The goal of this project was to study the frequency and natural history of perioperative ulnar neuropathy.
A prospective evaluation of ulnar neuropathy in 1,502 adult patients undergoing noncardiac surgical procedures was performed. Patients were assessed with a standard questionnaire and neurologic examination before surgery, daily during hospitalization in the first week after surgery, and by telephone if they were discharged before 1 postoperative week. Patients in whom ulnar neuropathy developed were followed for 2 yr.
Ulnar neuropathy developed in seven patients (0.5%; 95% confidence interval, 0.2% to 1.0%). Six of the seven patients were men. Symptoms of ulnar neuropathy began 2-7 days after surgery. Manifestations were mild and confined to sensory deficits in six patients. Symptoms resolved in four patients within 6 weeks. The remaining three patients had residual symptoms 2 yr later.
In this surgical population, ulnar neuropathy was an infrequent complication. It occurred primarily in men who were 50-75 yr old and was not symptomatic until several days after surgery. Gender-dependent differences in the anatomy of the ulnar nerve and related structures at the elbow may serve as risk factors for ulnar neuropathy in patients having surgery.
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ULNAR neuropathy may develop in anesthetized patients who undergo surgical procedures. [1-4]This neuropathy often is mild and resolves spontaneously, but it can be severe and associated with prolonged or permanent disability. One retrospective study of patients having surgery found a frequency of severe motor disability from ulnar neuropathy of 1 in 3,506, suggesting that a sustained motor dysfunction from this complication may develop annually in as many as 5,000 patients having surgery in the United States. 
Male gender, obesity, preexisting asymptomatic ulnar nerve dysfunction, and prolonged hospitalization have been identified as possible risk factors for perioperative ulnar neuropathy. [1-5]The mechanism of ulnar neuropathy in the perioperative setting often is unclear, although improper positioning or padding of the upper extremity during surgery leading to compression of the ulnar nerve in the region of the medial epicondyle or cubital tunnel frequently is implicated. [6-10]Little direct evidence, however, supports this hypothesis, because perioperative ulnar neuropathy occurs infrequently and extensive anatomic and sensorimotor nerve testing of a large surgical population would be needed to determine accurately the timing of onset, precise location, and potential causes of ulnar neuropathy.
The current study had two purposes: first, to prospectively study a large cohort of a wide spectrum of patients having surgery using simple neurologic evaluations to determine the frequency and natural history of symptoms of ulnar neuropathy after surgery; and second, to identify a surgical population at high risk for development of perioperative ulnar neuropathy.
The Mayo institutional review board for studies involving human subjects approved the study. Patients 18 yr and older undergoing elective ambulatory or inpatient surgery requiring anesthesia services at the two Mayo hospitals in Rochester, Minnesota, from June through August 1995 were asked to participate. All major surgical subspecialties and procedures are routinely performed at both hospitals involved in the study. A total of 1,506 patients were enrolled during the 3-month study period. Persons with preexisting ulnar neuropathy were excluded. Any or all of the following findings defined the condition: current symptoms of paresthesias in the ulnar distribution, signs of abnormal two-point discrimination in the volar surface of the distal fifth digit, or weakness of the first dorsal interosseous and abductor digiti minimi muscles. The diagnosis of preexisting ulnar neuropathy was confirmed by one of our study's neurologists for all patients who had any of these signs or symptoms. Patients undergoing cardiac surgery were excluded because of the association of brachial plexus injury with sternal retraction and physiologic changes common to cardiopulmonary bypass. [11,12]Patients undergoing upper extremity procedures that would likely involve ulnar nerve manipulation (e.g., elbow arthroplasty) also were excluded.
Anesthetic services were defined as the use of general or major regional anesthetic or monitoring with intravenous sedation. Patients who received local anesthetic block of major nerves of the upper extremities were evaluated for the development of ulnar neuropathy after the normal expected duration of the local anesthetic block had passed. Patients who underwent upper extremity surgical procedures were assessed for ulnar nerve dysfunction that was likely to be unrelated to the procedure.
Potential study participants were identified on the day of their surgery and enrolled after they gave informed consent. All participants answered a short, standardized list of questions designed to confirm the presence of ulnar neuropathy before operation or to identify potential risk factors for the development of ulnar neuropathy associated with surgery. Specifically, research assistants asked about preexisting numbness or tingling of their hands; previous carpal tunnel surgery; neck problems; arm or wrist fractures; the presence of diabetes for which they took insulin or oral hypoglycemic agents; their current and past smoking history; and any family history of peripheral nerve problems. These assistants also performed a standardized baseline neurologic examination of the upper extremities in all participants. This screening examination included bilateral dynanometric assessment of strength in the flexor muscles of both forearms (i.e., grip strength) and two-point discrimination in the digits. Our four research assistants underwent a prestudy course in peripheral neurologic examination taught by one of our study neurologists. To monitor the consistency and accuracy of their examinations, this neurologist randomly examined five patients of each assistant during the first 3 weeks of the study and compared his results with those of the assistants. There was general agreement of the comparative examination results of all randomly selected patients.
