Cauda equina syndrome is extremely rare accounting for 1 in 2000 presentations of low back pain however despite its low incidence serious morbidity is related to cauda equina syndrome if mismanaged. A treatment delay of more than 48 hours in acutely presenting cauda equina syndrome can result in loss of bladder function and sensory loss of the lower limbs. To minimise this risk of morbidity, the red flag symptoms of cauda equina syndrome which include saddle area anesthesia, loss of bladder and bowel function and bilateral sciatica should be well known by any of the healthcare team to whom these patients may present. There are numerous causes of cauda equina syndrome, the most common of which is a herniated lumbosacral disc. Other compressive etiologies include tumours and spinal stenosis, whilst non compressive etilogies include infections and inflammation. Despite the serious nature of this syndrome, it is not well known to many of the healthcare team and missed diagnosis or delayed treatment has given rise to many cases of litigation.
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Low back pain is a common musculoskeletal disorder that is experienced by up to 80% of the population by the age of 60 (Devlin 2003). The most common causes of low back pain are musculoligamentous injuries and age related degenerative processes in the intervertebral discs (Deyo and Weinstein 2001). However there are also other more serious causes of low back pain such as spinal stenosis and lumbar disc herniation which may injure nerve roots for example those in the cauda equina thereby causing Cauda Equina syndrome. The cauda equina is the collection of nerve roots caudal to the level of the spinal cord termination which provides the sensory nerve innervation of most of the lower extremities, pelvic floor and the sphincters (Cizkova et al 2001).
Cauda Equina syndrome is rare but serious and if it is of sudden onset, it is considered a medical emergency as if left untreated; it can lead to paralysis and loss of function of the lower limbs. Despite its serious nature, it can be easy to miss this syndrome due to its symptoms such as low back pain, bowel and bladder problems being very common and associated with a variety of causes. Therefore it is not surprising that many litigation issues have arisen in this area of medicine where clinicians have missed this syndrome. It is imperative that not only orthopaedic and neurosurgeons be aware of this, but anyone who may come in contact with patients presenting with this syndrome as timing is critical to avoid long term neurological damage. To fully comprehend this syndrome first the anatomy of the cauda equina region and thens the causes and symptoms will be discussed.
Anatomy of the Back
The vertebral column has many vital functions. It protects the spinal cord and spinal nerves, supports the weight of the body, provides an axis for the body and a pivot for the head and also plays an important role in posture and locomotion (Moore and Dalley 1999).
The vertebral column is made up of 33 vertebrae; 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal vertebrae. The sacral and coccygeal vertebrae fuse forming the sacrum and coccyx respectively and so there is no movement between these vertebrae. The highest mobility occurs in the cervical and lumbar regions, particularly the latter. The lumbar area is also subject to large forces and stresses during locomotion. Gravity acts directly downwards whilst the ground reaction force acts in the opposite direction and is of equal magnitude to the force the body applies to the floor (Webb 2009). Therefore it is not surprising that the lumbar region is the most common source of back pain and the most frequently injured area of the spine (Moore and Dalley 1999).
A typical vertebra consists of a vertebral body which is the anterior, larger, weight bearing component of the bone (Drake et al 2005) and a vertebral (neural) arch which is posterior to the body (Figure 1). The vertebral bodies of adjacent vertebra are separated and bound together by fibrocartilaginous intervertebral discs.
Figure 1 Axial and Side View of a typical vertebra (DeLisa and Stolov 1981)
Successive vertebrae bear increasing amounts of body weight as the column descends. To accommodate for this, the vertebrae become progressively larger from C1-L5; L5 is the largest vertebral body (Middleditch & Oliver 2005). After this, the size of the vertebrae decreases as the body weight is transferred to the pelvic girdle at the sacroiliac joints.
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The vertebral arch is formed by the right and left pedicles and laminae (Figure 1). The two pedicles form the lateral pillars of the arch and attach the vertebral arch to the vertebral body whilst the laminae are flat plates of bone that project from the pedicles to join in the midline and form the roof of the vertebral arch (Moore and Dalley 1999).
There are seven processes which project from the vertebral arch of a typical vertebra; a spinous process, two transverse processes and four articular processes (Figure 1). These processes serve as attachment sites for muscles and ligaments and sites of articulation with adjacent vertebrae (Drake et al 2005).
The vertebral arch and the posterior surface of the vertebral body form the walls of the vertebral foramen whilst the roof is formed by the notches on the superior and inferior vertebrae (Figure 5). When all the vertebral foramina align as in the vertebral column, a bony vertebral canal is formed within which the spinal cord along with its protective membranes, blood vessels, connective tissue, fat and proximal parts of the spinal nerves lie (Drake et al 2005). The spinal cord extends from the foramen magnum to the level of L1/L2 below which the collection of nerve roots is known as the cauda equina, a region which will be discussed later on.
