Plexus lumbosacralis
The lumbosacral plexus is the network of nerves arising from the lumbar and sacral spinal cord segments that innervates the hindlimb, pelvis, and perineum. The lumbar plexus (L4-L6) gives rise to the femoral and obturator nerves, while the sacral plexus (L6-S2) forms the massive sciatic nerve; the largest peripheral nerve in the horse. The sciatic nerve divides into the tibial and common peroneal (fibular) nerves at or above the stifle. The pudendal nerve (S2-S4) innervates the perineum, external genitalia, and urethral sphincter.
The lumbosacral plexus is formed by ventral branches of lumbar (L4-L6) and sacral (S1-S3) spinal nerves. It gives rise to the femoral, obturator, sciatic (and its divisions: tibial and peroneal), pudendal, and caudal rectal nerves. The sciatic nerve is the largest nerve in the body. Post-anesthetic neuropathy from prolonged recumbency on a hard surface is a significant risk in equine anesthesia.
Injury to the femoral nerve (L4-L5), most commonly from dystocia, pelvic fracture, or post-anesthetic neuropathy. Causes inability to fix the stifle in extension.
The common peroneal nerve is vulnerable where it crosses the lateral surface of the stifle. Commonly injured from lateral recumbency on hard surfaces (anesthesia, prolonged casting).
Stringhalt is a condition characterized by involuntary, exaggerated flexion of one or both hind limbs during movement. The limb is jerked upward toward the belly in a spasmodic manner, most noticeably
Polyneuritis equi is an immune-mediated demyelinating disease of the cauda equina and cranial nerves (especially trigeminal and facial). Granulomatous inflammation of nerve roots. Associated with EHV-
Injury to the femoral nerve (L4-L5), most commonly from dystocia, pelvic fracture, or post-anesthetic neuropathy. Causes inability to fix the stifle in extension. Signs include: Inability to bear weight on affected hindlimb; stifle collapses (cannot lock in extension); rapid quadriceps atrophy; reduced patellar reflex; horse cannot support weight on affected side
The common peroneal nerve is vulnerable where it crosses the lateral surface of the stifle. Commonly injured from lateral recumbency on hard surfaces (anesthesia, prolonged casting). Signs include: Knuckling of the fetlock; dorsal hoof dragging; inability to flex the hock and extend the digit; characteristic 'goose-stepping' gait when recovering; Knuckling of the hind fetlock; inability to extend the digit; toe dragging; dorsal hoof wear; may occur after surgery from positioning.; Inability to flex the hock actively; knuckling; often from bandage or cast pressure on the lateral stifle/gaskin area.
Stringhalt is a condition characterized by involuntary, exaggerated flexion of one or both hind limbs during movement. The limb is jerked upward toward the belly in a spasmodic manner, most noticeably at the walk. It can be idiopathic, acquired (often linked to toxic plants), or occur after trauma or surgery. ## What it is Stringhalt involves abnormal function of the nerves and muscles that control hind limb flexion, most classically the lateral digital extensor muscle and its tendon [VET REVIEW]. In acquired stringhalt, damage or irritation to this muscle-tendon unit leads to the characteristic hyperflexion. The Australian form is associated with ingestion of the weed *Hypochaeris radicata* (catsear), while other cases may follow trauma, surgery, or occur idiopathically with no clear cause. The condition is usually more obvious at the walk than at faster gaits and often improves when the horse is turned or backed. It can be unilateral or bilateral and ranges from mild to severely debilitating. ## Signs and symptoms - Exaggerated, spasmodic upward flexion of the hind limb during walking (snapping toward the belly) - Worse at walk than trot; often improves at canter - More noticeable when turning, backing, or in cold weather - Can be unilateral or bilateral - In severe cases, the horse may hit the abdomen or chest with the hoof - Australian stringhalt often follows ingestion of toxic weeds and may affect multiple horses in a group ## Causes and risk factors Stringhalt has several forms. The acquired form is often linked to ingestion of toxic plants (especially *Hypochaeris radicata* in Australia and New Zealand). Other cases follow trauma to the hind limb, surgery in the area, or occur idiopathically. There is no strong breed or age predisposition, though it can appear at any age. ## How it is diagnosed Diagnosis is primarily based on the characteristic gait abnormality observed during movement. A thorough neurologic examination helps rule out other causes of abnormal hind limb movement. In some cases, electromyography or ultrasound of the lateral digital extensor muscle and tendon is used. Ruling out other neurologic diseases is important. ## Treatment and management Treatment depends on the type and severity. In mild cases, many horses can continue light work. For more severe or acquired cases linked to toxic plants, removal from the offending pasture is essential. Surgical removal of a section of the lateral digital extensor tendon (myotenectomy) can provide significant improvement in many horses. Medical management with phenytoin or other medications has been tried with variable success. ## When to call the vet Contact your veterinarian if you notice sudden or progressive exaggerated hind limb flexion. While stringhalt itself is not an emergency, it is important to differentiate it from other neurologic conditions that may require urgent attention. ## Prevention For acquired stringhalt, prevention involves avoiding pastures containing known toxic plants such as *Hypochaeris radicata*. Regular pasture management and monitoring horses for early signs after moving to new properties can help reduce risk. ## Frequently asked questions **Is stringhalt painful?** Most horses with stringhalt do not appear to be in pain, though the gait abnormality can affect performance and, in severe cases, cause the horse to hit itself. **Does stringhalt get worse over time?** Idiopathic cases may remain stable or slowly progress. Acquired cases linked to toxic plants can improve once the horse is removed from the source, though some horses retain a degree of abnormality. **Can stringhalt be cured?** Surgical treatment (lateral digital extensor myotenectomy) provides good improvement in many horses, though complete resolution is not guaranteed in all cases. This information is for educational purposes only. Always consult with a licensed veterinarian for diagnosis and treatment of your horse. Signs include: Exaggerated, spasmodic upward flexion of the hind limb during walking (snapping toward the belly); worse at walk than trot, often improves at canter; more noticeable when turning, backing, or in cold weather; can be unilateral or bilateral; in severe cases, the horse may hit the abdomen or chest with the hoof; Australian stringhalt often follows ingestion of toxic weeds and may affect multiple horses in a group
Polyneuritis equi is an immune-mediated demyelinating disease of the cauda equina and cranial nerves (especially trigeminal and facial). Granulomatous inflammation of nerve roots. Associated with EHV-1 and P2 myelin protein. Signs include: Tail paralysis; perineal anesthesia; fecal and urinary incontinence; hindlimb weakness.; Loss of anal tone; fecal incontinence; urinary incontinence; flaccid tail; reduced perineal sensation; often from EHV-1, trauma, or polyneuritis equi.; Progressive tail and anal sphincter paralysis; fecal retention; perineal hypalgesia; rubbing tail base; if cranial nerves involved: facial paralysis, trigeminal dysfunction, dysphagia; Flaccid, dilated anus; fecal staining of the hindquarters; often accompanied by tail paralysis, urinary incontinence, and perineal anesthesia.; Flaccid, hanging tail with no voluntary movement; urine scalding of hindlimbs; fecal retention or incontinence; reduced anal tone; often from trauma, EHV-1, or polyneuritis equi.
The lumbosacral plexus is formed by ventral branches of lumbar (L4-L6) and sacral (S1-S3) spinal nerves. It gives rise to the femoral, obturator, sciatic (and its divisions: tibial and peroneal), pudendal, and caudal rectal nerves. The sciatic nerve is the largest nerve in the body. Post-anesthetic neuropathy from prolonged recumbency on a hard surface is a significant risk in equine anesthesia.
See the Lumbosacral Plexus on our interactive 3D horse model. Open 3D Model