Is the Rectal Exam Useful in Cauda Equina Syndrome?

I consulted recently in a medical malpractice case involving a patient who developed cauda equina syndrome (CES), a compression of the nerve roots in the lower spinal canal that can cause permanent neurologic and bladder or bowel dysfunction. The patient presented to an emergency department (ED) with low back pain. The patient was ambulatory and had no neurologic symptoms (eg, weakness, sensory disturbance, or gait disturbance) or urinary symptoms. A rectal examination was not performed. Unfortunately, his symptoms progressed a week later and included urinary hesitancy as well as bilateral lower-extremity pain and weakness. He then re-presented to an ED, had MRI of his lumbar spine, and he was found to have CES. A lawsuit ensued. The contention was made by the plaintiff expert that a rectal examination should have been performed during the first visit and that it likely would have demonstrated decreased rectal tone, which would have led to an emergent MRI and an earlier diagnosis of CES.This was not the first time I have heard others arguing for evaluation of rectal tone in patients with low back pain to determine the need for an MRI to rule in or rule out CES (although in these other cases, the argument for rectal tone assessment was always in the presence of other concerning neurologic symptoms as well — not purely in a patient with isolated back pain). I was taught many years ago that the rectal exam was a critical part of the assessment of CES, and I am sure that many others have been taught the same. But how reliable is rectal tone in determining the need for MRI testing? How useful are the other “classic” historical and examination features that we were taught in terms of predicting CES? Given how commonly patients present to the ED with low back pain and how difficult it can be to obtain an MRI, it is very important for us all to know which presenting features in these patients truly are concerning and also which features are less useful.A retrospective study examining various history and physical examination findings (including the rectal exam) in patients with atraumatic back pain offers some useful insights. Although limited by its design, the study was well done and provides data that should inform how we evaluate patients with back pain and possible CES.Which Findings Raise Concern for CES? Angus and colleagues performed a 4-year retrospective evaluation of 996 patients with atraumatic back pain who were already selected for urgent lumbar MRI because of clinical concern for CES. A total of 111 (approximately 11%) of these patients had MRI-confirmed CES. They found that the most useful patient-reported symptom was the presence of bilateral radicular pain into the legs. Unilateral radicular pain was less useful. Anatomically, this makes sense: Unilateral radicular pain suggests impingement laterally on a nerve root, whereas bilateral pain localizes centrally to the spinal cord or cauda equina, until proven otherwise. Bilateral pain might also serve as an early indicator of central impingement even before urinary or motor deficits begin. Bilateral sensory loss in dermatomal distribution was also concerning for central impingement. The key word here, whether we are finding pain, sensory, or motor findings, is “bilateral.”The strongest predictor of CES was absent ankle or knee reflexes. Sensitivity was noted to be low (~ 16%) but specificity was very high (~ 95%). In other words, normal reflexes do not rule out CES, but absent reflexes are very concerning. One caveat: Older patients often do have diminished reflexes, and so the finding of absent reflexes should be assessed in the proper context.The authors pointed out that there were several history and physical exam features that were less useful than commonly assumed.Subjective complaints of bilateral weakness, in contrast to objective findings of weakness, were not independently predictive of CES. Rely on your exam and assess the patient’s ability to walk!The digital rectal exam was not useful. Decreased rectal tone was found at similar rates in patients with and those without MRI-confirmed CES, with a sensitivity of only about 35% and specificity of approximately 69%. Anal squeeze and perianal sensation were also not useful in discriminating between those with positive vs negative MRIs.Bladder findings were variable. Urinary difficulty, overflow incontinence, and postvoid residual (PVR) ≥ 200 mL were not predictive of CES, although bladder scan results were limited by incomplete documentation. Authors suggest that a low PVR should not be considered reassuring if there are other findings of concern. However, they suggest that urine retention should still be a cause of concern in the setting of low back pain or other concerning symptoms or findings.Why the Rectal Exam Falls Short The findings from this trial are somewhat limited by its retrospective design. However, they are rational and support that there is no single historical or examination finding that definitively rules in or out CES, and that any findings should be considered in the context of the patient’s overall presentation. A recent JAMA clinical review of low back pain confirmed that the key features of possible CES that should prompt immediate spine imaging are “new bladder or bowel dysfunction, saddle anesthesia, [or] persistent or increasing lower motor neuron weakness.” These should be assessed in the context of a good history and physical examination. Of note, the authors of the JAMA review did not include any recommendation at all to perform a digital rectal exam.Angus and colleagues’ finding that the rectal exam lacks utility is not an outlier. A recent study in the neurosurgery literature evaluated 1005 patients with suspected CES and found that reduced rectal tone was present in 35% of MRI-positive and 31% of MRI-negative patients, concluding that there was no association between abnormal rectal tone and MRI-confirmed CES. They also found no consistent association between perianal sensation and CES. Another study from the neurosurgery literature evaluated 57 patients that had MRI testing for CES, 23% of whom were positive for CES. In this study as well, the rectal exam did not discriminate between those patients who ruled in or out for CES, and the authors concluded that the rectal exam “cannot be used as a discriminator to ration urgent MRI scanning.” Tabrah and colleagues performed a meta-analysis evaluating six studies, including 662 patients of whom 92 had confirmed CES. They evaluated rectal tone, perianal sensation, and anal squeeze, and they found low diagnostic value for each of these. Specifically, regarding rectal tone, they suggested that normal tone can provide the clinician a false sense of reassurance, concluding that assessment of rectal tone “is not recommended in any clinical setting.”The lack of utility of rectal tone assessment is in part probably due to relatively late degradation of S2-S4 motor function (ie, decreased rectal tone is a late finding). In addition, clinicians’ accuracy in assessing rectal tone appears to be limited. Sherlock and colleagues evaluated the accuracy of 75 physicians from various specialties and levels of training in assessing rectal tone using a “model anus” setup that incorporated a pediatric sphygmomanometer cuff in order to model the anal sphincter. Physician accuracy at identification of decreased (vs normal) “rectal tone” was only 64%, and there was no difference in accuracy between the physicians and a control group of healthcare assistants who had no prior training in assessing rectal tone. When to Suspect Cauda Equina Syndrome and Order MRI After reviewing this literature, I suggest the following points when evaluating patients with low back pain and possible CES:Heighten your concern for CES when a patient has:New bilateral lower-extremity pain or objective weakness (assess the patient’s ability to walk whenever possible)New urinary symptoms, including hesitancy or overflow incontinenceNew bowel incontinenceSaddle anesthesiaDecreased lower-extremity reflexesAssessment of rectal tone and perianal sensation can be deferred, because they should neither increase nor diminish your concern based on the factors above.Do not delay imaging while awaiting more classic findings, such as overflow incontinence, decreased rectal tone, or objective lower-extremity weakness.By the way, in the malpractice case, both the patient and the physician ended up with good outcomes.Amal Mattu, MD, is a professor, vice chair of education, and co-director of the emergency cardiology fellowship in the Department of Emergency Medicine at the University of Maryland School of Medicine in Baltimore. 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