Intervertebral discs lie between adjacent vertebrae forming a cartilaginous joint which facilitates movement of the vertebrae by separating the two vertebral bodies and therefore allowing rocking movements to occur without the two coming in contact (Bogduk 1997). Each intervertebral disc consists of an outer annulus fibrosus and a gelatinous inner nucleus pulposus (Figure 2).
Figure 2 Intervertebral disc
The annulus fibrosus surrounds the nucleus pulposus and consists of concentric layers of collagen fibres which are bound together by a proteoglycan gel. The collagen makes the annulus fibrosus a turgid, relatively stiff body capable of bearing weight (Bogduk 1997).
The arrangement of fibres in the annulus fibrosus is such that it permits angular movements such as flexion, extension and lateral flexion whilst providing stability against shear and torsion (Middelditch and Oliver 2005). During rotation of an intervertebral joint, the fibres of the annulus fibrosus inclined towards the direction of rotation are strained whilst the other half are relaxed, this restricts the range of rotation (Bogduk 2007). The posterior portion of the annulus has a different arrangement of fibres, more tightly packed lamellae and less binding gel; consequently this area is weaker than the rest of the disc (Middleditch and Oliver 2005).
The other constituent of the intervertebral disc, the nucleus pulposus is a semi fluid gel which fills the centre of the intervertebral disc. However, its position varies throughout the vertebral column, lying slightly more posteriorly in the cervical and lumbar regions (Middleditch and Oliver 2005).
Although the annulus fibrosus is able to bear some weight, sustained weight will cause the collagen lamellae to buckle. Therefore the intervertebral disc requires an additional mechanism to enable it to bear sustained weight without prolonged deformation; this is provided by the nucleus pulposus. Its fluid nature means that it can deform under pressure without compressing its volume. When force is applied, it is transmitted to the outer annulus fibrosus which, due to its stiff collagenous nature, opposes the outward pressure exerted by the nucleus pulposus and balances it out (Bogduk 1997).
The resilience of the intervertebral disc also enables it to act as shock absorber so that if a force is applied rapidly to a disc, it will be briefly diverted into stretching the annulus fibrosus and so slows the rate at which the force is transmitted to the next vertebra (Bogduk 1997).
The water content of the nucleus pulposus is maximal at birth and decreases with age whilst the collagen concentration increases. This limits its ability to act as a shock absorber and transmit forces and so there is greater stress on the annulus fibrosus (Kishner et al 2009). When this is combined with the degenerative changes that take place, it accounts for the weakening of the the annulus fibrosus which can lead to it tearing and therefore allowing the nucleus pulposus to bulge out resulting in protrusion of disc material into the vertebral canal. This is known as a herniated disc and most commonly occurs on the posterior aspect of the disc and in the lumbar region of the spine. It can therefore impinge on the spinal cord and nerve roots giving rise to symptoms such as pain and bladder or bowel dysfunction (Ramachandran et al 2008) which will be further discussed later on.
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Figure 3 Herniated Disc (Deyo & Weinstein 2001)
As well as the annulus fibrosus restricting some movements, the anterior and posterior longitudinal ligaments which attach to the surface of the outer annular fibres help prevent hyperextension and hyperflexion of the vertebral column respectively (Figure 3). The posterior longitudinal ligament also helps to prevent posterior protrusion of the intervertebral discs. However, it is not as strong as the anterior longitudinal ligament and there is thinning of the posterior longitudinal ligament as it extends caudally which partly accounts for the higher frequency of disc herniations in the lumbar region (Baldwin & Horwitz 2009). Another reason for this may be because of the higher stresses and more movement which occurs at the lumbar region of the spine.
The ligamenta flavum is another spinal ligament; it is a paired short but thick ligament that is the strongest of the spinal ligaments. It connects the laminae of adjacent vertebrae and so helps to stabilise the spine as well as protect the spinal cord and its roots. (Middleditch and Oliver 2005). The ligamentum flavum is different to the other spinal ligaments as it is made of elastin rather than collagen. An elastin ligament is less likely to buckle when the laminae draw together. The ligamentum flavum lies immediately behind the vertebral canal (Figure 3) and so this property reduces the chance of nerve root compression by the ligament (Bogduk 1997).
