Cauda Equina :

Cauda Equina


 

INTRODUCTION

The cauda equina (CE) is a collection of intradural nerve roots, beginning at the end of the spinal cord. "Cauda" is Latin for tail, and "equina" is Latin for horse, ie, the "horse's tail".

In this article, the primary focus is cauda equina syndrome (CES). However, as back pain is a major presentation of the syndrome, it is discussed briefly.

Back pain is among the most common of human conditions, approaching a 15-20% incidence in the population. Most causes of both acute and chronic back pain are benign. In a review of back pain cases in primary care practice, 4% were considered to result from compression fracture, 3% from spondylolisthesis, 0.7% from malignancy, 0.3% from ankylosing spondylitis (AS), and 0.1% from vertebral osteomyelitis.

Three basic approaches to the diagnostic evaluation of back pain are used. Most diagnostic decision making is based on the history and clinical presentation. The first approach could be termed the red-flag approach, in which certain historical and clinical clues are elicited to assess the probability of serious disease. This method is very useful because it can help to detect patients needing additional tests and can distinguish them from patients who may benefit (or at least not be harmed by) a trial of conservative care. This approach has been advocated for low back pain because it is efficient, and in part, because it is cost-effective.

The second approach to diagnosis could be termed the pattern recognition approach. This requires the clinician to match the patient's history and findings with a clinical picture of the various diagnostic possibilities. Based on the probabilities determined from such an analysis, additional testing may be suggested to document the existence of the condition. It also may lead to a trial of therapy for the specific disorder that, if successful, further validates the diagnosis.

The third approach to diagnosis is an algorithmic approach. Algorithms have proved very effective for making the differential diagnosis of many clinical syndromes and have been advocated by several authors for the investigation of low back pain. However, the utility of this approach may be less effective in the evaluation of low back pain than in other conditions because it rarely leads to a specific diagnosis. Algorithms may be helpful in determining the utility of particular diagnostic tests, with a goal of ruling out the presence of more serious causes of low back pain. In this regard, most of the algorithms that have been put forward for the diagnosis of low back pain consist of a screen for the presence of red flags, which are then used as decision points to determine which additional testing might be indicated.

Potential red flags

History

Present complaint

Physical examination and laboratory findings

Pattern of symptoms not compatible with benign mechanical disease

Lack of response to conservative measures

Red flags

Several symptoms and signs rarely encountered in benign forms of back pain exist that are useful for stratifying the risk of more serious underlying disease. These red flags can be divided into 3 basic types The first is based on history, the second on the characteristics of the presenting complaint, and the third on physical examination findings and simple laboratory investigations.

Several elements in the patient's medical history lead to some concern and, therefore, may be thought of as red flags. Among these historical elements, a history of cancer is the most important. Patients with cancer, even those without a history of metastasis, who have a new onset of back pain require a high index of suspicion for metastatic disease because neurologic symptoms can develop quite rapidly. Also, treat patients who have a history of a disorder that makes them prone to infection or hemorrhage or patients who have been using immunosuppressive or anticoagulant medications with a high index of suspicion.

Similarly, new back pain in patients older than 50 years who lack a prior history of similar back pain requires additional investigation because a higher incidence of serious medical causes of back pain exists in older individuals (eg, cancer, metabolic disorders of the spine) and most benign back pain initially presents in younger patients.

The second type of red flag may be raised by the characteristics of the present complaint. These characteristics include precipitation of the back pain by significant trauma; pain that is worse at night or not relieved by a position change; bilateral radiculopathy; numbness or paresthesias in the buttocks or the perianal region; a change in bladder or bowel habits; writhing pain; and significant lower limb weakness not explainable by pain or any progressive neurologic deficit. Any of these elements should result in consideration of more detailed imaging of the spine because of the increased likelihood of more serious disorders.

Simple physical and laboratory studies that can be considered red flags include a pulsatile abdominal mass or an enlarged shadow of the aorta on lumbar radiograph. Focus neurologic examination on the lower limbs, and determine any abnormality on motor or sensory examination. Static neurologic deficits explainable by the involvement of a single lumbar nerve root cannot be considered a red flag because this is a common presentation of benign disease (disk herniation, lateral recess stenosis). However, treat patients who have bilateral neurologic deficits, deficits involving multiple root levels, or progressive neurologic deficits as if they had red flags.

Of all of the simple laboratory tests that may be of use in low back pain, the erythrocyte sedimentation rate (ESR) is the most important. Not only is it generally elevated in metastatic disease, but it also elevates in infectious disorders of the spine (eg, osteomyelitis, discitis, epidural abscess). It may be the only abnormal test in epidural abscess because this may present without fever or an elevated white blood cell count. The ESR also may provide a clue as to the inflammatory nature of the complaint in spondyloarthropathies. Back pain with fever, as well as back pain with an elevated white blood cell count, requires additional consideration.

