Genetic Predisposition in Chondrodystrophic Breeds

Intervertebral Disc Disease (IVDD) is one of the most significant spinal disorders affecting dogs, and it is especially relevant for owners of chondrodystrophic breeds such as Dachshunds, French Bulldogs, Beagles, Corgis, and Pekingese. While many owners are familiar with the term “IVDD,” fewer understand that there are two distinct forms of the disease, known as Hansen Type I and Hansen Type II, each with different causes, progression patterns, age of onset, and treatment implications.

Understanding these two types—and the genetic factors that predispose certain breeds—is not just academic. It directly affects early recognition, prevention strategies, treatment decisions, prognosis, and long-term quality of life for affected dogs.

This article provides a detailed, evidence-based explanation of Hansen Type I vs. Hansen Type II IVDD, with a specific focus on genetic predisposition in chondrodystrophic breeds.

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1) What Is Intervertebral Disc Disease (IVDD)?

The canine spine is made up of vertebrae separated by intervertebral discs. These discs act as shock absorbers, allowing flexibility while protecting the spinal cord. Each disc has two main components:

• Nucleus pulposus: the inner, gel-like core that absorbs shock

• Annulus fibrosus: the outer fibrous ring that contains the nucleus

IVDD occurs when these discs degenerate, bulge, or rupture, leading to compression or contusion of the spinal cord or nerve roots. This compression disrupts nerve signaling and can result in:

• Back or neck pain

• Weakness or incoordination

• Difficulty walking

• Paralysis

• Loss of bladder or bowel control

The way a disc degenerates and fails is what distinguishes Hansen Type I from Hansen Type II.

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2) The Hansen Classification System: Why It Matters

The Hansen classification system, developed by Danish veterinarian Dr. Hans-Jørgen Hansen, categorizes disc disease based on pathophysiology and progression rather than just symptoms.

This distinction is critical because:

• The risk profile differs by breed

• The speed of onset varies dramatically

• Treatment urgency and prognosis are different

• Preventive strategies are not identical

In short, not all IVDD is the same disease.

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3) Hansen Type I IVDD: Acute Disc Extrusion

Definition and Pathophysiology

Hansen Type I IVDD is characterized by premature degeneration of the nucleus pulposus, which becomes dehydrated, mineralized, and brittle at an early age. Eventually, the weakened annulus fibrosus ruptures, allowing the nucleus material to extrude explosively into the spinal canal.

This sudden extrusion causes acute spinal cord compression and inflammation, often leading to rapid and severe neurologic deficits.

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Why Chondrodystrophic Breeds Are Predisposed

Chondrodystrophic breeds carry genetic mutations that affect cartilage development throughout the body—including the intervertebral discs. In these dogs:

• The nucleus pulposus loses its gel-like properties very early in life

• Disc degeneration can begin as early as 6–12 months of age

• By adulthood, many discs are already calcified

A major contributor is the FGF4 retrogene insertion on chromosome 12 (and chromosome 18), which has been strongly associated with premature disc degeneration and Hansen Type I IVDD.

This explains why Dachshunds and similar breeds may experience severe disc herniation at a relatively young age, sometimes with minimal trauma.

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Typical Age of Onset

• Most common between 3–7 years of age

• Can occur earlier in high-risk individuals

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Common Clinical Signs

• Sudden yelp or cry of pain

• Reluctance to move, jump, or be handled

• Arched back or neck

• Rapid onset of weakness or paralysis

• Loss of deep pain sensation in severe cases

These signs often appear within hours, not weeks.

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Progression and Urgency

Hansen Type I is considered a neurologic emergency when motor function or deep pain sensation is affected. The severity and speed of onset mean that early intervention can be the difference between recovery and permanent paralysis.

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Treatment Options

Conservative Management
Used in mild cases with pain only or minimal neurologic deficits:

• Strict crate rest (typically 4–6+ weeks)

• Anti-inflammatory medications

• Pain control

• Physical rehabilitation after stabilization

Surgical Intervention
Recommended when there is:

• Loss of motor function

• Progressive neurologic decline

• Loss of deep pain sensation

Procedures such as hemilaminectomy or ventral slot remove extruded disc material and decompress the spinal cord.

