Positioning in Anesthesiology: Toward a Better Understanding of Stretch-induced Perioperative Neuropathies

Twenty-five men with no history of cervicobrachial pain or orthopedic injuries volunteered to participate (mean height: 1.82 ± 0.06 m, weight: 76.2 ± 10.8 kg, and body mass index: 23.0 ± 2.9). Their age varied from 20 to 32 (23.4 ± 3.5) yr. To be suitable for inclusion, each subject had to have pain-free, full range of motion (ROM) of the cervical spine and all individual upper limb joints. Participants were excluded from the study if the combined shoulder position of abduction and lateral rotation was limited or painful. The subjects’ mean range of elbow extension was 183.4 ± 1.9°. Subjects who were aware of having diseases associated with a relatively high probability of neuropathy, such as diabetes or renal disease, were also excluded from the study. The participants signed an informed consent form, and the study was approved by the Ethical Committee for Rehabilitation Studies, University of Leuven (Leuven, Belgium).

To obtain better insight into the potentially hazardous positions, four variations of the BPTT were performed: elbow extension was performed (1) with the wrist and cervical spine in a neutral position (BPTT^NEUTRAL), (2) with wrist extension (BPTT^WE), (3) with cervical contralateral lateral flexion (BPTT^CCLF), and (4) with wrist extension and cervical contralateral lateral flexion (BPTT^{WE + CCLF}) (fig. 1).

These four variations were performed in three different shoulder positions: (1) with abduction and lateral rotation of the arm while the shoulder girdle was fixed in its neutral position (original BPTT 11,13), (2) with abduction of the arm and fixation of the shoulder girdle, and (3) with abduction only (fig. 2). Because positioning in lateral rotation is not routinely performed in anesthetic practice, rotation was left out of the second and third shoulder positions. The tests were performed with and without fixation of the shoulder girdle to simulate the presence and absence of a shoulder brace.

The principal measure during the tests was the available range of elbow extension. Elbow extension was stopped when the participant reported that the test caused substantial discomfort or when full range of elbow extension was reached. Substantial discomfort was defined before the start of the experiment as the maximal tolerance level for the test, knowing that the test had to be performed repeatedly. The available range of elbow extension at this point was considered to correspond with the submaximal extensibility of the nervous system and was used for further analysis.

Figure 2shows the experimental setup. The range of elbow extension was measured using an electrogoniometer (M180; Penny & Giles Biometrics Ltd., Blackwood Gwent, United Kingdom). To increase accuracy, the goniometer was calibrated separately for the tests with and without lateral rotation. The pain intensity elicited at the end of the test was measured using a numeric pain intensity rating scale, ranging from 0 (no pain) to 10 (worst pain possible). The types of elicited symptoms were recorded on a body chart. A load cell (535QD; DS Europe, Milan, Italy) was used to standardize the amount of depression force to reposition the shoulder girdle toward its neutral position and to block the shoulder girdle in this position. The load cell was positioned over the acromioclavicular joint, the advocated location for shoulder braces. 15 

The order of the four test variations was randomized to counterbalance possible effects of repeated testing. This sequence was repeated in the three different shoulder positions, which were also performed in different order. Two trial runs were performed to familiarize the subject with the procedure. To allow calculation of the reliability of the available range of elbow extension, three repetitions were performed by a first tester, followed by one repetition of a second tester. Throughout the experiment, the tester was blinded to the output of the electrogoniometer. A custom-made software program was used for data acquisition. A detailed description of the experimental devices and methodology has been reported elsewhere. 16 

Intraclass correlation coefficients (ICCs) were calculated as a measure of reliability for the available ROM (ICC[2,1] and ICC[2,3]). 17Intratester reliability was assessed by analyzing the three repetitions of the first tester; intertester reliability was assessed by comparing the first repetition of the first tester with the repetition of the second tester. In addition, the standard errors of measurement were calculated as a function of the pooled group SD and the reliability coefficient. 18 

Results are presented as mean amplitudes (± SD; range [minimum-maximum]). Two-way analysis of variance, with two repeated factors (shoulder position × variations), in combination with Tukey HSD post hoc  multiple comparison tests, was used to analyze differences in range of elbow extension. The number of levels coded under the shoulder position and variations factors were three and four, respectively. One-way analysis of variance, repeated-measures design, was used to analyze differences in pain intensity among the four different variations in the original BPTT position. The level of significance chosen was P < 0.05.

From a statistical as well as from a clinical point of view, the reliability of measurement of the available ROM was excellent. The mean intratester reliability among the 12 tests was 0.96, with a mean standard error of measurement of 1.8°. When three repetitions were used as the unit of analysis, the mean reliability increased to 0.99, with a mean standard error of measurement of 1.0°. The mean intertester reliability among the 12 tests was 0.93, with a mean standard error of measurement of 2.3°.