Figure 4 Ligaments of the vertebral column
In development, the vertebral column grows faster than the spinal cord and so the spinal cord does not extend the full length of the vertebral canal in the adult (Drake et al 2005). The spinal cord is a continuation of the medulla oblongata and extends from the foramen magnum to the level of the L1/2 disc, its tapering inferior end is known as the medullary cone and may extend to T12-L3 (Middleditch, Oliver, 2005). This is also known as conus medullaris as in Figure 5. From the apex of the conus, a strand of connective tissue called the filum terminale (Figure 5) extends and attaches to the dorsal surface of the first coccygeal vertebra (Crossman & Neary 2005).
The spinal cord bears two enlargements where the nerves associated with innervating the limbs enter and leave the spinal cord. The anterior rami of the spinal nerves arising from the cervical enlargement (C4-T1) provides innervation for the upper limb via the the brachial plexus (Moore and Agur 2007). Whilst the anterior rami of the spinal nerves arising from the lumbar enlargement (Figure 5) contribute to the lumbar plexus (L1-L4) and the sacral plexus (L4-S2) which are associated with innervation of the lower limb (Crossman & Neary 2005).
Due to the discrepancy in the lengths of the spinal cord and vertebral canal, below the cervical region, the spinal nerve roots course increasingly obliquely downwards to reach their respective intervertebral foramina. This is especially apparent in the lumbar and sacral nerve roots which descend below the termination of the spinal cord at L1/L2 forming the collection of nerve roots resembling a 'horse's tail' which is therefore known as the Cauda Equina (Crossman and Neary 2005). The nerve roots of this region are particularly susceptible to injury which can lead to symptoms associated
Figure 5 The spinal cord
Spinal cord & Its Membranes
The spinal cord and its associated nerves are of great importance as they receive afferent fibres from sensory receptors of the trunk and limb and control movement of these areas, and provide autonomic innervations for most of the viscera (Crossman & Neary 2000).
Each spinal nerve arises from the spinal cord and is formed by the union of a dorsal and ventral nerve root of which the dorsal contains afferent (sensory) fibres and the ventral contains efferent (motor) fibres. The spinal nerves exit the vertebral canal through the intervertebral foramina which are small spaces surrounded by bone and ligament. They are bounded by the ventral intervertebral joint and the dorsal zygapophysial joint (the joint between the articular processes) (Bhagia et al 2002). The roof of an intervertebral foramen is formed by the inferior vertebral notch on the pedicle of superior vertebra and the superior vertebral notch on the pedicle of the inferior vertebra (Badawi et al 1988) (Figure 4). In such a confined space, if there is any pathology such as herniation of an intervertebral disc, it can cause compression of the nerve passing through that particular intervertabral foramen.
Figure 6 Left view of the lumbar vertebral column A. Micheau, MD - E-anatomy
The spinal cord is lined by three membranes: the dura mater, the arachnoid and pia mater (Figure 6). These, together with cerebrospinal fluid (CSF) and the vertebral column, protect the spinal cord. The outermost dura mater is a tough fibrous membrane that is separated from the vertebra by the epidural space. It is continuous with the dura of the brain. The arachnoid mater is more delicate and is separated from the dura by the subdural space and separated from the pia by the subarachnoid space. It is in this space within the dural sac that CSF is found and the cauda equina lies (Cizkova et al 2001).
Figure 7 Spinal cord and its membranes (eOrthopod)
The cauda equina region of the spine is of particular interest as, when it is injured, it can give rise to a collection of symptoms which are associated with Cauda Equina Syndrome. A syndrome which is considered to be a clinical emergency if there is acute presentation. To fully appreciate this syndrome, firstly the formation of the cauda equina region and reasons why it is particularly susceptible to injury will be discussed, drawing from the knowledge of the anatomy which has been covered.
Formation of the Cauda Equina
In the human embryo, the cauda equina begins to develop soon after the post-somite phase at the beginning of the third month (Cizkova et al 2001) (Figure 7). In the embryonic period, the central nervous system (CNS) grows faster than any other organ system whilst in the fetal period, the vertebral column grows faster (Drews 1995). The resultant difference in growth leads to the spinal nerves shifting cranially in relation to the vertebral column and all the spinal nerves below the upper cervical region take a progressively more oblique direction. This results in the lumbar and sacral nerves passing almost vertically down within the dura before exiting from their foramina (Cizkova et al 2001), the roots of these spinal nerves which descend in the dura are called the cauda equina. At birth, the spinal cord terminates at L3 whilst in an adult it terminates at L2 due to continued growth of the vertebral column. Below L2, it continues into the filum terminale (Figure 7).