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Prognosis

• Excellent to good if treated early and deep pain is preserved

• Guarded if deep pain sensation is lost, though recovery is still possible with rapid surgery

• Recurrence risk exists, especially in chondrodystrophic breeds

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4) Hansen Type II IVDD: Chronic Disc Protrusion

Definition and Pathophysiology

Hansen Type II IVDD involves slow, progressive degeneration of the annulus fibrosus, rather than sudden nucleus extrusion. Over time, the disc bulges or protrudes into the spinal canal, causing gradual spinal cord compression.

Unlike Type I, the nucleus pulposus remains more fibrous and does not explosively herniate.

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Breeds Commonly Affected

Hansen Type II is more common in:

• Large and giant breeds (German Shepherds, Labradors, Dobermans)

• Non-chondrodystrophic dogs

However, chondrodystrophic breeds are not immune, especially as they age.

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Typical Age of Onset

• Usually older dogs, often 8 years and above

• Progression occurs over months to years

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Clinical Signs

• Gradual hind-limb weakness

• Stiffness or reluctance to exercise

• Subtle coordination deficits

• Worsening gait abnormalities

• Rarely sudden paralysis

Pain may be mild or absent compared to Hansen Type I.

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Progression Pattern

Because compression develops slowly, the spinal cord has time to partially adapt. As a result:

• Signs are often subtle early on

• Owners may misattribute symptoms to “aging”

• Diagnosis is frequently delayed

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Treatment Options

Medical Management

• Weight control

• Anti-inflammatory medications

• Physical therapy and rehabilitation

• Activity modification

Surgical Management

• Considered when neurologic deficits significantly impair quality of life

• Often more complex due to chronic compression and spinal cord changes

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Prognosis

• Variable

• Depends on severity, duration of compression, and response to therapy

• Improvement is often slower and less dramatic than in acute Type I cases

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5) Key Differences: Hansen Type I vs. Type II (Quick Comparison)

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6) Genetics and Chondrodystrophy: Why Breed Matters

The Role of FGF4 Retrogene Insertions

Research from UC Davis and other veterinary genetics laboratories has identified FGF4 retrogene insertions as a key genetic driver of:

• Shortened limbs (chondrodystrophy)

• Premature disc degeneration

• Increased risk of Hansen Type I IVDD

Dogs with one or two copies of these insertions have a dramatically higher risk of disc herniation compared to non-carriers.

This explains:

• Why Dachshunds can herniate discs with minimal trauma

• Why multiple discs may be affected over a lifetime

• Why prevention strategies must start early

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Implications for Breeders and Owners

• Genetic testing can identify risk status

• Breeding decisions can reduce population-level disease burden

• Owners of high-risk dogs should focus on prevention and early detection, not just treatment

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7) Prevention and Risk Reduction in Chondrodystrophic Breeds

While genetics cannot be changed, environmental and lifestyle factors strongly influence disease expression.

Key strategies include:

• Maintaining ideal body weight

• Avoiding jumping on/off furniture

• Using ramps instead of stairs

• Harness use instead of neck collars

• Core-strengthening and controlled exercise

• Early evaluation of subtle gait or posture changes

Preventive management is especially critical for dogs predisposed to Hansen Type I.

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8) Why Early Recognition Saves Mobility

In chondrodystrophic breeds, the difference between:

• Pain with walking
  and

• Permanent paralysis

can be measured in hours to days, particularly for Hansen Type I IVDD.

Owners who understand:

• The difference between acute and chronic disc disease

• The genetic risks of their breed

• The red-flag signs that require emergency care

are far better positioned to protect their dog’s long-term quality of life.

Sources & Further Reading

• American College of Veterinary Surgeons (ACVS). Intervertebral Disc Disease in Dogs

• Cornell University College of Veterinary Medicine. IVDD Overview and Treatment

• UC Davis Veterinary Genetics Laboratory. FGF4 Retrogene and IVDD

• Brisson BA. Intervertebral disc disease in dogs. Veterinary Clinics of North America: Small Animal Practice

• Dewey CW, da Costa RC. Practical Guide to Canine and Feline Neurology

• Veterinary Partner / VIN. Hansen Type I vs. Type II Disc Disease

• University of Missouri / OFA Canine Health Information Center