This report reviews the syndrome of entrapment of the superficial branch of the radial nerve (SBRN) in the forearm, and electrodiagnostictechniques to aid in diagnosis are presented. Normal mean radial sensory conduction in the forearm was found to be 61.4 i 3.1 m/sec. Three patients are presented. In two of these comparison of conduction in the SBRN to the lateral antebrachial cutaneous nerve (LACN) and contralateral SBRN was abnormal, whereas the absolute SBRN conduction appeared normal. Normal LACN-SBRN difference in the same arm was 1.9 2 1.6 m/sec, with a range of 0-7.0 m/sec; mean SBRN difference in opposite arms of the same subject was 1.8 t 1.6 m/sec. Key words: neural conduction superficial radial nerve MUSCLE & NERVE 13:l-5 1990
NERVE CONDUCTION STUDIES IN THE SUPERFICIAL RADIAL NERVE ENTRAPMENT SYNDROME HENRY A. SPINDLER, MD, and A. LEE DELLON, MD, FACS
Entrapment of the superficial branch of the radial nerve (SBRN) in the forearm between the tendons of the brachioradialis and extensor carpi radialis is a rarely recognized but definable syndrome. It is frequently misdiagnosed as DeQuervain's extensor tenosynovitis. Dellon and Mackinnon',* have presented a series of 51 patients with this syndrome. Electrodiagnostic studies were done on 19 of their patients with comparison made between the affected and unaffected SBRN. Sixteen of these were reported as abnormal, but three showed no apparent abnormalities. However, only distal latency and amplitude studies were done. In this study we have examined radial sensory conduction in the forearm and cornpared conduction in the contralateral SBRN. Since comparison studies of conduction in adjacent nerves have proven useful in evaluating entrapment syndrome^,^ we have examined the SBRN and lateral antebrachial cutaneous nerve (LACN).
From the Department of Rehabilitation Medicine, University of Maryland School of Medicine and Department of Medicine, Rehabilitation Medicine Section, Franklin Square Hospital (Dr. Spindler), and the Division of Plastic Surgery and Department of Neurologicai Surgery, Johns Hopkins University School of Medicine and the Curtis Hand Center, Union Memorial Hospital (Dr. Dellon). Baltimore, Maryland Address reprint requests to Dr. Spindler at the Franklin Square Medicine Arts Building, 9101 Franklin Square Drive, Baltimore, Maryland, 21237. Accepted for publication September 30, 1988. CCC 0148-639X19010101-05 $04.00 0 1990 John Wiley & Sons, Inc.
NCVs in the Superficial Radial
ANATOMIC CONSIDERATIONS
The radial nerve divides into the superficial and deep branches in the antecubital fossa at the level of the lateral epicondyle. The SBRN then traverses the length of the forearm deep to the brachioradialis. As the nerve travels distally, it passes between the tendons of the brachioradialis and extensor carpi radialis longus at the junction of the middle and lower thirds of the forearm. Just distal to this point, the SBRN pierces the deep fascia and becomes subcutaneous. It then divides into branches supplying the skin over the dorsum of the hand and thumb. Although superficial throughout its course in the forearm, the lateral antebrachial cutaneous nerve parallels the course of the SBRN and, in 75% of patients, a branch of the LACN crosses the SBRN in the distal forearm.4 METHODS
Conduction studies were performed on the SBRN and LACN in the forearms of 30 subjects with no evidence of neurological disease. Subjects ranged in age from 20 to 74 years, with a mean age of 39. Skin temperature at the wrist was maintained greater than 31°C in all patients. In a manner similar to that described by Shirali," the SBRN was stimulated medial to the brachioradialis in the antecubital fossa with surface recording at the base of the thumb. The SBRN was also stimulated 10 cm proximal to the recording electrodes, and conduction velocity in the forearm segment was calculated. Sensory con-
MUSCLE & NERVE
January 1990
1
duction in the LACN was performed as described by Spindler and Fel~enthal.~ The sensory nerve was stimualted at the biceps tendon at the elbow with recording of the sensory nerve action potential 12 cm distally. All latencies were measured from the onset of the stimulus to the onset of the negative phase of the action potential. Peakto-peak sensory nerve action potential amplitudes were recorded. Since the SBRN and LACN are close to each other at the elbow, following stimulation of either nerve at the elbow, recording was made from both nerves distally to look for overlap patterns or spread of the stimulus to the adjacent nerve. RESULTS
In all cases, with stimulation of the radial nerve at the elbow, an evoked response was recorded over the LACN in the forearm indicating that it too was being stimulated. This most likely occurs since the LACN is the more superficial of the two
nerves. In only one subject did LACN stimulation at the elbow evoke a response in the distal radial sensory nerve recording electrodes. This patient was not included in the normal group since it was felt that there might be an unusual plexus formation of the nerves at the wrist (Fig. l ) . Normal radial sensory conduction in the forearm was found to be 61.4 ? 3.1 m/sec, with a range of 54-70 m/secs. This is very similar to the 62.1 5 4.17 reported by Shirali.GThe difference between LACN and SBRN conduction in the same arm was found to be 1.9 t 1.6 mlsec, with a range of 0-7.0 ndsecs. T h e mean difference in conduction between the right and left SBRN in the same subject was found to be 1.8 2 1.6 misec, with a range of 0-7.0 m/secs. A Student’s t-test showed no significant difference in conduction between left and right arms. There appeared to be a slight increase in these means with advancing age, but this is not clear owing to the small number of individuals in each age group. The radial sensory
FIGURE 1. lntraoperative photograph of radial sensory nerve following neurolysis in a patient with radial sensory nerve entrapment in the forearm. The fascia between the brachioradialis muscle (bottom) and the extensor carpi radialis longus (top) has been released to expose the SBRN (center.) Note the small branch from the LACN joining the SBRN (distal to the left.) This anatomic variation may explain the occasional recording of a potential from the SBRN when stimulating the LACN at the elbow.