Potential intraoperative risk factors for ulnar neuropathy (e.g., surgical procedure, use and location of automatically inflated blood pressure cuffs, arm positions, anesthetic type, and the duration of surgery) were recorded for each patient. The upper extremities and specifically the elbows of all patients were padded with foam or gel pads. After surgery, each participant was assessed in the postoperative recovery room and then daily until they were discharged or for as long as 7 days after surgery using a standardized questionnaire and screening neurologic examination designed to detect manifestations of ulnar nerve dysfunction. Those who were discharged before 7 days were interviewed by telephone 7-9 days after surgery with a standardized questionnaire designed to screen for possible ulnar nerve dysfunction. Postoperative surveillance was successful in all but 4 of the 1,506 patients enrolled in the study. We excluded data for these four patients from this report.
A single neurologist evaluated all patients who were identified with signs or symptoms suggestive of ulnar neuropathy. A diagnosis of ulnar neuropathy was determined by the neurologist and based on physical findings and symptoms. Nerve conduction studies and needle electromyography were used at the discretion of the neurologist to confirm the diagnosis and guide treatment. Patients with ulnar neuropathy were contacted at various postoperative intervals, and specifically at 1- and 2-yr intervals after surgery, to determine their long-term outcomes and disabilities.
Patient and procedural characteristics of participants who developed ulnar neuropathy were compared with patients who did not have neuropathy using the rank sum test or Fisher's exact test for continuous and discrete variables, respectively. When appropriate, exact 95% confidence intervals were calculated for the frequency of ulnar neuropathy. In all cases, two-tailed tests with P values <or= to 0.05 were used to indicate significance. All calculated measures are reported as mean values +/− SD.
Complete outcome data are available for 1,502 of the 1,506 enrolled patients. Table 1shows patient and perioperative characteristics. The most commonly performed surgical procedures included operations on joint structures (17% of all cases), the urinary bladder (8%), the skin and subcutaneous tissues (7%), and the abdomen (7%). The primary anesthetic technique was general anesthesia in 64% of patients, major regional anesthesia in 11%, and monitoring with intravenous sedation in the remaining patients. Nearly all major regional anesthetic blocks were spinal or epidural; only 11 of the 167 major nerve blocks were upper extremity blocks. The duration of anesthesia was 2.4 +/− 1.6 h. Upper extremity arterial catheters were used in 5% of all patients; automated blood pressure cuffs were used in all other patients.
Twenty-seven patients developed symptoms or signs that were suspicious for ulnar neuropathy within 7 days after surgery. After evaluation by a neurologist, 20 of these patients were determined not to have ulnar neuropathy. They were found to have carpal tunnel symptoms (16), median neuropathy at the wrist (2), brachial plexopathy (1), and symptomatic migraine with hemiparesthesia (1). All 20 patients who had carpal tunnel symptoms after operation had similar symptoms within 5 yr before their surgical procedures. The diagnosis of postoperative ulnar neuropathy was confirmed by the study neurologist in seven patients (1 in 215 patients [0.5%]; exact 95% confidence interval, 0.2% to 1%;Table 2, Table 3). Six of the seven patients were men, and the mean age was 60 yr (range, 50-74 yr). Their preoperative patient characteristics and perioperative characteristics were similar to the remaining patients in the study population. Three of the men in whom ulnar neuropathies developed were part of a group of 88 men (1:29) who underwent intraabdominal or intrapelvic procedures.
Five of the seven patients with postoperative ulnar neuropathy had unilateral paresthesias of the ring and little fingers without objective motor or sensory deficits evident on the neurologic examination. One patient had objective findings confined to sensory loss of the ring and little fingers, and mild (4/5) weakness of the first dorsal interosseous and abductor digiti minimi muscles and objective sensory loss of the little finger developed in another patient.
The onset of symptoms was documented readily through daily postoperative visits and telephone calls. The median onset of symptoms was 4 days (range, 2-7 days). Patients tended to underestimate the time to onset of their symptoms when questioned at a later date. For example, 8 weeks after their procedures two patients reported that their symptoms began “immediately” after surgery, although their study records documented an actual latency to symptoms of 4 and 6 days.
Electrophysiologic studies were performed on five of the seven patients with ulnar neuropathy from 6 to 16 weeks after surgery at a time when two patients were still symptomatic. One of the remaining two patients died within 6 weeks of his surgery, and the other chose not to participate in the study. The results of bilateral nerve conduction studies were normal in four of five patients. The remaining patient had nonfocal slowing of ulnar motor conduction velocity without focal conduction block. He had no ulnar sensory nerve potential on the symptomatic side.
Four of the seven patients having surgery, including the patient who died, had complete resolution of their symptoms within 6 weeks of surgery (Table 2). The symptoms of two of the remaining patients improved during the ensuing 26 weeks, but mild residual paresthesias persisted. These symptoms did not improve further during the next 18 months. The patient with the most severe neuropathy (a motor deficit) underwent an ulnar nerve submuscular transposition 8 weeks after his initial surgery. He noted slight symptomatic improvement during the subsequent 8 weeks but no further improvement during the following 20 months.
