Canine Congenital Spinal Malformations Study Guide

Overview and Clinical Importance

Congenital spinal malformations (CSMs) represent a group of developmental abnormalities affecting the vertebral column that are present at birth. These conditions range from clinically insignificant incidental findings to severe, life-threatening spinal cord compression. Understanding the pathophysiology, breed predispositions, clinical presentation, and treatment options for these conditions is essential for the NAVLE examination and clinical practice.

The major congenital spinal malformations covered in this guide include hemivertebrae, butterfly vertebrae, block vertebrae, transitional vertebrae, atlantoaxial instability (AAI), and cervical spondylomyelopathy (CSM/Wobbler syndrome). These conditions have significant breed predispositions, with brachycephalic screw-tailed breeds and toy breeds being most commonly affected by certain malformations, while large and giant breeds are predisposed to others.

Embryological Basis of Vertebral Development

Understanding vertebral embryology is essential for comprehending the pathogenesis of congenital malformations. Each vertebral body develops from two paired sclerotomal masses that migrate from the somites to surround the notochord. These paired structures normally fuse and ossify during fetal development to form normal vertebrae.

Normal Vertebral Development Process

• Resegmentation: Sclerotomal cells from adjacent somites combine
• Chondrification: Cartilaginous vertebral precursors form
• Ossification: Primary ossification centers develop (one centrum, two neural arch centers)
• Fusion: Complete fusion of ossification centers occurs postnatally

Classification of Congenital Vertebral Malformations

Hemivertebrae

Definition and Pathophysiology

Hemivertebrae are the most common congenital vertebral malformation in dogs. They result from asymmetrical development or failure of one half of the vertebral body to form properly. The resulting wedge-shaped vertebra causes angulation of the spine (kyphosis, lordosis, or scoliosis) depending on the location and orientation of the defect.

Types of Hemivertebrae

• Dorsal hemivertebra: Ventral portion absent, causes kyphosis (most clinically significant)
• Ventral hemivertebra: Dorsal portion absent, causes lordosis
• Lateral hemivertebra: One lateral half absent, causes scoliosis

Breed Predispositions

Clinical Signs

Most hemivertebrae are incidental findings and do not cause clinical signs. However, when clinical signs occur, they typically present:

• Age of onset: Usually less than 1 year of age; signs plateau around 9 months when vertebral growth stops
• Neuroanatomical localization: T3-L3 myelopathy (upper motor neuron signs to pelvic limbs)
• Pelvic limb paresis/paralysis: Progressive weakness, ataxia, inability to walk
• Urinary/fecal incontinence: UMN bladder dysfunction in severe cases
• Spinal pain: Variable; may or may not be present
• Visible spinal deformity: Kyphosis may be palpable or visible

Diagnosis

Treatment

Atlantoaxial Instability (AAI)

Definition and Pathophysiology

Atlantoaxial instability (AAI) is a condition characterized by excessive movement between C1 (atlas) and C2 (axis), resulting in dorsal displacement of the axis and spinal cord compression. The atlantoaxial joint is normally stabilized by the dens (odontoid process) of the axis fitting against the atlas, along with transverse, apical, and alar ligaments.

Causes of Instability

• Dens abnormalities (most common): Aplasia, hypoplasia, or dorsal angulation of the dens
• Ligamentous abnormalities: Aplasia of transverse ligament, weak or absent apical/alar ligaments
• Trauma: Fracture of dens or ligament rupture (can affect any breed/age)

Breed Predispositions

AAI predominantly affects toy and miniature breeds due to congenital abnormalities:

• Yorkshire Terrier - Most commonly affected breed
• Chihuahua
• Toy/Miniature Poodle
• Pomeranian
• Pekingese
• Large breeds (Rottweilers, Dobermans) - less common, usually traumatic

Clinical Signs

• Neck pain: Most common sign; crying when touched, reluctance to move head
• Cervical myelopathy (C1-C5): UMN signs to all four limbs
• Gait abnormalities: Tetraparesis, ataxia, proprioceptive deficits
• Ventroflexion of head: Patient holds head down to minimize pain
• Acute presentation: Can follow minor trauma (jumping, rough play)
• Severe cases: Respiratory paralysis, sudden death