2
NCVs in the Superficial Radial
MUSCLE & NERVE
January 1990
nerve action potential amplitude with elbow stimulation was never less than half that with wrist stimulation, and the amplitude with elbow stimulation was never less than half of the opposite side’s. Detailed results are presented in Table 1.
sory nerve in the forearm in August 1984. She was last seen in December 1984, at which time the numbness, tingling, and pain had cleared in both hands. In February, 1984 a 36-year-old female fell from a ladder, twisting her left upper extremity, causing hyperpronation of the forearm. She was treated conservatively, but continued to complain of aching and a burning sensation over the dorsal-radial aspect of the left hand. On physical examination there was diminished sensation over the dorsum of the radial aspect of the left hand. There was also a positive Tinel’s sign over the left radial sensory nerve in the forearm. There appeared to be pain-reduced effort when measuring left grip strength. Block of the radial sensory nerve was carried out with Xylocaine and caused a decrease in the patient’s discomfort with a resultant increase in her grip strength. Electromyography was performed on the left upper extremity and was normal. Radial, median, ulnar, and musculocutaneous sensory and motor conduction studies were performed. Left radial sensory conduction in the forearm was normal at 58 mhec but was significantly slower than right radial sensory conduction at 68 m/sec and left LACN sensory conduction at 67 m/sec. T h e clinical and electrical studies were felt to be compatible with a localized entrapment of the left superficial radial nerve in the forearm. In July 1984 the patient underwent neurolysis of the left radial sensory nerve in the Case 2.
CASE REPORTS Case 1. In November 1982 a 41-year-old female bookkeeper was seen complaining of pain and numbness in the hands, more on the left than right. There was a positive Tinel’s sign over the left radial sensory nerve. She was treated conservatively without improvement. In November 1983 a diagnostic nerve block of the left radial nerve gave symptomatic improvement, and the patient underwent neurolysis of the left radial sensory nerve. Following this, there was definite improvement in her symptoms. Electrodiagnostic studies were not performed prior to surgery on the left. In February 1984 the patient was seen for electrodiagnostic studies because of increased symptoms in the right hand. Median and ulnar motor and sensory conduction studies were normal. Right superficial radial sensory conduction in the forearm was slow at 53 mlsec, whereas it was normal on the left at 61 m/sec. Right radial sensory conduction velocity from the wrist to the base of the first web space was normal, as were all amplitudes. Musculocutaneous conduction studies were not done. Because of the lack of improvement with conservative treatment on the right, the patient underwent neurolysis of the right radial sen-
Table 1. Normal radial and lateral antebrachial cutaneous sensory conduction velocity and amplitude. Velocity Nerve Radial Radial Radial LACN LACN-Radial Difference-same side
Segment
Mean velocity t SD (m/sec)
Range
Wrist-1st web space Elbow-wrist Right-left difference Elbow-wrist
564 2 4 0 614t31 18k16 617t29
48-67 54-70 0-7 0 56-70
1 9 t 1 6
0-7 0
Amplitude Nerve Radial Radial Radial Amplttude with elbow Stimulation Amplitude with wrist stimulation Amplitude with elbow Stim (smaller of L or R) Amplitude with elbow Stim (larger of L o r R) ~~~
~~
NCVs in the Superficial Radial
Segment
Mean Amplitude 2 SD (kV)
Range
Wrist- 1st web space Elbow-1st web space
305 t 5 9 1 9 2 -C 6 4
20-50 10-30
0.5-1 .O 0.5- 1 .O ~
~~
MUSCLE & NERVE
~
January 1990
3
forearm. Postoperatively there was loss of pain and improvement in sensation. Case 3.
