Sidewinder Syndrome (Crab Walking) in Horses: Causes, Risk Factors & Severity

Sidewinder Syndrome
Causes
Risk Factors
Symptoms
Severity
Diagnosis
Treatment
Prognosis

Sidewinder syndrome, also known as sidewinder gait or crab walking, is one of the more perplexing and challenging equine health conditions for horse owners and veterinarians alike.

This syndrome is characterized by a sideways drift of the horse’s trunk and hind limbs while moving forward, resulting in three distinct hoof tracks instead of the usual two.

Affected horses exhibit a range of symptoms from awkward movement and difficulty standing still to severe cases where the horse cannot walk forward at all, often spinning or circling in place.

Understanding the anatomy of a horse and the potential musculoskeletal and neurological causes of Sidewinder syndrome is important for diagnosis and treatment, although the prognosis remains poor.

If your horse’s movement seems abnormal, it’s best to contact your veterinarian promptly, as early intervention provides the best chance of a positive outcome.

Read on to learn more about the anatomy involved, risk factors, causes, symptoms, and diagnostic challenges of equine Sidewinder Syndrome.

What is Sidewinder Syndrome?

Sidewinder syndrome (also known as sidewinder gait, side walking, and crab walking) is a condition in which the horse’s trunk and hindlimbs drift to one side while they are walking forwards. ^[1]^[2]

Horses with this condition walk on three tracks meaning that the hind feet do not follow directly in the tracks of the front feet as normal. This results in three distinct lines of hoof prints instead of the usual two. ^[2] The action of the body is variously described as leaning, swaying, drifting, and pivoting. ^[1]

Horses with sidewinder syndrome also have difficulty standing still or loading their weight symmetrically on their pelvic (hind) limbs. ^[1] Horses with severe symptoms are not able to walk forward at all and tend to spin or circle in one spot. ^[1]^[2]

The Equine Skeleton and Neurological Control

Sidewinder syndrome has a variety of causes that relate to several parts of the horse’s body. To understand this condition, it’s useful for horse owners to familiarize themselves with the relevant aspects of equine anatomy.

Spine and Vertebrae

The spinal cord is a bundle of nerves that connect the brain to the rest of the body. The vertebral column is a stack of interlocking bones called vertebrae which surround the spinal cord, protecting it while also allowing some degree of flexibility as the body moves.

Nervous signals from the brain are communicated to the body by the spinal cord, which governs all major bodily functions including:

Movement
Locomotion
Sensation
Balance
Coordination

The thoracic and lumbar vertebrae are sections of the vertebral column that support the back. These play a significant role in the horse’s movement and balance. Horses with sidewinder syndrome may have injuries or diseases that affect these structures. ^[3]

Pelvis and Hip Joint

The pelvic bone supports the hindquarters. The pelvis articulates with the spine at the sacroiliac junction and forms the socket side of the hip joint.

The three iliopsoas muscles of the pelvis stabilize but also allow flexion and rotation of the hip. They also stabilize the thoracolumbar junction, lumbar vertebrae, lumbosacral junction and sacroiliac joints. Hindleg movement and stability depends on the strength and stability of these structures.

Misalignment or injury to the pelvis can result in changes to the horse’s gait, including sidewinder syndrome. ^[3]

The hip joint is the connection between the pelvis and the femur – the bone in the horse’s upper hind leg. The hip joint is necessary for locomotion. Damage or disease in the joint can cause changes to the gait like sidewinder syndrome. ^[3]

Hind Limbs

Damage to the hind limbs can cause changes to the gait including sidewinder syndrome. ^[3] The bones of the pelvic limbs above the hock include: ^[3]

Femur
Tibia
Fibula

The muscles of the pelvic limbs include: ^[3]

Gluteus maximus
Superficial gluteal
Iliopsoas group
Semimembranosus
Semitendinosus
Gastrocnemius
Biceps femoris
Gracilis
Quadriceps

Tendons in the pelvic limbs allow the horse to stand and walk. Injury in any of these structures may contribute to sidewinder syndrome.

Neurological System

The neurological system is comprised of the peripheral nerves and the central nervous system.