Figure 8 Development of the Cauda Equina (Drews, 1995)
Susceptibility of Cauda Equina to injury
As previously mentioned, there is a difference in strength between the posterior region of the intervertebral disc and the rest of the disc. This difference is accentuated in the lower lumbar region as the anterior and lateral regions of the disc are approximately twice as thick as the posterior region, thereby predisposing it to degenerative changes and trauma.
Another difference in the lumbar region of the vertebral column (where the cauda equina lies) is that the posterior longitudinal ligament which protects the nucleus pulposus from protruding is thinner in here than in other spinal regions (Baldwin & Horwitz 2009). As the posterior longitudinal ligament helps to prevent posterior disc protrusions, it is not surprising that approximately 90% of disc herniations occur in the lumbar/sacral region (Gleave and Macfarlane 2002). A massive midline disk herniation of the cauda equina region where the disc material protrudes centrally and impinges on the nerve roots can lead to the symptoms associated with cauda equina syndrome.
As well as a weaker spinal ligament in this region, the cauda equina is also more susceptible to injury due to its sparse connective tissue network and differing microanatomy in terms of its blood supply compared to the peripheral nerves (Rydevik 1993).
Although it is protected by the vertebral canal and meninges, the cauda equina does not have the same connective tissue protection as the peripheral nerves. Peripheral nerves are fairly resilient as they are covered by three connective tissue layers. These are the endoneurium, the perineurium and the epineurium. The epineurium is the outermost layer and is a thick sheath of loose connective tissue that surrounds and encloses the nerve bundles, it protects against compressive stresses on the nerves (Moore and Dalley 1999). The nerve roots of the cauda equina are particularly susceptible as they have a poorly developed epinerium (Qureshi et al 2010). Each nerve bundle is instead protected and bound by a layer of pia mater which allows movement of the nerve roots which is required of the lumbar region but means that these nerves in particular are vulnerable to injury (Parke et al 1981).
The blood supply to the cauda equina is derived from central and peripheral sources but there is no segmental supply of this region unlike in the peripheral nerves (Rydevik 1993). Additionally, there is a region of relative hypovascularity at the proximal portion of the cauda equina as demonstrated by Parke et al (1981). Therefore a change in the blood supply to this region may be of greater significance than in other regions of the cauda equina (Parke et al 1981). For example, a change in blood supply is seen in spinal stenosis where there is narrowing of the vertebral canal, and especially in a two level stenosis which can cause an increase in pressure such that blood flow is reduced giving rise to a nutritionally impaired region between the two compression sites as there are no regional arteries which could compensate. This can give rise to considerable blood pooling with a build up of metabolites and lack of oxygenated blood leading to damage to these nerve roots and therefore symptoms associated with cauda equina syndrome (Porter 1993).
However, the arterial supply is not the principal source of nutrition for the nerve roots, there is an increased vascular permeability and diffusion from the surrounding CSF to the cauda equina which allows nutrients to be absorbed directly (Bogduk 1997). However this increased permeability may result in the tendency towards oedema formation of the nerve roots which can complicate what would have otherwise been minor injuries (Beeson 2009).
CAUDA EQUINA SYNDROME
Cauda equina syndrome is caused by significant narrowing of the vertebral canal compressing on the cauda equina nerve roots. Although a rare condition occurring in less than 1 in 2000 patients with severe low back pain (Bavetta et al 2004), when cauda equina syndrome presents acutely, it is considered a surgical emergency as it can lead to loss of bowel and bladder control and paralysis of the legs if left untreated (Eck 2007). The term Cauda Equina Syndrome is used to describe the collection of symptoms when one or more of bowel, bladder or sexual dysfunction or reduced sensation in the saddle area is present (Fraser et al 2006). These symptoms present as the cauda equina region provides sensory and motor innervation to most of the lower extremities, the pelvic floor and the sphincters (Cizkova et al 2001).
Multiple causes of the narrowing which impinges on the cauda equina region to give rise to the symptoms of cauda equina syndrome have been found and can be separated into compressive and non compressive etiliogies. Compressive causes of cauda equina syndrome include herniated lumbar intervertebral discs, spinal stenosis and spinal neoplasms whilst non compressive causes include inflammatory conditions, iatrogenic causes and infections (Storm et al 2002).
COMPRESSIVE ETIOLOGIESFull-size table
A herniated disc can be caused by general wear and tear of the disc or injury to the spine causing weakening of the annulus fibrosus. With age, the nucleus pulposi lose their turgor and become thinner due to dehydration and degeneration causing deterioration of the annulus fibrosus. Also, trauma or additional stress can cause the annulus fibrosus to tear and lead to protrusion of the nucleus pulposus. (Moore and Dalley 1995).