Compared with patients without neuropathy, those in whom ulnar neuropathy developed had a higher mean body weight (93 +/− 17.8 kg versus 77.1 +/− 18.7 kg, respectively; P = 0.019). The frequency of ulnar neuropathy, however, was significantly (P = 0.048) higher for men (6 of 653 [0.9%]; exact 95% confidence interval, 0.3% to 2.0%) than for women (1 of 849 [0.1%]; exact 95% confidence interval, 0.0 to 0.6%). When the body weight analysis was restricted to men, there was no significant difference in weight between those in whom ulnar neuropathy developed and those in whom it did not (93.5 +/− 19.5 kg versus 84.9 +/− 16.1 kg, respectively; P = 0.264). No other patient or procedural characteristic was found to be associated with ulnar neuropathy. The sample size of only seven cases, however, precludes definitive statements regarding the identification of risk factors.
Our finding that patients having surgery commonly are free of symptoms or signs of ulnar nerve dysfunction until 2-7 days after surgery suggests that postoperative ulnar neuropathy may be caused by factors other than improper patient positioning and padding of extremities during surgery. This finding may explain why the frequency and severity of postoperative ulnar neuropathy has not changed significantly in the past two decades despite widespread attempts by anesthesia providers to avoid positions and extremity placements that may stretch or compress the ulnar nerve during surgery. [1,3,5]
This study was performed as an intermediate step in a process to identify the etiologic factors associated with perioperative ulnar neuropathy. Retrospective studies and this and other prospective studies clearly show that (1) patient characteristics are associated with the problem and (2) simple neurologic screening tests and survey tools (e.g., two-point discrimination, grip strength) are insufficient to provide accurate data on either the timing of onset or specific nerve location of perioperative ulnar neuropathy. Importantly, the current study identified a group of high-risk patients and procedures (i.e., 50-75-yr-old men undergoing intraabdominal and pelvic procedures) that will be the focus of future studies in which more detailed evaluations such as long-term continuous nerve conduction evaluations and unique imaging techniques will be used.
Our patients in whom perioperative ulnar neuropathies developed and those in other studies were predominantly men between the ages of 50 and 75 yr. Alvine and Schurrer found that 15 of the 17 patients they prospectively identified to have perioperative ulnar neuropathies were middle-aged men. In an earlier retrospective study of this problem, we found the male-to-female ratio to be 2:1. Why is this complication more likely to develop in men? We speculate that several anatomic differences between men and women increase the likelihood that perioperative ulnar neuropathy will develop in men. First, two anatomic differences may increase the chance of ulnar nerve compression in the region of the elbow. The tubercle of the coronoid process is approximately 1.5 times larger in men than in women. In addition, there is less adipose tissue over the medial aspect of the elbow of men compared with women of similar body fat composition. [14-16]Second, we speculate that men are more likely to have a well-developed cubital tunnel retinaculum than women and that the retinaculum, if present, is thicker. A thicker cubital tunnel retinaculum may increase the risk of ulnar nerve compression within the cubital tunnel when the elbow is flexed. [17,18]
Carpal tunnel symptoms often mimicked ulnar neuropathy in our patients after operation. We found that 16 of the 25 patients who had tingling in their fourth and fifth digits or grip weakness in the postoperative period actually had exacerbated symptoms of a preexisting carpal tunnel syndrome that was asymptomatic during their preoperative evaluation.
One especially interesting finding of our study was the difference in the time that symptoms of ulnar neuropathy were first reported by several patients during daily visits compared with their later recall of the time of onset of their symptoms. During postoperative follow-up telephone calls at 8 weeks, two of our seven patients in whom ulnar neuropathy developed commented that they had received legal advice about potential malpractice litigation against their anesthesiologists. Both patients were adamant that they had noticed their symptoms immediately after awakening from their general anesthetics and while still in the postoperative recovery room (within approximately 1 h of their procedures). However, when provided with detailed descriptions of the multiple visits after operation in which the results of their neurologic examinations were normal and in which they repeatedly denied having any symptoms of ulnar neuropathy until postoperative days 4 and 6, both patients and their legal counsels decided that they would not pursue malpractice suits. These anecdotes suggest that patient recall of events that lead to this perioperative complication and the time of its onset are not always reliable.
In conclusion, signs or symptoms of ulnar neuropathy developed within 7 days of their procedures in approximately 1 of every 200 of our patients who underwent surgery and anesthesia. None of the neuropathies were present within the first 2 postoperative days. Although more than one half of these neuropathies resolved within 6 weeks, the rest were still present 2 yr after they developed. In our study population, men 50-75 yr old who undergo intraabdominal or intrapelvic procedures have a high frequency of developing symptoms of ulnar neuropathy. Our findings suggest that detailed neurologic and anatomic evaluations of the ulnar nerves of this group of men will be useful in more extensive future studies that are designed to evaluate etiologic factors for this problem.