Peripheral nerves innervate the muscles in the body. These nerves are responsible for communicating signals between the brain and spinal cord and the muscles that direct muscle function and coordination.

There are many peripheral nerves that innervate the hind limbs including the sciatic nerve, one of the largest and most important nerves in the horse’s body. Damage or compression of nerves can lead to changes in the gait including sidewinder syndrome. ^[3]

The central nervous system comprises the brain and spinal cord. These structures control the movement and coordination of the muscles. Damage or disease in the horse’s brain or spinal cord can lead to the changes associated with sidewinder syndrome. ^[3]

Causes

Sidewinder syndrome has a variety of causes that can relate to several parts of the horse’s body. ^[1] The causes of sidewinder syndrome are broadly categorized as either neurological or musculoskeletal based on the origin of dysfunction within the body. ^[1]

In cases where the cause is musculoskeletal, the sideways gait may be a response to weakness, limitation of mobility, or pain. Damage or disease of the pelvis, hip joints, back or legs are all possible musculoskeletal causes. ^[1]

Specific musculoskeletal causes include: ^[1]^[2]

Osteoarthritis
Muscle strains
Displaced pelvic fractures or other pelvic injury
Rupture of the ligamentum capitis ossis femoris – the ligament that connects the femur to the pelvis
Muscle tissue death (necrosis) in the pelvic limbs
Vertebral/Back injuries (may also have a neurological component)

In cases of sidewinder syndrome that have a neurological underlying cause, the sideways gait may be a response to signals from the brain either misfiring or not reaching the limb. ^[1] This may be caused by compression or damage to the peripheral nerves, or due to disease within the central nervous system. ^[1]

Specific neurological causes include: ^[1]^[2]

Spinal cord compression
Equine protozoal myeloencephalitis
Thoracic myelopathy
Gliosis
Thrombosis (blood clots) in the thoracic spinal cord
Intervertebral disc disease
Nerve inflammation
Viral myeloencephalitis

Some horses have a combination of musculoskeletal and neurological issues that result in sidewinder syndrome. ^[1] In some cases, the cause of the syndrome is never found, even during postmortem evaluation. ^[1]

Risk Factors

Sidewinder syndrome typically affects older horses. The risk factors for this condition include age-related degenerative changes such as arthritis. ^[1]

Horses that suffer pelvic misalignment or other injuries or diseases of the hips, legs, or pelvis are at risk. Horses with neurological conditions that affect muscle coordination and nerve function are also more likely to develop sidewinder syndrome. ^[1]

Symptoms

The symptoms of sidewinder syndrome relate to the way the horse holds or carries its body. Symptoms include: ^[1]^[2]

Moving awkwardly
Hindquarters that drift or lean to one side while walking
Trouble standing in one place
Spinning or circling with the hindlimbs while the forelimbs move to balance the body
Dropping the hindquarters in a “dog sitting” position while walking
Hind feet do not follow directly in the tracks of the front feet
Reluctance to move
Stiffness
Stilted gait
Lameness
Pain when touched
Difficulty lying down
Decreased muscle tone
Weakness

Severity

Sidewinder syndrome is a serious condition with limited treatment options. Generally, the horse does not recover function and euthanasia is often necessary. ^[1]

The severity of sidewinder syndrome depends on several factors, including: ^[1]

Underlying cause
Rate of onset
Severity of lameness
Severity of neurological disease, if present

Primary Causes

The underlying causes of sidewinder syndrome can range in severity from mild muscle strain to serious conditions such as neurological disorders, spinal abnormalities, or infections. ^[1] Veterinary attention is required to determine the cause. In some cases, no cause can be found until postmortem examination has been conducted. ^[1]

Rapid vs. Progressive Onset

How quickly symptoms of sidewinder syndrome develop can help indicate the underlying cause, and therefore the severity.

Rate of onset is generally described as: ^[1]

Acute: symptoms develop rapidly, in under 48 hours
Subacute: symptoms take between 2 and 7 days to surface
Insidious: symptoms take at least one week to develop

Lameness

Lameness in horses varies in severity. A basic framework for categorizing lameness is: ^[1]

Mild lameness: is exhibited inconsistently, usually only under specific circumstances
Moderate lameness: is seen consistently at higher speeds
Severe lameness: is obvious when the horse is walking
Extreme lameness: the horse has difficulty bearing weight at motion, at rest, or both.