Most disc herniations occur postero-laterally as this is where the annulus fibrosus and posterior longitudinal ligament are particularly thin especially in the lumbar region. Also, flexion of the vertebral column pushes the nucleus pulposus further posteriorly. If there has been degeneration of the annulus fibrosus and posterior longitudinal ligament, the nucleus pulposus may herniate into the vertebral canal and compress the spinal cord or nerve roots (Parke et al 1981).
Disc herniations which cause impingement on the nerve roots L5 or S1 give rise to sciatica. This is the term used for symptoms including pain in the lower back and hip radiating down the back of the thigh and into the leg (Moore and Dalley 1995).
Unilateral or bilateral sciatica is one of the symptoms of cauda equina syndrome (Cizkova et al 2001). Like sciatica, the most common cause of cauda equina syndrome is intervertebral disc herniation. Indeed, this syndrome seems to be more prevalent in men in their fourth or fifth decade of life as they are the most prone to disc herniations (Shapiro 2000).
Although most disc herniations occur postero-laterally, cauda equina syndrome is usually due to a large central lumbar disc herniation (James et al 2009) as this protrudes into the central part of the canal where it is likely to impinge on a greater number of nerve roots (Figure 8). When this occurs in the particularly vulnerable cauda equina region of the spinal cord it can lead to the symptoms associated with cauda equina syndrome (Parke et al 1981).
This syndrome accounts for 1%-16% of cases of lumbar disc herniations (Gleave and Macfarlane 2002) and approximately 90% of disc herniations involve the discs between the L4-S5 or the L5-S1 levels (Gleave and Macfarlane 2002).
Large amounts of disk material can extrude and take up at least one third of the canal diameter (Eck 2007). Cauda equina syndrome from lumbar herniated discs is considered to be the only absolute indication for surgery (Elwood et al 2009) and most surgeons feel that the spine should be decompressed within 48 hours of onset and diagnosis (Gitelman et al 2008) to prevent permanent damage.
Figure 9 Lavy et al 2009| Left: MRI scan showing compression of the cauda equina (arrow) due to a large posterior disc herniation at L4/5.
Right: MRI scan showing a large disc herniation at
L5/S1 (arrow) bulging posteriorly and compressing the cauda equina.
Spinal stenosis is narrowing of the vertebral canal which can be caused by congenital abnormalities such as dwarfism or it can be acquired which includes degenerative processes such as herniated intervertebral discs (Storm et al 2002). In spinal stenosis, which most commonly affects the L4-5 lumbar vertebrae (Strömqvist 2006), there is thickening of the ligamentum flavum causing narrowing of the canal which can impinge on the cauda equina nerve roots (Lavy et al 2009). Patients with both a congenital spinal stenosis and a herniated disc are likely to develop cauda equina syndrome as even a small herniated disc can limit the space for the nerve roots, causing impingement of them (Eck 2007). It has been suggested that stenosis at two levels of the vertebral canal is more likely to produce the symptoms of cauda equina syndrome (Cizkova et al 2001) as a one-level compression will affect only a small section of the nerve root at the site of the stenosis, with adequate arterial supply and venous drainage on both sides of the lesion. However, a two-level compression, would affect a much larger segment of the nerve root causing venous congestion and pooling of all the roots of the cauda equina between the blocks (Cizkova et al 2001).
A tumour or neoplasm is the second most commonly identified pathology resulting in cauda equina syndrome (Fraser et al 2009). It can compress on the nerve roots of the cauda equina and can be a primary or metastatic spinal neoplasm. Primary neoplasms are those which remain in the original site from where they arose therefore there are specific tumours which occur in the specific regions of the spinal cord. These include myxopapillary ependymomas, paraganglipmas and shwannomas.
Of the primary tumours that arise from the conus medullaris or the filum terminale, the most common one is the myxopapillary ependymoma (Bagley and Gokaslan 2004) however tumours of this region account for less than 6% of all spinal tumours (Fearnside and Adams 1987). An ependymoma is a tumour of the ependymal cells which are the epithelial cells that line the ventricles of the brain and the central canal of the spinal cord. They are a type of glioma which is the term used to describe tumours arising from glial cells (such as ependymal cells) in the brain or spine (Cotran et al 1999).
Myxopapillary ependymomas are a subclass of ependymomas; they are slow growing gliomas which occur most commonly in the cauda equina , originating from the conus medullaris and filum terminale regions for which they have a high affinity (Meneses et al 2008) (Figure 8).
