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Brachycephalic syndrome
Synonym(s): Brachycephaly
Introduction
- Cause: congenital shortening of skull bones with widening of the maxilla.
- Signs: short and broad facial structure, dental malocclusion, nasolacrimal duct disease, respiratory infections, ear infections.
- Diagnosis: predisposed breed, clinical signs, oral endoscopy, radiography, computed tomography, ultrasonography.
- Treatment: dietary modification, ear cleaning, dental surgery, nasolacrimal duct cannulation.
- Prognosis: good for mild cases with early intervention – poor if advanced disease or severe malformations.
Presenting signs
- Often ‘dwarf’ breeds with short, broad facial structure Dwarf lop:
- Mandibular prognathism/maxillary brachygnathism.
- Many rabbits have concurrent lop-ear deformity.
- Reduced stature, stocky and rounded body structure.
- True dwarf rabbits often <1 kg in size but lop rabbits vary depending on type of lop and crossbreeding.
- Dental disease Teeth: dental disease - overview:
- Primary incisor malocclusion due to maxillary brachygnathism.
- Cheek tooth malocclusion with or without secondary incisor malocclusion Dental malocclusion / overgrowth.
- Tooth root elongation, anorexia Anorexia, ileus Gastrointestinal: ileus.
- Facial, dental and orbital abscessation Abscess.
- Respiratory disease: sneezing, nasal discharge Nose: nasal discharge - overview.
- Ocular disease: dacryocystitis Eye: dacryocystitis.
- Ear disease: otitis externa Otitis externa, otitis media Otitis media/interna, head tilt Heat tilt, neurological signs.
- Dermatological disease:
- Moist dermatitis Moist dermatitis.
- Perineal soiling Sticky bottom syndrome.
- Myiasis Fly strike.
- Cellulitis Cellulitis.
- Neurological disease:
- Head tilt Head tilt.
- Dwarfs have increased susceptibility to E. cuniculi Encephalitozoonosis.
- Reproductive disease:
- Increased neonatal mortality.
- ‘Peanut’ malformation.
Acute presentation
- Severely affected rabbits may have incisor malocclusion and maxillary brachygnathism apparent from birth.
- Lethal dwarf gene in neonates causes fading and death within a few days post-partum.
Geographic incidence
- Worldwide due to owner and breeder preference for animals with juvenile features.
Age predisposition
- Neonates that are homozygous for the dwarf allele have malformations present from birth and deteriorate within a few days.
- Juveniles with severe brachycephaly have advanced craniofacial malformation and typical present with incisor malocclusion and jaw misalignment at birth or within the first few weeks of life.
- Rabbits with mild or moderate brachycephaly develop signs such as dental changes and ocular disease due to the progressive nature of these conditions in rabbits.
- Presentation of disease can be affected by diet as low-fiber (limited grass or hay) diets tend promote earlier onset of disease.
Breed/Species predisposition
- ‘True’ dwarf breeds such as the Polish Polish, and the Netherland dwarf Netherland dwarf.
- Dwarf lops Dwarf lop.
- Lionhead Lionhead and Angora Angora rabbits (dwarfs contributed to breed development).
- Crossbred rabbits often have dwarf ancestry.
Public health considerations
- None reported.
Cost considerations
- Disease associated with brachycephaly is chronic and progressive so lifelong management is required for clinical cases.
- Many diseases require surgical intervention, and this may be recurrent, eg dental malocclusion Dental malocclusion / overgrowth and ear abscessation Abscess.
- Anorexia Anorexia and ileus Gastrointestinal: ileus associated with disease may require hospitalization Hospitalization/nursing care.
Special risks
- Increased anesthesia risk due to:
- Respiratory compromise:
- Rabbits are obligate nasal breathers and the nasal passages may be occluded due to tooth root abscessation or elongation.
- Iatrogenic compression of the nares when a facemask is used in short-faced animals.
- Anorexia Anorexia and ileus Gastrointestinal: ileus:
- Anesthetic agents can further inhibit peristalsis Gastric dilation and stasis.
- Dehydration and electrolyte disturbances can be significant due to sequestration of fluid in the gastrointestinal lumen.