Horses with sidewinder syndrome typically have moderate to extreme lameness. ^[1]

Neurological Disease

The degree of neurologic disease depends on the significance of the deficits and how they impact overall quality of life. Horses with mild neurologic symptoms have subtle deficits that are only detectable to trained observers and appear only during special circumstances. ^[4]

Moderate symptoms are noticeable to the untrained eye and occur constantly or consistently. Severe symptoms are very obvious and result in the risk of the horse falling when standing. Extreme neurologic symptoms mean that the horse is unable to stand. ^[4]

Horses with sidewinder syndrome related to the nervous system typically have moderate to severe neurological symptoms. ^[1]

Diagnosis

Horses affected by sidewinder syndrome present a diagnostic challenge because they are often unable to stand still for examination. ^[1] In some cases, they collapse if they are sedated. ^[1]

Diagnosis of this condition requires a full neurologic examination, which includes a physical examination of the head, neck and trunk and perineum. The horse’s gait and posture are also evaluated. ^[5]

During this evaluation, the veterinarian examines the horse’s: ^[4]^[5]

Reflexes
Posture
Response to moving in a circle or on slopes
Motor function
Proprioception – the sense of the body’s position in space and where the limbs are relative to each other
Movements of the body while walking
Movements of the feet
Heart rate
Breathing rate

Diagnostic Tests

Additionally, diagnosis requires information on internal functions of the horse’s body. Diagnostic tools include: ^[1]^[2]^[6]

Bloodwork
X-ray
Ultrasound
Nuclear scintigraphy (X-ray with radioactive contrast dye)
CT scan
Transcranial magnetic stimulation (nerve stimulation using magnetic fields)
Cerebrospinal fluid analysis (“spinal tap”)
Electromyography (sending electrical activity into the muscles to see which nerves are functioning)
Muscle biopsy
Immunofluorescent antibody test
Intra-articular anesthesia of the coxofemoral joint
Rectal/pelvic examination

Treatment

Sidewinder syndrome is difficult to treat successfully. Treatment aims to resolve the underlying cause and depends on the cause of the condition.

Treatments may include: ^[1]^[2]

Even if treatment is successful in resolving some of the symptoms of sidewinder syndrome, improvement tends to be temporary. Horses with sidewinder syndrome often retain the peculiar gait for the rest of their lives. ^[1]

If the horse is unable to lie down, stand still, or stop circling, euthanasia is recommended. ^[1]

Due to the wide array of possible causes, diagnosis of sidewinder syndrome is often elusive. The cause is commonly not confirmed until a postmortem evaluation (necropsy) is performed. In some cases, even with a postmortem evaluation, no cause is ever verified. ^[1]

Prognosis and Prevention

Horses with sidewinder syndrome have a poor prognosis for both function and survival. ^[1]

There are no specific preventive measures against sidewinder syndrome, other than general best management and training practices to avoid injury and infectious disease. General best practices include:

Staying up to date with all recommended vaccinations, especially when traveling for sports or events
Keeping a robust parasite control regimen
Providing a free-choice, forage-based diet
Following veterinary recommendations for routine screenings and evaluation
Working with qualified professionals, including but not limited to trainers, farriers, and body work practitioners

Summary

Sidewinder syndrome is a condition in which the horse’s trunk and hindlimbs drift to one side while they are walking forwards. There are a range of potential causes stemming from the musculoskeletal or neurologic systems or both.

Symptoms include difficulty walking, inability to stand, stiffness, and weakness
In severe cases, the horse is unable to stand still and continuously circles or spins
Diagnosis requires a full neurological and physical examination as well as blood work and imaging
Treatment options are limited and include anti-inflammatory drugs, physiotherapy, and antioxidants
Prognosis for horses with sidewinder syndrome is poor. Affected horses usually never return to a normal gait and are euthanized if symptoms are severe