Congenital and Inherited Anomalies of the Musculoskeletal System in Dogs and Cats

Dysostoses in dogs and cats are congenital abnormalities that result in abnormal bone development—in most cases, a partial or complete absence (agenesis) of bones. The prevalence of these anomalies in dogs and cats is low, but this may be due to underreporting in the literature.

Therapeutic interventions (ie, surgical and nonsurgical) are reserved for congenital dysostoses that result in mobility impairment; however, treatment success depends on the condition's severity.

Some conditions have hereditary etiology with different modes of inheritance (eg, autosomal dominant).

Diagnosis is based on physical examination and radiographic imaging of the affected limb(s).

Hemimelia

Hemimelia is a developmental abnormality characterized by complete or partial absence of one or more bones in a limb. This condition is further subdivided by the location of the absent bone(s) on the limb. (See photograph and radiograph of hemimelia in dogs.)

Hemimelia, dog

Image

Courtesy of Dr. R.D. Montgomery.

Hemimelia, lateral radiograph of forelimb, dog

Image

Courtesy of Dr. R.D. Montgomery.

Absence of middle and distal bony segments of the limb is referred to as terminal hemimelia. Absence of an intermediate segment of bone with proximal and distal limb segment present is referred to as intercalary hemimelia. Both varieties of hemimelia have been reported in dogs and cats. Additional subdivisions in hemimelia include transverse (complete absence of bone along the width of the limb) and longitudinal (absence of bones along the long axis of the bone, either medially or laterally).

The most common form of hemimelia in dogs and cats is in the radius.

Ectrodactyly

Ectrodactyly is also known as lobster-claw deformity and involves a digital cleft between the metacarpal bones of the fore- or hindlimb. This condition has been reported in both dogs and cats. (See ectrodactyly syndrome image.)

Ectrodactyly syndrome, dog

Image

Courtesy of Dr. R.D. Montgomery.

Pectus excavatum is a congenital abnormality, also known as funnel chest, that has been reported in dogs and cats (3,4).

Affected animals typically have a dorsal deviation of the caudal aspect of the sternum with associated costal cartilages. Dorsoventral flattening of the thorax may be observed in these patients as well.

The compressive impact of this anomaly is associated with secondary changes to the intrathoracic structures that may lead to respiratory and cardiovascular dysfunction. The resulting cardiopulmonary abnormalities may become life-threatening if left untreated.

Predisposed dog breeds include Maltese and brachycephalic dogs (eg, English Bulldogs). Predisposed cat breeds include Burmese and Bengals. The etiology of the condition is not well understood; however, a hereditary component is highly suspected in certain breeds.

Diagnosis is based on palpation of the depression in the sternal area in addition to results of diagnostic imaging (eg, thoracic radiographs demonstrating the defect).

Treatment options in early stages include passing sutures around affected, yet pliable, sternebrae that are affixed to a rigid external splint. The position of the splint ventral to the chest provides an outward traction of the sternebrae, allowing immediate correction. The splint is removed in a few weeks to a month, once the correction is considered irreversible.

In older patients, open surgical techniques with osteotomies and expansion of the thoracic cavity, followed by internal stabilization or external coaptation, have been reported; however, these procedures are associated with higher morbidity and mortality rates.

Pectus carinatum is a rare congenital abnormality of the thoracic wall that manifests as protrusion of the sternum (the opposite of pectus excavatum). This abnormality, also known as pigeon chest, may result in transient respiratory distress and has been reported in dogs.

Also known as atlantoaxial subluxation, the congenital form of atlantoaxial instability is more common than the acquired form in dogs. Toy breed dogs (eg, Yorkshire Terriers, Chihuahuas, Pomeranians) are frequently affected, but the instability can occur in large breed dogs and cats as well.

The instability is the result of one or more abnormalities, such as second cervical (C2) vertebral malformation (aplastic or hypoplastic dens) or absence or laxity of supporting ligaments (ie, transverse or alar ligaments).

In mild cases, the instability may not manifest in clinical signs until a traumatic event. Clinical signs vary in severity, but lesion localization in the neurological examination points to C1-C5 myelopathy, and signs vary from neck pain to ataxia, paresis, and paralysis.

Diagnostic workup often includes radiographs and CT.

Surgical intervention, typically with a ventral approach, is the most common treatment. The prognosis depends on severity of malformations, concurrent spinal cord injury, and surgical techniques used (ie, dorsal versus ventral, implants used for stabilization) (5).

