CLINICAL RISK REDUCTION
Cauda equina syndrome
Consultant neurosurgeon Robert Macfarlane highlights the need for early diagnosis and treatment to avoid irreversible nerve damage in cauda equina syndrome
T
HE diagnosis and management of cauda equina syndrome (CES) can be fraught with potential difficulties. Back pain and sciatica are common conditions, but an average GP will
probably encounter only one or two cases of CES in their professional lifetime. A patient in pain from a disc prolapse may have difficulty
passing urine purely for mechanical reasons. Te analgesics used in treatment almost invariably cause constipation. Tis situation is entirely different from CES where, instead of a lumbar disc protruding to one or other side of the spinal canal and compressing nerve roots to the lower limbs, it prolapses centrally. Here it impinges on the nerves subserving sensation to the saddle region, bladder, urethra and rectum, as well as the parasympathetic motor innervation to the bowel and bladder. It is critical to diagnose CES at an early stage because these
nerves have characteristics which make them both vulnerable to injury and unlikely to recover from a severe insult. Firstly, they comprise small myelinated and unmyelinated nerves which are less resilient to compression than larger fibres. Secondly, because compression occurs proximal to the cell body, axons will not regenerate if Wallerian degeneration develops. CES may be subdivided into two categories. At first there is
impairment of bladder/saddle sensation and difficulty with micturition, but the patient remains continent (CESI – an incomplete lesion). Te syndrome becomes complete when the bladder is no longer under voluntary control and the patient has painless urinary retention with dribbling overflow incontinence (CESR). At the outset the patient will be constipated through loss of the parasympathetic innervation to the descending colon, even although anal tone may be lax. Faecal incontinence is generally a very late sign in CES and its absence should not be regarded as reassuring. Although there remains controversy regarding management of
CESR, many studies have concluded that, once this state is reached, the opportunity has been lost to reverse the situation by
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emergency decompression. In contrast, the outcome for CESI is usually favourable; therefore it is important to achieve decompression before the patient has progressed to CESR. Any perceived delay in diagnosis and treatment, or failure to warn the patient of the need to seek urgent attention should CES symptoms develop, may lead to allegations of negligence.
Differentiating CESI A detailed history is needed to differentiate between CESI and bladder disturbance secondary to pain and constipation. Te patient in pain who is having difficulty voiding purely for mechanical reasons is aware that the bladder is full, retains the desire to micturate, has normal sensation in the saddle region, and a tender bladder. Urethral sensation is preserved and the patient can differentiate flatus from faeces. In contrast, the patient developing CES will experience some or all of the following:
• altered saddle and/or urinary sensation • perineal/rectal pain • reduced awareness of bladder filling • the need to strain to maintain urine flow.
On abdominal palpation the bladder may be distended but not
tender. Saddle sensation may be reduced to light touch and/or pinprick. In the early stages, anal tone will remain normal. Unfortunately, the distinction between the two is not always
clear. Some patients will complain of altered saddle sensation but an MRI will show no compression. Conversely, a person with CESR may remain continent by toileting regularly to avoid over-distension of the bladder, and micturate by straining or applying lower abdominal pressure. Although the presence of bilateral sciatica is well-known as a ‘red flag’ for CES, many cases will only ever have unilateral sciatica. Very occasionally, an L5/S1 central disc may compress the cauda equina without involving the laterally-placed nerve roots. CES can therefore occur without sciatica. Neither is report of an improvement in back pain/ sciatica always reassuring. When the disc fragment migrates centrally, pressure may be relieved from the laterally-placed nerve roots. Tis results in relief of sciatica at the time that CES occurs. If doubt exists about the diagnosis, the only way in which this can be resolved is by emergency MRI.
SUMMONS
PHOTOGRAPHS: SCIENCE PHOTO LIBRARY
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