The lack of a pattern of symptoms compatible with benign disease and the lack of response to the usual measures of conservative care also may be considered types of red flags. Reconsideration of the diagnosis is important in such cases after a reasonable period of several weeks of conservative management.

Neoplastic disease

Neoplasms of the spine may be either primary or metastatic. Primary tumors include neoplasms of the marrow, especially multiple myeloma, or tumors of the bone or the cartilage of the spine. These neoplasms usually produce progressive indolent back pain, which typically is unresponsive to rest and actually may be most severe at night. Some of these neoplasms are benign, such as osteoid osteoma (with pain that usually is remarkably responsive to aspirin); however, most of these tumors can result in neurologic compromise by direct involvement from the tumor mass or by compromising the structural integrity of the spine.

Multiple myeloma is the most common primary malignancy of the bone in adults. It is very rare in people younger than 40 years, and back pain is the presenting symptom in 35% of cases. Initially, this pain may be relieved by rest. Various laboratory tests, including ESR, white blood cell, serum calcium, and uric acid levels, frequently are elevated. Electrophoresis of serum and urine (for light chains) is diagnostic for the condition by the demonstration of paraproteins produced by myeloma cells.

Metastatic spinal disease is 25 times more common than primary tumors. In part, this results from the large volume of blood that courses slowly through bidirectional venous channels (Batson plexus) in the epidural space, which communicates with blood spaces in the vertebral marrow. Spinal metastasis is found in as many as 70% of patients who have primary tumors on autopsy. The most common tumors to metastasize to the spine include tumors of the breast, lung, prostate, kidney, lymphoma, melanoma, and GI tract. Back pain is the presenting symptom in 90% of patients who have spinal metastasis. Frequently, early metastasis is not visualized on plain radiographs, though bone scans are positive in 85% of patients who have spinal metastasis. MRI may identify spinal metastasis in patients who have normal radiographs and bone scans.

In a survey of 1975 patients presenting for the evaluation of low back pain, Deyo and Diehl found that an age greater than or equal to 50 years, a previous history of cancer, duration of pain greater than one month, failure to improve with conservative therapy, an elevated ESR, and anemia were significantly associated with the presence of neoplastic disease. The pain produced by neoplasm often is worst while the patient is at rest, and it may even awaken patients from a sound sleep. Both of these symptoms are atypical for benign disease. The back pain usually is focal and worsens with percussion.

Assume new back pain is neoplastic in any patient who has a history of cancer or unexplained weight loss, and there should be a low threshold for radiographic studies in such patients. Plain radiographs may be reassuring; however, if neurologic compromise occurs, if the patient with cancer is not responding to conservative care, or if a compelling story for benign mechanical disease does not exist, perform and MRI or CT scan. The best predictors of the spinal level of the abnormality are location of the focal pain, location of percussive tenderness, or location of origin of any nerve roots or lower motor neurons that may be clinically affected.

Problem: The CE is formed by nerve roots caudal to the level of spinal cord termination. CES has been defined as low back pain; unilateral or, usually, bilateral sciatica; saddle sensory disturbances; bladder and bowel dysfunction; and variable lower extremity motor and sensory loss.

Lesions involving the termination of the spinal cord (conus medullaris) and the CE are not discussed here. Please refer to Spinal Cord Injuries.

Frequency: In the United States, CES is uncommon, both atraumatically and traumatically, accounting for approximately 1-3% of patients who undergo spinal surgery. No racial or sexual predilection exists for CES. Traumatic CES is not age specific. Atraumatic CES occurs primarily in adults.

Etiology:

Pathophysiology: CES may result from any lesion that compresses the nerve roots of the CE. These nerve roots are particularly susceptible to injury because they have a poorly developed epineurium. When they are well developed, as in peripheral nerves, they protect against compressive and tensile stresses. The microvascular systems of nerve roots have a region of relative hypovascularity in their proximal third. Increased vascular permeability and subsequent diffusion from the surrounding cerebrospinal fluid (CSF) supplement the nutritional supply. This property of increased permeability may be related to the nerve root tendency toward edema formation, which may result in the edema compounding initial and, occasionally, seemingly slight injury.

Clinical: Radicular pain is a frequent presentation of the cauda equina syndrome, usually in association with radicular sensory loss (saddle anesthesia), asymmetrical paraplegia with loss of the tendon reflexes, muscle atrophy, and bladder dysfunction. The principal causes of this syndrome are tumors, lumbar spinal stenosis, ruptured lumbar disk, arachnoiditis, and spinal fracture.