Congenital osteopetrosis is a rare group of heterogenous skeletal disorders called osteopetroses. The congenital form has been reported in both dogs and cats.

A decrease in normal osteoclast-mediated bone resorption results in increased skeletal mass. Animals develop hard but brittle marble-like bones that are prone to pathological fractures with minimal trauma. Variations in mutations and differences at a cellular level exist in different subtypes in animal models of osteopetroses. Affected animals can be prone to anemia that is attributed to limited intramedullary canals in the abnormal bones; animals typically have visible deformities in maxillofacial bones and clublike long bones.

Management strategies include lifestyle modifications to decrease the risk of trauma that can lead to pathological fractures. Fracture repair in affected animals involves a higher risk of causing additional iatrogenic fractures and difficulty in penetrating the dense bone with drill bits. Bone healing after fracture involves less than normal vascularity, and remodeling is not observed.

The prognosis is variable and depends on caregivers’ ability to manage the patient and on the severity of bony malformations that can affect daily function (eg, difficulty in jaw movement).

Osteogenesis imperfecta, an inherited, rare congenital disease, has been reported in several dog breeds (eg, Beagles, Collies, Dachshunds) and in cats.

Gene mutations in type I collagen structure result in excessively fragile and osteopenic bones that are prone to pathological fractures and inefficient bone healing.

The prognosis is poor, and animals that are prone to repetitive, potentially fatal fractures are often euthanized.

Dwarfism is a form of chondrodysplasia that can be either proportionate or disproportionate. The most commonly encountered pathological form of proportionate dwarfism is known as juvenile-onset panhypopituitarism in German Shepherd Dogs but is also reported in other breeds (eg, Spitzes, Miniature Pinschers) (6).

Disproportionate dwarfism is characterized by decreased limb length relative to the trunk and is also referred to as rhizomelic or micromelic dwarfism. Disproportionate dwarfism can be due to selective breeding strategies in which dogs are bred for shorter and smaller limbs (eg, Dachshunds, Welsh Corgis, Pekingese). Disproportionate forms that are considered pathological are reported in Alaskan Malamutes, Norwegian Elkhounds, Great Pyrenees, and Labrador Retrievers (7).  

These musculoskeletal abnormalities can affect one or more developing areas of subarticular cartilage or endochondral ossification centers. Dwarfism can also be associated with other abnormalities such as ocular-skeletal dysplasia in Labrador Retrievers (8) or glycosuria in conjunction with Norwegian Elkhound chondrodysplasia.

The prognosis is dependent on the degree to which musculoskeletal abnormalities cause mobility impairment and the impact of other, concurrent abnormalities.

Mucopolysaccharidoses (MPSs) are a group of lysosomal storage diseases that result in abnormal metabolism mainly of glycosaminoglycans due to abnormal activity of lysosomal enzymes. Multiple variants of these hereditary metabolic diseases, which have different modes of inheritance, have been reported in dogs and cats:

• MPS I (eg, Rat Terrier, domestic shorthair cat)

• MPS II (eg, Labrador Retriever)

• MPS VII (eg, German Shepherd Dogs)

The MPS variants can result in a range of musculoskeletal abnormalities, including disproportionate and decreased growth patterns, decreased mobility, and decreased flexibility, in addition to other systemic abnormalities. Genetic tests for specific MPS variants are becoming more widely available for dogs and cats. The disease is progressive, and animals are often euthanized at a young age.

Congenital shoulder luxation, a nontraumatic condition, has been reported in miniature breeds (eg, Miniature and Toy poodles, Chihuahuas) (9). Abnormal glenohumeral joint formation may be bilateral or unilateral, but it occurs exclusively with medial luxation of the humeral head.

Early diagnosis may allow successful surgical intervention; however, development of osteoarthritis in more severe and chronic cases is inevitable.

Congenital elbow luxation, a nontraumatic hereditary condition, has been reported in small and toy breed dogs (eg, Miniature Poodles, Pekingese, Pugs, and Pomeranians), large breed dogs (eg, Bouviers des Flandres), and cats (10, 11, 12).

Three variations of this condition have been described where elbow luxation is a result of an abnormal proximal radial epiphysis (type I; most common), an ulnar luxation (type II), or luxation of both the radius and ulna (Type III; 13). The condition may occur unilaterally or bilaterally. The resulting elbow abnormality results in mild to severe joint dysfunction and development of osteoarthritis.