Table 1summarizes the mean range of elbow extension for the four different variations in the three different shoulder positions. In general, the available ROM decreased, and the variability among subjects, represented by the standard deviations, increased when more test components were added. The results are graphically presented in figure 3.

The analysis of variance revealed a significant interaction for the ROM between the shoulder positions and test variations (F^6,144= 41.5;P < 0.0001). A significant main effect was present for the shoulder positions (F^2,48= 90.2;P < 0.0001). Additional analysis revealed significant differences in the available range of elbow extension for all three shoulder positions (P < 0.05). A significant main effect was also present for the test variations (F^3,72= 102.8;P < 0.0001). The following significant differences were found: (1) the addition of wrist extension (BPTT^WE) or cervical contralateral lateral flexion (BPTT^CCLF) resulted in a significantly reduced ROM when compared with BPTT^NEUTRAL(P < 0.05); (2) the simultaneous addition of wrist extension and cervical contralateral flexion (BPTT^{WE + CCLF}) resulted in a significantly smaller ROM when compared with BPTT^WEand BPTT^CCLF(P < 0.05); (3) no significant overall difference in ROM was observed between the addition of wrist extension (BPTT^WE) and cervical lateral flexion (BPTT^CCLF) (P = 0.90).

A selection of results of the multiple comparison procedure analyzing the differences among individual tests is reported. These results are given in figure 3and table 1.

With the shoulder positioned in the original BPTT position, pain intensity increased significantly when more components were added to the test (BPTT^NEUTRAL: 4.1 ± 1.5, BPTT^WE: 5.1 ± 1.9, BPTT^CCLF: 5.7 ± 1.4, BPTT^{WE + CCLF}: 6.4 ± 1.3;P < 0.05). In addition, the number of subjects reporting paraesthesia increased when more components were added (BPTT^NEUTRAL: 12%, BPTT^WE: 36%, BPTT^CCLF: 48%, BPTT^{WE + CCLF}: 72%).

The BPTT was analyzed to quantify the impact of various arm and neck positions on the upper quadrant nervous system to obtain better insight into potentially dangerous positions associated with stretch-induced neuropathies in anesthetic practice. Cervical contralateral lateral flexion, depression of the shoulder girdle and lateral rotation of the shoulder in combination with shoulder abduction, and wrist extension all had a significant effect on the available range of elbow extension. Furthermore, because all starting positions were pain-free and because a substantial discomfort was elicited when the elbow was extended, it could be inferred that elbow extension also substantially challenged the nervous system. In addition, a higher pain intensity and a larger number of subjects reporting paresthesia could be observed when more test components were simultaneously applied.

The findings of this study support the experientially based recommendations regarding potentially dangerous positions. 5,7–10Based on our findings, we suggest that the dangerous positions mentioned herein should be avoided whenever possible. Because various perioperative neuropathies have been reported when shoulder braces were used, 6,10the impact of shoulder fixation deserves special attention, especially because one third of the members of the American Society of Anesthesiologists either do not know or disagree with the opinion that the use of shoulder braces impacts the risk of peripheral neuropathy. 7When fixation was added, all tests resulted in a significantly reduced ROM, even when the fixation force was applied over the advocated region, i.e. , the acromioclavicular joint. Therefore, we strongly discourage use of shoulder braces whenever possible in patient positioning. However, if braces must be used, as for example in steep head-down tilt during laparoscopic surgery, nonsliding mattresses should be added. Furthermore, the pressure of the braces on the shoulder girdle should be controlled regularly, and if necessary, the position of the braces should be adjusted during anesthesia.

Based on the results of the different tests, the continuity of the nervous system can be regarded as the common denominator for a cumulative and neutralizing effect. The cumulative effect indicates that when different positions that individually load the nervous system are simultaneously added, their impact on the nervous system is larger than when added separately. This can be clearly observed when wrist extension and cervical contralateral lateral flexion were added in the original BPTT position (fig. 3). When the wrist, shoulder, and cervical spine were simultaneously positioned in a nerve bed-lengthening position, the mean maximal tolerable range of elbow extension corresponded with an extension deficit of no less than approximately 40°. This cumulative effect is supported by anatomic studies in which a much more pronounced excursion of the median nerve was observed when joint movements were combined. 19 

The neutralizing effect indicates that the impact of a position can be reduced if a neighboring region allows for unloading of the nervous system. For example, the impact of cervical lateral flexion, even when added simultaneously with wrist extension, decreased significantly when the shoulder was not laterally rotated or fixed in a neutral position (table 1). Although both effects are important to consider in patient positioning, extrapolation of the neutralizing effect into anesthetic practice should be made carefully. The BPTT is a dynamic test of short duration, and it is unlikely that the neutralizing effect will hold to the same extent with sustained arm positions in prolonged operations. However, if one of the mentioned hazardous positions cannot be avoided, it may be worth considering positioning the adjacent joints in an unloading position.