Diagnosis

• Radiography: Increased atlantodental interval; decreased C1-C2 overlap; CAUTION: flexed views may worsen subluxation
• MRI: Spinal cord compression at C1-C2; intramedullary signal changes; syringomyelia
• CT: Best for assessing dens morphology; fractures; surgical planning

Treatment

Cervical Spondylomyelopathy (Wobbler Syndrome)

Definition and Pathophysiology

Cervical spondylomyelopathy (CSM), commonly called "Wobbler syndrome," is characterized by compression of the cervical spinal cord and/or nerve roots secondary to developmental abnormalities and degenerative changes of the cervical vertebrae. It is one of the most common causes of non-infectious spinal cord disease in large and giant breed dogs.

Two Forms of CSM

Clinical Signs

• "Two-engine" gait: Pelvic limbs hypermetric/ataxic, thoracic limbs short/choppy
• Proprioceptive ataxia: Wobbly gait worse on slippery floors; knuckling
• Neck pain: Ventroflexion of head; reluctance to move neck
• Scuffing nails: Worn dorsal nail surfaces from dragging feet
• Difficulty rising: Weakness getting up, may buckle over with front legs
• Progression: Usually chronic and progressive; can have acute episodes

Diagnosis

• MRI (Gold Standard): Best for spinal cord assessment; shows compression, signal changes; no contrast needed
• Dynamic studies: Flexion/extension views show dynamic compression
• CT/Myelography: Alternative if MRI unavailable; good bony detail
• Radiography: Limited value; may show spondylosis, stenosis; cannot confirm diagnosis

Treatment and Prognosis

Other Congenital Vertebral Malformations

Butterfly Vertebrae

Butterfly vertebrae result from persistence of the notochord preventing fusion of the lateral halves of the vertebral body. They appear as a sagittal cleft through the vertebral body with a funnel shape at the ends, resembling a butterfly on VD radiographs. Most are incidental findings and rarely cause clinical signs. They are most common in screw-tailed breeds (French Bulldogs, English Bulldogs, Pugs) and are often found alongside hemivertebrae.

Block Vertebrae

Block vertebrae occur when there is failure of segmentation between adjacent vertebrae, resulting in complete or partial fusion. The intervertebral disc space is absent or reduced. These are usually incidental findings but can cause increased stress on adjacent segments, potentially leading to degenerative changes or instability at neighboring levels.

Transitional Vertebrae

Transitional vertebrae display characteristics of two adjacent spinal segments. The most clinically significant are lumbosacral transitional vertebrae (LSTV), which occur when L7 partially or completely fuses with the sacrum or when S1 is partially separated from the sacrum. German Shepherd Dogs with LSTV are at increased risk for cauda equina syndrome. Pugs have a high prevalence of LSTV (54%).

Neuroanatomical Localization Review

Accurate neuroanatomical localization is critical for identifying the location of spinal cord lesions:

Exam Focus: Most thoracic hemivertebrae cause T3-L3 myelopathy with UMN signs to pelvic limbs and normal thoracic limbs. AAI causes C1-C5 myelopathy with UMN signs to all four limbs. CSM/Wobblers causes variable signs depending on compression level but typically affects cervical segments with the classic "two-engine" gait.

Summary: Key Points for NAVLE

• Hemivertebrae: Screw-tailed breeds (French/English Bulldogs, Pugs), most are incidental, T3-L3 signs if symptomatic, surgery if severe
• Atlantoaxial Instability: Toy breeds (Yorkshire Terrier, Chihuahua), dens abnormalities, C1-C5 signs with neck pain, surgical stabilization preferred
• CSM/Wobblers: Large/giant breeds; Doberman (disc-associated, older) vs Great Dane (osseous, younger), "two-engine" gait, MRI gold standard
• MRI is gold standard: For assessing spinal cord compression and planning treatment
• CT is best: For bony detail and surgical planning
• Radiographs: Screening tool; cannot assess spinal cord; may show incidental findings