Conservative management is reserved for mildly affected cases, whereas open or closed reduction, in addition to surgical stabilization methods, is used for patients with clinical signs. Surgical intervention has a variable reported success rate that depends on severity and chronicity of the case as well as on the surgical technique used.

Also known as swimming puppy, flat-puppy, turtle pup, and twisted legs, swimmer syndrome has been described in newborn dogs and cats; however, its exact etiology remains unknown. Smaller litter size and early weight gain have been suggested as contributing factors in puppies (14, 15, 16, 17).

Clinically, affected animals are unable to stand or walk due to laterally splaying limbs (hindlimbs are more affected); paddling limbs mimic a swimming motion as these animals attempt to move. Affected animals have joint stiffness and hypoplastic limb musculature.

Case reports have described intensive physical rehabilitation and splinting/bandaging of the limbs with some successful longterm outcomes. However, no standardized treatment regimen exists. Most animals are euthanized at an early age due to mobility issues and excessive home-care requirements.

Swimmer syndrome can manifest with other concurrent congenital abnormalities, such as pectus excavatum (15).

Congenital myasthenic syndromes were historically referred to as congenital myasthenia gravis. However, variations of this disease have been identified that are referred to together as CMSs (18). CMSs are divided into three types:

• presynaptic (defect in acetylcholine synthesis)

• synaptic (acetylcholinesterase deficiency)

• postsynaptic (primary deficiency of the acetylcholine receptor)

Breed predisposition to CMSs varies. For example, in dogs, the synaptic type has been reported in Labrador Retrievers and Golden Retrievers; in cats, occurrences in Sphynxes and Devon Rexes have been more commonly reported. The postsynaptic form of CMS is more commonly reported in Springer Spaniels, Jack Russell Terriers, and Smooth Fox Terriers.

CMSs must be distinguished from acquired myasthenia gravis, the more common form. In acquired myasthenia gravis, which is an autoimmune disease, acetylcholine receptors at the neuromuscular junction are decreased due to development of autoantibodies against these receptors.

Clinical manifestation of myasthenia gravis is generalized muscle weakness that affects voluntary (ie, skeletal) muscles. CMSs are typically detected around 5–8 weeks of age, as puppies become more active.

Diagnosis is based on clinical observation of collapse, exercise intolerance, and decreased postural reflexes after short exercise. IV edrophonium chloride injection, resulting in rapid movement, has been used to diagnose the condition. Test injection effects wear off within minutes, but risk of cardiac arrest can be high.

Serum autoantibody testing is not useful because CMSs are not autoimmune diseases. As a result, anticholinesterase treatment is ineffective in most cases, and the prognosis is generally poor.

Myotonia congenita is due to a genetic defect in sodium channel function that results in prolonged muscle contraction or delayed relaxation after voluntary or simulated contractions. This condition has been reported in a variety of dog breeds, including Chow Chows and Miniature Schnauzers, as well as in cats.

Clinical signs manifest when puppies and kittens begin to walk and is associated with a variety of clinical signs including stiffness, bunny-hopping gait, and percussion myotonia. Abnormalities in esophageal function and abnormal bark also may be noted.

Treatment uses antagonists to voltage-gated sodium channels (eg, procainamide). However, responses to treatment and general prognosis are variable.

Glycogen storage disease in dogs and cats is a group of autosomal recessive disorders of glycogen metabolism characterized by excessive accumulation of glycogen in the liver and muscle, the two primary tissues with high glycogen metabolism. Stunted growth and progressive myopathy resulting in muscle weakness have been reported with certain forms of glycogen storage disorders in dogs (eg, type III in German Shepherd Dogs) and cats (eg, type IV in Norwegian Forest Cats) (19, 20).

• What are dog dewclaws? American Kennel Club.

• Find tests by species. UC Davis Veterinary Genetics Laboratory.

