Regional Anesthesia: Research Report
The cephalic vein is particularly well suited for IV drugs: its constant presence and large size make it easy to cannulate. Anatomically, it is difficult to give a systematic description of vein positional variations of the forearm: the dorsal superficial network of veins of the hand converge to form the cephalic vein on the posterior lateral aspect of the hand and receive the dorsal vein of the thumb (1–3). Although the sensory branch of the radial nerve has been described many times (4–6), its relationships with tributary cutaneous veins have not been documented.
Lesions of the sensory branch of the radial nerve are a complication of pin insertion in the distal end of the radius (7–9). Identical lesions have been reported after venous puncture on the lateral aspect of the wrist. Three cases of painful neuromas of the superficial branch of the radial nerve have been reported in the literature after IV catheter insertion in the cephalic vein (10–12). Two additional cases (personal data) have not been reported yet. The aims of this study were to verify by a descriptive anatomic study whether there is a significant risk of nerve lesion when cannulating veins on the lateral aspect of the wrist and to identify potential anatomic landmarks for safe puncturing of the cephalic vein.
This study was approved by our local IRB. We used 33 forearms and wrists (16 left and 17 right) from cadavers of undetermined age and sex that had been embalmed with Winckler’s classic method (13). Veins were injected with blue latex in four specimens to facilitate dissection and increase topographic precision. Each specimen was then dissected: first, a dorsolateral incision was made to isolate the subcutaneous area of interest, from the elbow to the metacarpophalangeal joints; the cephalic vein, which is the most superficial element, was subsequently identified and carefully dissected. The superficial fascia was then incised to reach the sensory branch of the radial nerve as it emerges from under the brachioradial muscle, at the upper third of the forearm. The nerve was then dissected distally to the emergence of its terminal branches.
Six measurements were made along the axis of the forearm (xOy) for each specimen (Fig. 1). O was a reference point located on the styloid process or radius; we arbitrarily defined a positive measure as being proximal to the styloid process of radius and a negative measure as being distal from it. N was the point where the sensory branch of the radial nerve emerged between the tendons of the brachioradial muscle and the long radial extensor muscle of the wrist. The ON distance represented the distance between the styloid process of the radius and the emergence of the nerve. N1 marked the division of the superficial radial nerve into a lateral branch that continued on the dorsal aspect of the thumb and a medial branch coursing on the dorsal aspect of the wrist. ON1 was the distance from the styloid process of the radius to this first division. N2 was the point where the medial branch of the superficial radial nerve divided into a lateral branch running on the lateral aspect of the index (dorsal digital nerve of the radial nerve distributing to the thumb and part of the index finger) and a medial branch (dorsal digital nerve of the radial nerve distributing to the index finger and part of the middle finger). ON2 was the distance between the styloid process of the radius and this second division. V was the point where the dorsal vein of the thumb merged with the cephalic vein. OV was the distance from the styloid process of the radius to the venous confluence. C1 was the point where the vein (or one of its tributaries) crossed the superficial radial nerve (or one of its branches). On some specimens, a second intersection was observed and likewise called C2. OC1 and OC2 were the distances between the styloid process of the radius and the C1 or C2 points, respectively.
Specimens were photographed for subsequent tracing with a 2-cm ruler and three colored-end pins indicating the styloid process of the radius, the dorsal aspect of the metacarpophalangeal joint of the thumb, and the dorsal aspect of the metacarpophalangeal joint of the index finger.
Specimens were all in a strict lateral position, index fingers were lined up with the axis of the forearm, and the axes of the thumbs were maintained at a 45° angle with the index finger. The scale used for tracing was 1:1 (on the basis of rulers on the photographs). The use of anatomic landmarks enabled us to superimpose different venous and nervous courses in a reliable and reproducible way.
The sensory branch of the radial nerve emerged proximally at an average distance of 8.16 cm (range, 6–11 cm) from the styloid process of the radius in the 33 studied specimens (Fig. 2). In each specimen, the nerve crossed the inferior face of the cephalic vein or one of its tributaries at least once. In 14 of 33 cases, the nerve and vein crossed in two different places (Fig. 3), and in one case, they crossed in three places. The intersection occurred in highly variable positions. When data were pooled, the crossing point ranged from 9 cm proximally from the styloid process of the radius to 4.5 cm distally. On one specimen, we found a double emergence of the sensory branch of the radial nerve under the tendon of the brachioradial muscle. This was because the medial and lateral branches of the radial nerve originated high up in the forearm (Fig. 4).