There is a high incidence of myxopapillary ependymomas in young adults. Due to the slow growing nature of these tumours, patients present with a long history of back, leg, or sacral pain and weakness, or sphincter dysfunction (Bagley and Gokaslan 2004).
Figure 10 Left: Surgical operating microscope view of the filum terminale ependymoma after opening of the dura mater. Right: Magnified view depicting the filum terminale next to the tumour
Paragangliomas are similar to ependymomas in their location, encapsulation and arrangement of cells (Mardi and Thakur 2009) and are not easy to differentiate until surgical intervention. Paragangliomas are benign, slow growing neuroectodermal tumors (Sankhla and Khan 2004) that are widely distributed throughout the body. Spinal paragangliomas are relatively uncommon but when they do occur, they usually present as intradural tumours in the cauda equina region (Makhdoomi et al 2009).
Schwannomas of the conus medullaris or filum terminale region are another type of primary tumour known to cause cauda equina syndrome. The transition zone from the central (Schwann cells) to peripheral (oligodendricytes) myelin is known as the Obersteiner Redlich zone and it is in this region that schwannomas are likely to occur.
Figure 11 Schwannoma (Bagley and Gokaslan 2004)
Schwannomas are slow growing benign tumours that usually present with similar symptoms to disc herniation (Osbourne 1994) and are relatively easy to remove due to their well circumscribed nature (Figure 9).
Other rarer types of primary tumour which have also been reported to cause cauda equina syndrome include dermoid tumours, hemangioblastomas, lipomas and astrocytomas (Harrop and Schneiderman 2009).
Metastatic or secondary tumours are those which have spread from other parts of the body. They comprise the bulk of all spinal tumours however, of those occurring in the cauda equina region, primary tumours are more common (Bagley and Gokaslan 2004).
Metastatic spine tumours are most commonly from the lung and breast (Eck 2007). They can involve bone, be intradural, epidural or both. If there are signs of severe pain preceding motor weakness this indicates epidural compression (Gilbert et al 1978), whilst muscle weakness and sphincter dysfunction with little pain points to an intramedullary metastatic location (Grem et al 1985). However, intramedullary metastases of this region are rare (Bagley and Gokaslan 2004).
Lung tumours usually spread to the thoracic region with compression at the level of the cauda equina being relatively rare. Likewise, breast cancer can also present with diffuse metastatic cancer in the cauda equina region (Bagley and Gokaslan 2004).
NON COMPRESSIVE ETIOLOGIES
The inflammatory causes of cauda equina syndrome include Paget's disease and Ankylosing spondylitis cause narrowing of the spinal canal leading to CES (Eck 2007).
Paget's disease is a chronic metabolically active bone disease where there is abnormal bone remodelling due to dysfunction of osteoblasts and osteoclasts (Dell'Atti et al 2007). The vertebra is the most common site of Paget's disease after the pelvis.
Cauda equina syndrome from Paget's disease is caused not by gross external deformity of the spine but is due to slow progressive narrowing of the vertebral canal from enlargement of the vertebral bodies (Klenerman 1966). Causing compression of the nerve roots of the cauda equina, leading to the symptoms associated with cauda equina syndrome.
Ankylosing spondylitis is a chronic inflammatory arthritis that mainly affects the lower back, neurological complications of long standing ankylosing spondylitis include lumbosacral or thoracic nerve root lesions, injury to the spinal cord due to the susceptibility of the rigid spine to trauma and cauda equina syndrome amongst others. The latter is a rare but disabling complication of advanced ankylosing spondylitis (Sant and O'Connell 1995). Although the pathophysiology isn't entirely clear, numerous cases have reported that spinal arachnoiditis, which is where the arachnoid membrane becomes inflamed, in ankylosing spondylitis to be the cause of cauda equina syndrome. The arachnoiditis may be the result of an inflammatory response in ankylosing spondylitis to the meninges (Mitchell et al 1990). Findings have shown that in patients with long standing ankylosing spondylitis complicated with cauda equina syndrome and spinal arachnoiditis there is an enlarged caudal dural sac with dorsal arachnoid diverticula, which may erode the lamina and spinous processes of the bony lumbosacral spine (Sant and O'Connell 1995).
A spinal epidural abscess (Figure 10) is an accumulation of pus in the epidural space that usually occurs in the posterior epidural space of the thoracic or lumbar regions (Rubin 2007). If there is an abscess in the cauda equina region that is left untreated, it can impinge on the spinal cord and present with sensory and motor symptoms and signs consistent with cauda equina syndrome (Huff 2009). Symptoms of severe back pain with rapidly worsening muscle weakness are characteristic of spinal epidural abscess (Cizkova et al 2001).
