English
Stringhalt, or equine reflex hypertonia, is a neuromuscular condition that causes abnormal hindlimb movement in the horse.
Horses with stringhalt have excessive and prolonged flexion of the pelvic limbs while in forward movement, showing signs of the condition at most gaits. [15]
One (unilateral) or both (bilateral) legs may be affected. Some horses experience mild cases characterized by involuntary jerking of the hindlimb, while others experience lameness and difficulty standing up.
Horses of all ages and breeds can be affected by stringhalt. In some cases, it is caused by ingesting toxic plants at pasture, but other cases develop quickly without apparent cause.
Diagnosis is complicated due to limited research on equine neurological movement disorders. [4] The prognosis and treatment of stringhalt depend on the individual case.
Contact your veterinarian if your horse shows signs of a neuromuscular movement disorder.
Stringhalt in Horses
Equine reflex hypertonia or stringhalt is characterized by involuntary jerking or flexing of the hind limb.
This condition can be acquired and intermittent (temporary), or chronic with progressive symptoms. It is often idiopathic meaning it has no known cause and arises spontaneously.
Mild cases involve sporadically lifting and grounding the foot while walking. More severe cases involve full leg spasms in which the foot is raised to the horse’s stomach and then dropped to the ground.
Other progressive movement disorders with similar presentation include Shivers and Stiff-horse syndrome (SHS).
Clinical Signs of Stringhalt
Horses can develop clinical signs of stringhalt suddenly or gradually over time. These symptoms can range from mild to very severe, where a secondary injury is possible due to involuntary kicking.
Horses that are nervous, excited or agitated may demonstrate more gait abnormality. Some movements may exacerbate signs, such as walking downhill, turning sharply or suddenly stopping. Cold weather, hard exercise and sudden movement after rest can intensify symptoms.
The most common signs of the condition are: [5][15]
- Involuntary, exaggerated upward movement of the hindlimb
- Hopping or jerking
- Kicking upwards towards the belly
- Incoordination, dragging hind hooves
- Muscle atrophy of the lower hindlimb
- Inability to stand up without assistance
The clinical presentation of stringhalt varies greatly between individuals, which makes the disorder difficult to diagnose. If your horse is showing signs of stringhalt, contact your veterinarian immediately.
Idiopathic Stringhalt
Idiopathic stringhalt is sometimes referred to as classic or true stringhalt. The exact cause of this condition is unknown, and it can develop overnight.
The condition tends to affect one hind leg (unilateral), and hyperflexion is particularly evident when the horse is walked, trotted or turned in a tight circle. [15]
In horses with idiopathic stringhalt, the nerves that convey impulses and trigger contraction of the lower hindlimb are affected, although the exact cause of neurological dysfunction is unknown.
Damage to the peripheral nerves leads to abnormal smooth muscle contraction. This explains the jerky, random contractions observed in the hindlimb.
Injury to the muscles of the lower hind limbs, back or neck may also lead to stringhalt. [15] Unfortunately, the signs of the condition can progress over time, and horses rarely recover from idiopathic stringhalt without surgical intervention.
Pasture-Associated Stringhalt (PSH)
Pasture-Associated Stringhalt (PSH), or sporadic stringhalt, results when horses are exposed to a plant neurotoxin that damages the peripheral nervous system. This is generally defined as neuropathy. [5]
This condition usually occurs in an outbreak when horses are kept on poor-quality pasture during late summer and autumn and thus forage on weeds they otherwise wouldn’t be interested in..
The onset of PSH is sudden, and signs can worsen quickly, although the condition is usually temporary. Depending on the severity, recovery can take between 6 – 24 months. Some horses never fully recover.
Horses with PSH usually have bilateral hyperflexion of the hindlimbs. This results in a bunny-hopping gait when in movement.
In extremely rare cases of PSH, the forelimbs can be affected in what is called atypical bilateral stringhalt syndrome. [12]
PSH can also affect nerves in the larynx (voice box). Approximately 20-60% of PSH-affected horses demonstrate abnormal vocalization due to laryngeal dysfunction.
This condition limits airflow to the lungs and produces an abnormal, high-pitched noise during inhalation (stridor). [7]
Hypochaeris radicata
The pasture weed hypochaeris radicata (flatweed, catsear, false dandelion) is known to cause bilateral stringhalt in horses. This perennial weed is native to Europe, but has been introduced to North and South America, Japan, Australia and New Zealand where it is considered invasive.