- Prolonged anorexia predisposes to hepatic lipidosis Liver: hepatic lipidosis and can impede metabolization and excretion of drugs.
- Hypothermia Hypothermia: many rabbits with chronic disease are in poor body condition with limited muscle mass.
- Respiratory compromise:
Pathogenesis
Predisposing factors
General
- Failure to clean ears in lop-eared rabbits predisposes to cerumen impaction.
- Muesli-based diets or insufficient roughage Nutrition: dietary requirements predispose to early development of tooth elongation and malocclusion Dental malocclusion / overgrowth.
Specific
- The dwarf allele has been shown to be responsible for the craniofacial changes noted in dwarf breeds:
- 25% heritability estimate.
- Simple autosomal recessive inheritance with incomplete penetration.
- Associated with a deletion mutation in the coding sequence.
- Homozygous dw/dw is lethal:
- ‘Peanut’ deformity with tiny ears, swollen head and markedly reduced birth weights to littermates.
- Death inevitable within a few days following birth due to neurological and gastrointestinal dysfunction.
- Heterozygous Dw/dw:
- Reach 2/3 size of ‘wild-type’ litter mates at birth.
- Typically <1 kg in adulthood but given incomplete penetrance can be larger.
- More compact, rounded body shape with shortened facial structure and wider, broader skull that is disproportionately large.
- Homozygous wild-type rabbits are DW/DW:
- Some dwarf rabbits have this genotype but retain the brachycephalic phenotype.
- Other gene loci are likely to produce dwarf phenotypes alone or in combination with this gene, eg mutations in the HMGA2 transcription regulator gene has also been associated with brachycephalic traits.
Pathophysiology
- Inherited gene mutation (one or more) → altered transcription of genes encoding development of cranium → maxilla develops abnormally → maxillary brachygnathism rather than true mandibular prognathism → dental arcades in abnormal alignment (malocclusion) leading to:
- Insufficient attrition of occlusal surface → spur formation and root elongation with oral soft tissue trauma → progressive worsening of malocclusion → loss of apical bone as roots continue to elongate → secondary tooth root abscessation → jaw osteomyelitis, tooth resorption and tooth loss → Ptyalism → moist dermatitis of mandible and neck.
- Compression of nasolacrimal ducts by elongating tooth roots → obstructed tear drainage → epiphora → infection of duct causes dacryocystitis → moist dermatitis of face.
- Extension of tooth root abscesses into nasal cavity → rhinitis → extension into sinuses →rhinosinusitis with nasal turbinate destruction.
Timecourse
- Progressive and lifelong
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed Papers
- Recent references from PubMed and VetMedResource.
- Johnson J C & Burn C C (2019) Lop-eared rabbits have more aural and dental problems than erect-eared rabbits: a rescue population study. Vet Rec 185 (24), 758 PubMed.
- O’Neill D G, Craven H C, Brodbelt D C et al (2019) Morbidity and mortality of domestic rabbits (Oryctolagus cuniculus) under primary veterinary care in England. Vet Rec. 1-8 PubMed.
- Harvey N D, Oxley J A, Miguel-Pacheco G et al (2019) What makes a rabbit cute? Preference for rabbit faces differs according to skull morphology and demographic factors. Animals 9 (10), 728 PubMed.
- Carneiro M, Hu D, Archer J et al (2017) Dwarfism and altered craniofacial development in rabbits is caused by a 12.1 kb deletion at the HMGA2 locus. Genetics 205 (2), 955-965 PubMed.
- de Matos R, Ruby J, Van Hatten R A & Thompson M (2015) Computed tomographic features of clinical and subclinical middle ear disease in domestic rabbits (Oryctolagus cuniculus): 88 cases (2007-2014). JAVMA 246 (3), 336-43 PubMed.
Other sources of information
- McInnes E F (2011) Background Lesions. In: Laboratory Animals E-Book: A Color Atlas. 1st edn. Elsevier Inc, USA. pp 87-92.
- Vennen K M & Mitchell M (2008) Rabbits. In: Manual of Exotic Pet Practice. 1st edn. Elsevier Inc, USA. pp 375-405.