The large interindividual differences in reaction to elongation of the nerve bedding were another key finding of this study. Maneuvers had only a minor impact in one subject, whereas the ROM was strongly limited in another subject. Noteworthy is that the variability among subjects increased as more components were added, i.e. , as the nervous system was loaded to a greater extent. This shows that subjects respond differently to nerve bedding elongation. Consequently, the question arises as to whether subjects with an adverse BPTT response are more prone to development of stretched-induced perioperative neuropathy. Only a large prospective study, in which neural provocation tests are included as preanesthetic screening tests, will show the additional value of the BPTT and similar tests for other major peripheral branches, e.g. , the ulnar nerve. 13If these tests are able to identify patients at risk, e.g. , patients with an occult neuropathy before surgery, these simple and quick screening tests could become valuable tools toward prevention, as well as for medicolegal justification. Triggering a preexisting subclinical neuropathy is considered to be a possible mechanism for development of perioperative ulnar neuropathy. 20–22 

Numerous studies 19,23–26have shown that the different components of the BPTT cause elongation, tension, and compression of the brachial plexus and median nerve. In addition, it is well-established that not only low-grade compression 27but also elongation of merely 10–15%28,29is sufficient to cause important alterations in vital physiologic processes, compromising the condition of the peripheral nerve. Changes include reduced intraneural blood flow, resulting in nerve ischemia and endoneurial edema, 30–32suppression of axonal transport, 29,33and changes in conduction characteristics. 28,34These mechanical and physiologic alterations probably have an important role in the genesis of stretched-induced neuropathies and in the symptoms elicited during the BPTT. However, apart from the nervous system, many nonneural elements are also challenged during the BPTT, and the question arises as to what their contribution is in the outcome of the BPTT. There are sufficient indications in this study to assume that the more positions were simultaneously applied, the smaller the importance of nonneural elements in the test result became and that the responses were indeed predominantly neurogenic. For example, bearing in mind that the starting position was symptom-free, many of the articular and muscular structures around the cervical spine, shoulder, and wrist could not have had a direct contribution to the test response because these structures do not span the elbow joint and therefore were not challenged during the test movement of elbow extension. In addition, the subjects experienced increasing pain intensity although the loading of muscular and articular structures around the elbow decreased when more components were added to the test as the tolerance level for the test was elicited earlier in range. Preloading a continuous structure, such as the nervous system, by the addition of components to the starting position seems to be an obvious and adequate explanation. Furthermore, the increasing amount of subjects reporting paresthesia when more test components were added supports the opinion that the upper quadrant nervous system was substantially challenged during the test. Therefore, we conclude that the limitation in ROM and many of the elicited symptoms can be ascribed to the peripheral nervous system, which is unable to comply sufficiently with the elongation of its nerve bedding, even in healthy subjects.

Another important aspect to consider in patient handling and positioning is the absence of protective reflexes in the anesthetized and sedated patient. Throughout progressive stages of the BPTT, Balster and Jull 35demonstrated increasing muscular activity as loading of the nervous system increased. It is often hypothesized that this muscular activity acts as a nociceptive-mediated withdrawal reflex to protect the nervous system from harmful elongation. 11,14,35In the absence of this protective muscle activity, the nervous system may be subject to a larger elongation perioperatively than that which would occur in activities of daily living. Suppression of this protective mechanism may also be a consideration in the selection of short-and long-acting muscle relaxants, especially in patients at risk, such as patients with diabetes mellitus and renal or vascular disease.

Based on the findings of this study and others, we advocate the following perioperative positioning of the upper quadrant to optimally protect the peripheral nervous system in the supine position. Shoulder abduction and lateral rotation should be kept to a minimum and should never be combined with the use of shoulder braces. Ideally, the head should remain in the midline position, the arm should be kept at the side, the elbow should be gently flexed to unload the median nerve, and the forearm should be supinated to protect the ulnar nerve. 36The wrist is best positioned in the neutral position because it does not load the nervous system and approximates the position associated with the lowest carpal tunnel pressure. 37Shoulder braces and elbow and wrist extension should be avoided whenever possible.

The authors thank Koen Janssens, P.T. (Department of Rehabilitation Sciences, University of Leuven, Leuven, Belgium), for his assistance during the measurements and Hedwig Bogaerts, Ph.D. (Department of Kinesiology, University of Leuven, Leuven, Belgium), for statistical advice.