The spine consists of 27 bones, not including those in the tail. Disks are located between the bones of the spine and serve as shock absorbers. Each disk consists of an outer fibrous ring (annulus fibrosus) that surrounds inner pocket of gelatinous material (nucleus pulposus). Ligaments and many muscles support each of the bones of the spine. The spinal cord runs through a large canal within the spinal bones, thereby protecting it from injury. Each disk is located beneath the spinal cord, where each of the spinal nerves exits the spine.
The spinal cord terminates at approximately the sixth lumbar vertebral bone (lower back). Nerves to the hind limbs, bladder, rectum and tail extend off the terminal part of the spinal cord and exit the spine to their respective areas. The area where all of the nerves come off the terminal spinal cord looks like a horse’s tail, hence it is called the cauda equina.
Causes of cauda equina syndrome
Cauda equina syndrome is a condition in which the nerves of the hind limbs and rear end are compressed. Disease processes that can compress the sciatic and sacral nerves include degenerative arthritis of the spine; congenital deformity of the bones; acute intervertebral disk herniation; chronic bulging disk; infection in the disk and adjacent spinal bones; fractures; partial dislocation of the spinal bones; tumors; inflammation of the nerves; and infection in the spine.
Degenerative lumbosacral stenosis, the most common form of cauda equina syndrome, is associated with a number of pathologic changes in the spine that result in compression of nerves. Thickening of the intervertebral disk, thickening of the ligament within the spinal canal (interarcuate ligament), thickening of the joints of the spine due to arthritis, and partial dislocation (malalignment) of the spinal bones are common features of this condition.
Other diseases that can mimic lumbosacral stenosis include fibrocartilaginous embolic myelopathy (spinal cord stroke), degenerative spinal cord disease (myelopathy), muscle weakening disease (myopathy), myasthenia gravis, blood clot in the arteries of the hind limbs, hip dysplasia, cruciate ligament rupture and polyarthritis.
Clinical signs and diagnosis
Most commonly affected dogs are large breed, older dogs with German shepherds over- represented. Other reported breeds include great Danes, Airedale terriers, Irish setters, English springer spaniels, boxers, Labrador retrievers and golden retrievers. The typical age is about six to seven years, with males more commonly affected.
Clinical signs of cauda equina syndrome may include a prolonged period of intermittent or continuous weakness of the hind limbs. As time progresses, so do the clinical signs. Some dogs show lameness of one of the hind limbs if an intervertebral disk is bulging one side of the spinal canal, thereby primarily compressing one of the nerve roots to a hind limb; this is termed root signature. Pain may be manifested by intermittently crying out for no apparent reason. Other dogs exhibit their pain when the lower spine is palpated. Patients may have a crouched stance of the body with flexion of the hips, knees and ankles. When walking, affected patients have a choppy movement of the hind limbs. In more severe cases, the patient may knuckle the paws over or walk on the top of the paws. Jumping, rising from a sitting position, and climbing stairs are also common findings. Leakage of urine during sleep and dropping feces are seen as in the later stages of the disease. Defecation can also be affected due to difficulty with posturing. Severely affected pets will also lose the ability to wag or raise the tail while urinating or defecating. Some dogs will self-mutilate the tail, presumably due to a tingling sensation in this region.
The diagnosis of this condition includes finding neurological deficits of the nerves that are compressed by the stenotic spinal canal and pain in the lumbosacral junction. A test called an electromyogram detects abnormal signals from the muscles due to damage of nerves. Imaging tests such as X-rays, myelogram, epidurogram, computed tomography (CT) and magnetic resonance imaging (MRI) are useful to confirm the diagnosis. If a tumor is the suspected cause of cauda equina syndrome, then the patient should have chest X-rays to help rule out visible spread of tumor to the lungs; it is important to note that microscopic spread of cancer to the lungs will not be detectable with X-rays.
Medical treatment includes exercise restriction for a period of four to six weeks. Anti-inflammatories are commonly administered for a period of four to six weeks. Cortisone is commonly prescribed if the patient has more severe neurological signs for a period of two to four weeks. One report indicated a 50% long-term response to epidural injection of a long-acting steroid (Depo-medrol®), but some patients required multiple injections. The efficacy of medical therapy may only be seen in patients that have minimal neurological deficits. In general, about half of the patients may respond to treatment.
Surgery is commonly recommended in dogs that do not respond to medical treatment, have progressive clinical signs, or have more severe neurological deficits. Surgery, called a laminectomy, involves making an opening in the top of the spine over the area of nerve compression and removing the bulging disc and thickened ligaments. If the spine is unstable, a fusion surgery is performed. After surgery, the patient will likely have a urinary catheter in place and will be given narcotics to control pain.
For more information on this subject, speak to the veterinarian who is treating your pet.