When the conus medullaris is impaired, radicular pain is less prominent. The principal and early disturbances are urinary retention and constipation. In addition, there may be loss of pinprick sensation in the perianal region, impotence, and sometimes muscle weakness of the lower limb.

Table 1. Cauda Equina versus Conus Medullaris Syndrome
  Cauda Equina Conus Medullaris Syndrome
Pain Severe radicular pain Back pain, less severe radicular pain
Hypesthesia Radicular sensory loss
(saddle hypesthesia)		 Usually restricted to perianal region
Motor deficits Asymmetrical areflexic
paraplegia		 Distal paresis of lower limbs
Sphincters Urinary retention Urinary retention plus atonic anal Sphincter
Sexual dysfunction Sometimes impotence Impotence frequent


History

Physical

INDICATIONS

Lumbar laminectomy (for removal of the cause of nerve root compression) is indicated for patients with a history, physical examination, and diagnostic imaging (magnetic resonance [MR] lumbar spine stenosis) consistent with a spinal pathology explaining the presentation with CE.

RELEVANT ANATOMY AND CONTRAINDICATIONS

Relevant Anatomy: The spinal cord is the downward continuation of the medulla, starting at the foramen magnum. It descends to about the level of the second lumbar vertebra, tapering to a structure called the conus medullaris. A thin threadlike continuation of the conus, the filum terminale, extends as far as the coccyx and is composed of nonnervous tissue.

The cord serves as a conduit for the ascending and descending fiber tracts that connect the peripheral and spinal nerves with the brain. The cord projects 31 pairs of spinal nerves on either side (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) that are connected to the peripheral nerves. A cross section of the spinal cord demonstrates a butterfly-shaped gray matter in the middle surrounded by white matter. As in the cerebrum, the gray matter is composed of cell bodies. The white matter consists of various ascending and descending tracts of myelinated axon fibers with specific functions.

Contraindications: An uncertain diagnosis or an unclear clinical presentation is a definite contraindication to the surgery. The same contraindications as for any other surgery (eg, fever) also apply.

Observation for a variable period of time may be a better treatment option for patients with high surgical risk and/or mild neurologic dysfunction.

Patients in whom biopsy reveals malignant neoplasm may be best treated with radiotherapy since they are expected to have a rapid downhill course.

WORKUP

Lab Studies:

Imaging Studies:

TREATMENT

Medical therapy: No proven medical treatment exists. Generally, therapy is directed at the underlying cause of CES. Some might suggest methylprednisolone in a regimen similar to that administered for traumatic spinal cord injury or another regimen of steroids for the acute syndrome.

For penetrating trauma, steroids have not shown significant benefit. Surgery is controversial. The timing of decompression is controversial, with immediate, early, and late surgical decompression showing varying results.

Consultations

Early neurosurgical, neurological, or orthopedic consultations are recommended, depending on the suspected etiology of CES.

Medication

Steroids may be recommended in acute or traumatic CES. Early consultation regarding the use of steroids and any specific regimen is encouraged. Steroids have anti-inflammatory properties and cause profound and varied metabolic effects. In addition, these agents modify the body's immune response to diverse stimuli. They may decrease edema around nerve root segments.

A regimen of methylprednisolone (Solu-Medrol, Depo-Medrol) may be used. One possible regimen may be the dose employed in traumatic spinal cord injuries, but no studies exist to support this over any other regimen.

Surgical therapy:

Surgical objectives

The lumbar laminectomy is one of the simplest neurosurgical operations performed with the patient under general anesthesia. Another simple neurosurgical procedure, insertion of a ventriculoperitoneal shunt, is like lumbar laminectomy in that it is fraught with a number of untoward outcomes and complications disproportionate to the ease with which it is performed. The objective of lumbar laminectomy is to open up the spaces that nerve roots travel through on their way from inside to outside the vertebral canal. The site along the course of the nerve root most liable to compression is where it passes through the opening from the canal to the surrounding paraspinous back muscles; this opening is known as the neural foramen.

Lesion considerations

Degenerative osteoarthritis causes foraminal narrowing by increasing the amount and extent of inflamed arthritic bone at and immediately adjacent to the facet joint, the site of movement in the spine. Also, as a consequence of movement, friction and irritation generate inflammation, which ultimately leads to the changes in joints and bones called arthritis. Relieving pressure on nerves at and near the neural foramen requires removal of some of the material pathologically narrowing the neural foramen. To reach the neural foramen and remove some of the pathological bony material, the surgeon must remove the bones and ligaments of the posterior vertebral canal, laminectomy. Once the lumbar dura has been exposed, the surgeon needs to inspect the spaces on the sides (lateral) of the inside the vertebral canal, ie, the so-called recesses, along which the nerve root travels just prior to entry into the neural foramen on its way out of the canal.