1. Kim J, Blevins WE, Breur GJ. Morphological and functional evaluation of a dog with dimelia. Vet Comp Orthop Traumatol. 2006;19(4):255-258. doi:10.1055/s-0038-1633009

2. Towle HA, Blevins WE, Tuer LR, Breur GJ. Syndactyly in a litter of cats. J Small Anim Pract. 2007;48(5):292-296. doi:10.1111/j.1748-5827.2006.00232.x

3. [[Komsta et al 2019]] Komsta R, Osiński Z, Dębiak P, Twardowski P, Lisiak B. Prevalence of pectus excavatum (PE), pectus carinatum (PC), tracheal hypoplasia, thoracic spine deformities and lateral heart displacement in thoracic radiographs of screw-tailed brachycephalic dogs. PloS One. 2019;14(10):e0223642. doi:10.1371/journal.pone.0223642

4. [[Yoon et al 2008]] Yoon HY, Mann FA, Jeong SW. Surgical correction of pectus excavatum in two cats. J Vet Sci. 2008;9(3):335-337, doi:10.4142/jvs.2008.9.3.335

5. Slanina MC. Atlantoaxial Instability. Vet Clin North Am Small Anim Pract. 2016;46(2):265-275. doi:10.1016/j.cvsm.2015.10.005

6. Greco DS. Pediatric endocrinology. Vet Clin North Am Small Anim Pract. 2006;36(3):549-556. doi:10.1016/j.cvsm.2005.12.005.

7. [[Sande, Bingel 1983]] Sande RD, Bingel SA. Animal models of dwarfism. Vet Clin North Am Small Anim Pract. 1983;13(1):71-89, doi:10.1016/s0195-5616(83)50005-3

8. [[Sebbag et al 2023]] Sebbag L, Riggs A, Carnevale J. Oculo-skeletal dysplasia in five Labrador Retrievers. Vet Ophthalmol. 2020;23(2):386-393, doi:10.1111/vop.12715

9. Read RA. Successful treatment of congenital shoulder luxation in a dog by closed pinning. Vet Comp Orthop Traumatol. 1994;7:170-172. doi:10.1055/s-0038-1633091

10. Temwichitr J, Leegwater PA, Auriemma E, et al. Evaluation of radiographic and genetic aspects of hereditary subluxation of the radial head in Bouviers des Flandres. Am J Vet Res. 2010;71(8):884-890, doi:10.2460/ajvr.71.8.884

11. Valastro C, DiBello A, Crovace A. Congenital subluxation in a cat. Vet Rad Ultrasound. 2005;46(1)63-64, doi:10.1111/j.1740-8261.2005.00012.x

12. Verdese P, Manfredini S, Formaggini L. Surgical treatment of congenital radial head luxation in a cat by ostectomy of radial head. Vet Comp Traumatol Open. 2022;5(1):e1-e5, doi:10.1055/s-0042-1742473

13. Kene R, Lee R, Bennett D. The radiological features of congenital elbow luxation/subluxation in the dog. J Small Anim Pract. 2008;23(10):621-630, doi:10.1111/j.1748-5827.1982.tb02055.x

14. Gomes LCVM, Santana VL, Farrias LA, Souza AP. Swimming puppy syndrome in cats. Acta Sci Vet. 2015;43(Suppl 1):109.

15. Nganvongpanit K, Yano T. Prevalence of swimming puppy syndrome in 2,443 puppies during the years 2006-2012 in Thailand. Vet Med Int. 2013;2013:617803. doi:10.1155/2013/617803

16. Tomihari M, Nobutoki Y, Nakajima N, et al. Factors contributing to the swimmer puppy syndrome found in Labrador retrievers. BMC Vet Res. 2022;18(1);120, doi:10.1186/s12917-022-03226-3

17. Verhoeven G, de Rooster H, Risselada M, Wiemer P, Scheire L, van Bree H. Swimmer syndrome in a Devon rex kitten and an English bulldog puppy. J Small Anim Pract. 2006;47(10):615-619, doi:10.1111/j.1748-5827.2006.00069.x

18. Mignan T, Targett M, Lowrie M. Classification of myasthenia gravis and congenital myasthenic syndromes in dogs and cats. J Vet Intern Med. 2020;34(5):1707-1717. doi:10.1111/jvim.15855

19. Brooks ED, Yi H, Austin SL, et al. Natural progression of canine glycogen storage disease type IIIa. Comp Med. 2016;66(1):41-51.

20. Fyfe JC, Giger U, Van Winkle TJ, et al. Glycogen storage disease type IV: inherited deficiency of branching enzyme activity in cats. Pediatr Res. 1992;32(6):719-725. doi:10.1203/00006450-199212000-00020