Figure 12 Spinal epidural abscess (Huff 2009)
Other infectious causes reported to have caused cauda equina syndrome include vertebral osteomyelitis and diskitis. Vertebral osteomyelitis is an infection of the vertebral bodies that most commonly affects the lumbar area whilst diskitis is an infection of the nucleus pulposus of intervertebral discs (Harrop and Schneiderman 2009).
It is possible that cauda equina syndrome can be the result of medical intervention which is often preventable. Therefore it is imperative that great care is taken to ensure that procedures where the patient is at risk of developing such a debilitating disease as cauda equina syndrome are undertaken in the least risky manner possible.
One of these is spinal anesthesia, neurological complications following this have serious consequences such as spinal cord ischaemia, arachnoiditis and cauda equina syndrome (Loo and Irestedt 1999). Spinal anaesthesia is administered into the subarachnoid space to allow surgical procedures to take place without causing pain to the patient. It can cause cauda equina syndrome indirectly through the development of a haematoma in the subdural or epidural space which can compress the nerves of the cauda equina or directly due to local anaesthetic neurotoxicity (Ã-zgen et al 2004).
There is particular concern over the potential effect of hyperbaric 5% lidocaine in causing cauda equina syndrome due to a number of cases reporting this (Auroy et al 1997). These reports propose that the cause of cauda equina syndrome in these cases is sacral orientation of the catheter which results in restricted distribution and overdose of a concentrated hyperbaric local anesthetic agent (Rydevik et al 1991). Loo and Irestedt (1999) recommend that hyperbaric lidocaine should be administered in concentrations not greater than 2% and a total dose preferably not exceeding 60 mg to avoid any potential risks associated with it. Other iatrogenic causes of cauda equina include poorly positioned screws in the spine which can compress the spinal nerves (Eck 2007) and following a lumbar puncture.
Cauda equina syndrome is a very rare disorder with fewer than 1 in 2000 clinical presentations in patients with low back pain (Bavetta et al 2000). Also, it presents with symptoms similar to those of other disorders such as herniated discs so it can be difficult to diagnose.
Clinicians must therefore be well aware of the 'red flag' symptoms which can distinguish cauda equina syndrome from other diseases. Red flags are 'key historical and clinical clues that increase the likelihood of a serious underlying disorder' (Arce et al 2001) and should be recognised in both the clinical examination and history of a patient to help guide the clinician to identifying the syndrome as well as its cause.
The red flag symptoms of cauda equina syndrome include saddle anaesthesia, recent onset of bladder and bowel dysfunction and motor weakness of the lower extremities (Kinkade 2007). Saddle anaesthesia is a pattern of sensory loss restricted to the medial buttocks and perianal area and is typically associated with cauda equina syndrome (Fowler 1999). It is imperative that all of these symptoms are well known by anyone to whom these patients may present so that when they do present, immediate action is taken to minimise the risks of permanent neurological damage. As well as hospital clinicians, general practitioners must be fully aware of this syndrome. In a time period of five years (2003-2007), more than two thirds of the cauda equina syndrome cases reported to the Medical Protection Society (MPS) were in relation to General Practice issues (Gardner & Morley 2009).
The most common cause of cauda equina syndrome is large acute central disc herniations impinging on the cauda equina nerve roots usually presenting with bilateral sacral, buttock and posterior leg pain with sphincter dysfunction. On examination, weakness in the gastrocnemius, hamstrings and gluteal muscles is usually shown as these are innervated by S1 and S2 which may be the most damaged owing to their location near the central of the cauda equina. Loss of the bulbocavernous reflexes may also be seen on examination as well as sensory loss from the soles of the feet to the perianal region (Fowler 1999).
As well as loss of the bulbocavernous reflex, other reflexes which may be diminished in cauda equina syndrome include the patellar, ankle, detrusor and cremasteric reflexes (Fraser et al 2009) and so lower limb reflexes should be examined.
Another symptom in cauda equina syndrome is sexual dysfunction, however, although patients may be reluctant to offer this information, it is vital for clinicians to inquire regarding this as sometimes other symptoms such as bowel dysfunction may not be evident and so a diagnosis may have to be made based on these other findings (Fraser et al 2009).
Red flags may also be present in the patient's history and include recent injury to the back, recent spinal surgery, recent severe infection or a history of cancer. In addition, approximately 70% of patients with cauda equina syndrome present with a history of chronic back pain (Shapiro 2000).
According to Della-Giuistina (1999), initial evaluation of urinary retention, saddle anaesthesia, rectal tone and bulbocavernous reflex are the most important to assess and failure of assessing these is the most common cause of litigation due to missed cauda equina syndrome.