To evaluate for compression in the lateral recesses and on through the neural foramen, the surgeon needs to tactilely and visually inspect these structures. The thecal sac must be pulled (retracted) medially to create a space between it and the inner wall of the vertebral canal to allow the surgeon to inspect and work. A variety of probes are used to feel for spurs and other excrescences of bone that can be removed using rongeurs and curettes designed to address the anatomic particularities of surgical access and manipulation in the posterior lumbar vertebral canal.

With curettes and rongeurs, excess bone, inflamed joint material, and hypertrophied ligamentum flavum are removed. (Hypertrophy [abnormal thickening] of the ligamentum flavum is a frequent accompaniment to lumbar stenosis.)

Laminectomy facetectomy

Once, by feeling with probes, the surgeon is convinced that the space of the lateral recesses and neural foramina is sufficiently open to allow for unrestricted passage of the exiting nerve root, the surgeon allows the dura to return to its position up against the wall (except for an intervening layer of epidural fat and the epidural venous plexus) of the vertebral canal.

Hemostasis usually is simple as long as muscles have not been divided during the laminectomy. Closure is the same as for laminectomy. For lumbar stenosis, a frequent occurrence during lumbar laminectomy is violation of the dura mater (a tear) associated with escape of CSF from the subarachnoid space into the epidural space; occasionally, this occurs in such volume and escapes rapidly enough that it begins to fill the laminectomy wound.

A persistent CSF leak can become a significant complication of lumbar laminectomy associated with formation of pseudomeningoceles and, possibly, even wound dehiscence. The yellow ligament in elderly patients with lumbar stenosis, facet hypertrophy, lateral recess stenosis, and neural foraminal stenosis, also is frequently hypertrophic and closely adherent (stuck) to the underlying dura. To avoid tearing the dura, the surgeon must carefully develop a plane between the dura and the yellow ligament (ligamentum flavum). This is usually the most difficult and time-consuming part of a lumbar laminectomy for decompression of degenerative arthritic lumbar stenosis (vertebral canal narrowing due to lumbar spondylitic [degenerative arthritic] disease).

Surgical considerations include the following:

Preoperative details: Stabilize acute life-threatening conditions. Immobilize the spine if the injury is traumatic.

Postoperative details: After a decompressive lumbar laminectomy, the patient usually is taken to the recovery room. Immediately postoperatively and preoperatively, patients are checked to make sure they are moving and can feel their legs. The back also may be inspected to make sure no seepage of blood occurs through the dressings. Once recovered, the patient usually is taken to a regular ward bed.

In the recovery phase after decompressive lumbar laminectomy, rehabilitative physical therapy or rehabilitation is not indicated in patients without preoperative neurologic deficit.

Follow-up care: After discharge following a lumbar laminectomy, the patient should be seen in the outpatient (ambulatory) clinic in 7-10 days. Perform a follow-up MRI.

Admit patients to appropriate service (usually neurology, neurosurgery, or orthopedic surgery) with frequent neurologic checks. Ideally, the admitting physician or service should examine the patient at the time of admission.

Patients in whom acute CES is being considered should not be managed or investigated on an outpatient basis without evaluation by a consultant and/or appropriate imaging specialist.

Consider patients with CES for transfer if appropriate subspecialty care is not available.

COMPLICATIONS

If therapy is delayed, potential problems include residual weakness, incontinence, impotence, and sensory abnormalities. However, these problems can persist even with prompt decompression.

Risks and complications of lumbar laminectomy include dural tear (occasionally with pseudomeningocele), "failed back syndrome" (scar, residual stenosis), and instability.

OUTCOME AND PROGNOSIS

CES is not fatal. Morbidity from CES is variable, depending on the etiology of the syndrome.

The prognosis for CES improves if a definitive cause is identified and appropriate management occurs early in the course. Surgical decompression may be performed emergently, or, in some cases, delayed, depending on the etiology.

The prognosis following decompressive lumbar laminectomy is related to the length of time that the patient was symptomatic or with deficit.

FUTURE AND CONTROVERSIES

Reoperation may be necessary for recurrence of stenosis or to extend the levels of decompression after a patient fails to improve after an initial limited procedure. Reoperation is usually more difficult than the first procedure because of scar tissue.

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Constructed by Dr N.A. Nematallah Consultant in perioperative medicine and intensive therapy, Al Razi Orthopedic Hospital , State of Kuwait, email : razianesth@freeservers.com