bone
  • Parietal bone
  • Frontal bone
  • Ethmoid bone
  • Vomer
  • Nasal bone
  • Lacrimal bone
  • Maxilla
  • Incisive bone
  • Palatine bone
  • Zygomatic bone
  • Mandible
  • Hyoid apparatus
  • Vertebral column
  • Humerus
• Sutures of the head
• Joints
• Muscles
  • Muscles of the head
  • Muscles of the eyeball
  • Masticatory muscle
  • Muscles of neck
  • Muscle of back
  • Fasciae
• Digestive system
  • Oral cavity
  • Teeth
  • Teeth - Numbering
  • Salivary glands
  • Tongue
  • Pharynx (naso-, oro-, laryngopharynx): while from a functional point of view, the nasal part of the pharynx (nasopharynx) is part of the respiratory system, from an anatomical point of view, it is part of the digestive system.
  • Oesophagus
• Respiratory system
  • External nose
  • Nasal cavity
  • Paranasal sinuses
  • Larynx
  • Trachea
• Eye
• Ear
  • External ear
  • Middle ear
  • Inner ear
• Arteries
• Veins
• Lymph nodes
• Nervous system

Results

During the identification of the anatomical structures, the following variations were identified:

• The condylar canal of the occipital bone, which exhibits significant variation between species (and even within the same species), is very short in this dog, reduced almost to a foramen.

Transverse image of the condylar canal of the occipital bone (Brachycephalic dog of the study).	Transverse image of the condylar canal of the occipital bone (Mesocephalic dog).
Sagittal image of the condylar canal of the occipital bone (Brachycephalic dog of the study).	Sagittal image of the condylar canal of the occipital bone (Mesocephalic dog).
Dorsal image of the condylar canal of the occipital bone (Brachycephalic dog of the study).	Dorsal image of the condylar canal of the occipital bone (Mesocephalic dog).

• The left temporal meatus divides proximally into two roughly parallel meatus. These meatus merge distally before opening through the retroarticular foramen.

Sagittal image of the subdivision of the left temporal meatus	Sagittal image of the right temporal meatus

• No caudal alar foramen is present on the right side. The right maxillary artery does not traverse the alar canal to enter the pterygopalatine fossa; instead, it courses ventrolaterally along the basisphenoid bone. 

Transverse image at the level of the left caudal alar foramen.  The right caudal alar foramen is not identified.	Transverse image at the level of the cranial alar foramina. The maxillary artery travels inside the alar canal on the left side and ventrolaterally to the basisphenoid bone on the right side.

• At the base of the skull, between the hypophysial fossa and the right foramen rotundum, an aberrant foramen opens up. This foramen gives rise to a canal: initially oriented ventrally, it turns 90° rostro-laterally and opens at the level of the rostral alar foramen. Blood vessels, probably the emissary vein of the foramen rotundum and/or the middle meningeal vein, travel along the canal.

Transversal image at the level of the foramen rotundum. An aberrant foramen is identified.	Dorsal image at the level of the hypophysial fossa. An aberrant canal is identified.

• The right zygomatic gland extends rostrally beyond the orbital surface, passing dorsally to the zygomatic bone. It also extends rostrally through the infraorbital canal.

Sagittal image of the right zygomatic gland extending rostrally beyond the orbital surface.	Dorsal image of the right zygomatic gland extending rostrally through the infra-orbital canal.

• The transverse foramen of C2 is absent on the right.

Transverse image of the axis. The right transverse foramen is absent.

• An aberrant caudal endoturbinate is visible in the nasal part of the pharynx, on the right. Bilateral aberrant cranial endoturbinates are visible in the middle nasal meatus.

Dorsal image of the nasal part of the pharynx. A right aberrant caudal endoturbinate is visible.	Transverse image of the nasal cavity. Bilateral aberrant cranial endoturbinates are visible.

• The rostral displacement of the tympanic bulla, compared to a mesocephalic dog, is noticeable. This position creates an overlap of the tympanic bulla and the temporomandibular joint, as described by Mielke et al. 2017 in French Bulldogs and Pugs. 

Position of the temporomandibular joint and tympanic bulla as described by Mielke et al. 2017, on a 3D computed tomographic image illustrating skull morphology (Brachycephalic dog of the study).	Position of the temporomandibular joint and tympanic bullas described by Mielke et al. 2017, on a 3D computed tomographic image illustrating skull morphology (Mesocephalic dog).

• Moderate sclerosis of the tympanic bulla’s wall is observed (score 2, according to Mielke and al. 2017).

Transverse image at the level of the tympanic bulla: its wall has a thickness of 2 to 4 mm (Brachycephalic dog of the study).	Transverse image at the level of the tympanic bulla: its wall has a thickness of 1 mm (Mesocephalic dog)

• The cricoid cartilage height-to-width ratio (height: 15.73 mm, width: 9.60 mm, ratio: 1.64) was similar to that reported by Rutherford et al. (2016) in French Bulldogs. This finding suggests a more vertically ovoid cricoid cartilage compared to that of mesocephalic dogs, supporting the presence of airway hypoplasia. Similarly, a relatively small tracheal width was observed, consistent with the findings of Rutherford et al. (2016).

Transverse image of the cricoid cartilage. Internal height and width dimensions are shown.	Transverse image of the trachea at the level of C4. Internal width dimension is shown.

• Teeth 305, 306, 311, 405 and 411 are absent (there is major apical osteolysis and rotations of teeth 110, 210 and 410).
• The different compartments of the frontal sinus (rostral, medial and lateral frontal sinuses) are difficult to differentiate and vary greatly in size.