Also, to avoid missing diagnosing cauda equina syndrome, patients who present with the red flags should have an emergency MRI scan performed in order to confirm the suspected diagnosis and to localise the lesion (Bin 2009).
If the MRI confirms a compression on the cauda equina region which could be due to a herniated intervertebral disc or spinal stenosis; emergency spinal decompression should be performed. The exact timing of the surgery is the subject of controversy; most researchers believe that surgery within 48 hours of onset is best to prevent permanent neurological damage (Bin 2009).
McCarthy et al (2007) found no significant difference between timing of surgery and outcome for the patient. Whereas Shapiro et al (2000) found that if early decompression within 48 hours was performed, this led to reduced chronic sciatica and good sexual potency compared with later decompression. Ahn et al (2001) also found better outcomes in sensory, motor, bladder and bowel functions in patients who underwent decompression within 48 hours compared to those after. The general consensus is that there is less risk to the patient of permanent neurological damage if surgery is performed within 48 hours of symptoms onset.
The treatment for cauda equina syndrome is directed to its cause, so if the cause is a tumour in the spine, radiotherapy is usually commenced to try and destroy the cancer cells and decompress the nerve roots (Arce et al 2001). However the exact treatment will usually depend on the type of tumour taking factors such as its location and stage of progression into account.
Vasodilatory treatment is another form of treatment and is used for the nerve root ischemia which may be responsible for the symptoms of pain and motor weakeness associated with cauda equina syndrome. It has been advised that mean arterial blood pressure should be maintained above 90 mm Hg to maximize blood flow to the spinal cord and nerve roots (Eck 2007). After systemic treatment of cauda equina syndrome with vasodilatatory agents, patients reported significant relief of lower extremity pain. However, it should be noted that these patients did not have severe cauda equina nerve root compression, for whom this treatment is not be that beneficial (Ordenacova et al 2001).
If a definitive cause is identified early on and treatment is carried out accordingly and without delay, patient suffering from cauda equina syndrome should recover. The relief of the symptoms of pain, sensory and motor dysfunction usually occurs first whilst recovery of bladder, bowel and sexual dysfunction takes longer and is often incomplete. Full recovery really depends on the etiology of the syndrome and when the symptoms began (Qureshi 2010). To give the patient every chance possible of minimal morbidity, it is imperative that the clinician does not miss the diagnosis of cauda equina syndrome and begins the appropriate treatment quickly; however, all too often this is not the case.
Medico legal implications
Failure to diagnose cauda equina syndrome can result in serious morbidity such as loss of bladder, bowel, and sexual function and therefore has the potential for serious medicolegal consequences. The claims made and large settlement fees paid are disproportionate when compared with its extremely low incidence in the population. The percentage of reports becoming claims for cauda equina syndrome is 65%, compared to 25% of all UK cases becoming a claim (Markham D 2004).
In a period of five years (2003-2007) it was reported by the MPS that of the concluded cases of cauda equina syndrome, damages were paid in 55%, with an average payment of £117,331 per case representing a total payout of £1,290,641 over this time period (Gardner & Morley 2009).
Claims may relating to delayed diagnosis are based upon the reasoning that treatment within 48 hours is necessary for a good prognosis (Ahn et al 2001). Some of the major causes of delay in diagnosis are GPs not recognising the urgency of red flag symptoms and A&E staff missing the diagnosis (Gardner & Morley 2009). Lavy et al (2009) reported that the average delay to diagnosis was 67 hours and to treatment 6.14 days in 22 medical negligence cases relating to cauda equina syndrome. These delays were attributed to orthopaedic surgeons in 32% of cases and to GPs in 18%. All of the patients involved had moderate or severe bowel and genitourinary symptoms and most had persisting severe back pain (Lavy et al 2009).
The level of expenses and number of claims reflects the distressing nature of this syndrome and so it is vital that general practitioners, NHS Direct advisers, triage nurses and medical staff within A&E departments are fully aware of the signs and symptoms as well as the complications of cauda equina syndrome which may arise and the need for urgent investigation and treatment when it presents (Markham 2004).
The timely diagnosis and treatment of cauda equina syndrome is essential to minimise morbidity in patients and provide them with the best chance of recovery. To achieve this, red flags must be ascertained in both the clinical examination and past medical history of the patient. This will also minimise the risk of litigation against the healthcare team which is unusually high in cauda equina syndrome considering its low incidence. This therefore suggests that clinicians are currently not as aware of this syndrome, its causes signs and symptoms as they should be.