We superimposed the 33 tracings of the courses of radial nerves (one for each specimen we dissected) on a drawing of the distal end of an upper limb in a strict lateral position (Fig. 5). We superimposed the 33 tracings of veins (one for each specimen we dissected) on a drawing of the distal end of an upper limb in a strict lateral position (Fig. 6). Both superimpositions illustrate the highly variable course of cephalic veins and radial nerves on the lateral aspect of the wrist.
The sensory branch of the radial nerve emerged between the tendons of the brachioradial muscle and the long radial extensor muscle of the wrist at a somewhat variable distance from the styloid process of the radius in all 33 specimens studied. The distance ranged from 6 to 11 cm, with an average of 8.16 cm. In the study of Abrams et al. (7), it reached 11.6 cm and averaged 9.0 cm. Auerbach et al. (4) reported similar distances, with an average of 8.6 cm (range, 6.1–11.8 cm). Our results are consistent with these two studies and therefore confer a greater degree of certainty to the level of subcutaneous emergence of the superficial branch of the radial nerve.
Despite the more or less easy to systematize nervous divisions described by Abrams et al. (7) and Chevaleraud et al. (5), which we also observed, the course of the nervous branches was highly variable, as shown by the superimposed tracings.
In every specimen, we found a large cephalic vein on the lateral aspect of the forearm collecting blood from the superficial venous arch on the back of the hand. The dorsal vein of the thumb, another tributary of the cephalic vein, was also systematically found. It merged proximally to the styloid process of the radius, ranging from 5 to 12.5 cm (OV distance).
By injecting veins with blue latex, we were able to observe the many anastomoses between the cephalic vein and the superficial dorsal venous network of the hand and the dorsal veins of the thumb. But because the small size of the veins in the venous network makes them difficult to puncture or cannulate, we considered only the vein-nerve intersections involving the cephalic vein and the dorsal vein of the thumb.
As was the case for the nerve, the course of venous elements was highly variable, as illustrated by their superimposition on the tracings in Figure 6. Although the confluence between the dorsal vein of the thumb and the cephalic vein was consistently found, as shown on the tracings, the extremely variable courses of the underlying nerves make it very difficult to predict where the vein and nerve will cross. On every specimen, we found that veins and nerves intersected at least once, and sometimes twice or even three times.
Our results show that the risk of damaging a nerve when accidentally puncturing the posterior wall of a vein on the lateral or dorsal aspects of the wrist is constant: the cephalic vein (sometimes the dorsal vein of the thumb at its terminal end), usually used for punctures, always crosses over the sensory branch of the radial nerve or one of its two main branches. Because the deeper side of the vein and superficial side of the sensory nervous branches are very close to each other, the risk of damaging a nerve is great if the vein is transpierced right where nerve and vein intersect.
Our study led us to two conclusions: First, there is a tiny but real risk of nerve lesion when puncturing a vein, because veins and nerves always intersect at some point. Second, it seems difficult to determine a point where there would be no such risk, because the intersection may occur anywhere, from the emergence of the nerve between the tendons of the brachioradial muscle and long radial extensor muscle of the wrist to its terminal branches on the back of the hand. Although it may be impossible to determine a safe area for nerve lesions, obviously there is no risk of damaging the superficial branch of the radial nerve above its emergence in the subcutaneous fascia. According to our findings, it emerges between 6 and 11 cm above the level of the styloid process of the radius.
Our results are consistent with those of other studies, according to which the sensory nerve may emerge up to 12 cm proximal from the styloid process of the radius. Because each of these studies was conducted on 20 specimens, their results, coupled with ours, confer a greater degree of certainty to the level of subcutaneous emergence of the superficial branch of the radial nerve.
For the risk of nerve lesion to be minimal when puncturing the vein on the lateral or dorsal aspects of the wrist, we suggest that punctures should be made at least 12 cm (e.g., approximately eight finger breadths) above the styloid process of the radius.
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© 2001 International Anesthesia Research Society
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