A 65-year-old female was seen on August
27, 1986 because of hand paresthesias. The patient had previously been seen in August 1981 with the finding of a right carpal tunnel syndrome. She eventually underwent bilateral carpal tunnel release. The patient had been doing well but recently had begun complaining of recurrent pain in her right wrist and thumb. She also complained of tingling in the radial distribution of her right hand. On sensory examination there was decreased sensation over the radial aspect of the right hand as well as over the palmar aspect of digits 1,2,and 3 bilaterally. Grip strength was decreased on the right, probably owing to pain. Electromyography performed on both upper extremities was normal. Ulnar, median, radial, and musculocutaneous sensory and motor conduction studies were performed. Right radial sensory conduction in the forearm was normal at 56 misec but was significantly slower than in the LACN at 67 m/sec and the contralateral SBRN at 68 mlsecs. All other values were normal. The clinical and electrical findings were felt to be indicative of entrapment of the SBRN in the forearm, and release of the nerve alleviated the symptoms. DISCUSSION
Dellon and Mackinnon’x2have proposed that the SBRN may become entrapped in the forearm at the level of its passage between the brachioradialis and extensor carpi radialis longus tendons, where the nerve transits from deep to superficial. They feel that this is a true entrapment neuropathy and may be treated by release of the fascia and neurolysis where indicated. Pathological evidence of entrapment of the SBRN has been presented in support of this t h e ~ i s .In ~ Dellon and Mackinnon’s series, patients presented with pain, numbness, or paresthesias over the dorsal-radial aspect of the hand, usually aggravated by ulnar flexion of the wrist. A history of trauma was common, and there was frequently job-related activity requiring pronation-supination andlor hyperpronation of the forearm. Patients in their series had a positive Tinel’s sign in the area of the brachioradialis tendon insertion, and in 98% there was altered sensation in the dorsal-radial aspect of the hand as compared with the opposite hand.
4
NCVs in the Superficial Radial
On electrodiagnostic evalution Dellon and MackinnonLS2 reported absent or prolonged distal radial sensory latency or decrease in the amplitude of the evoked response, comparing side to side. Three of 19, or 16%, of the clinically affected nerves showed no electrical abnormalities. However, only the distal radial sensory nerve was examined. As demonstrated in case 1, distal conduction may be normal while conduction in the forearm segment is impaired. This is the expected finding in an entrapment neuropathy, i.e., segmental conduction velocity slowing. In more advanced cases, slowing of conduction in the distal portion of the nerve would not be unexpected, nor would complete loss of conduction. In clinically mild cases, one would also suspect that normal conduction might sometimes be seen, as in other entrapment neuropathies. Cases 2 and 3 illustrate that comparison studies may increase the diagnostic yield when individual studies appear to be “normal.” Comparison of LACN to SBRN conduction may be helpful when bilateral entrapment is present o r when there is a unilateral amputation. Side-to-side or wrist-to-elbow amplitude comparison might also be expected to be helpful, but these were normal in the patients presented. We have previously noted, in one of our control patients, that stimulation of the LACN at the elbow illicited an evoked response from the recording electrodes over the radial nerve at the wrist. Anatomic studies4 have shown that in 75% of patients a branch of the LACN crosses the SBRN in the distal forearm, and there may be overlap of the dermal innervation (Fig. 1). As we have seen, stimulation of the SBRN in all patients illicited a response from the LACN-recording electrodes, most likely owing to the superficial postition of the LACN as compared with the SBRN. It is therefore possible that in some of the previously reported “normal” radial sensory conduction studies in clinically involved patients, the examiner may have actually been recording from a branch of the LACN at the wrist rather than the SBRN. We therefore recommend that during all examinations for possible entrapment of the radial nerve in the forearm, the musculocutaneous nerve at the elbow also be stimulated. If a response is recordable from the radial sensory pickup electrodes while stimulating the musculocutaneous nerve, radial sensory conduction studies should be interpreted with great caution.
MUSCLE & NERVE
January 1990
REFERENCES
1. Dellon AL, Mackinnon SE: Entrapment of the radial sensory nerve in the forearm.] Hand Surg 1986;11:199-205. 2. Dellon AL, Mackinnon SE: Radial sensory nerve entrapment. Arch Neurol 1986;43:833-835. 3. Felsenthal G : Median and ulnar distal motor and sensory latencies in the same normal subject. Arch Phys Med Rehahd 1977;58:297-302. 4. Mackinnon SE, Dellon AL: The overlap pattern of' the lateral antebrachial cutaneous nerve and the superficial branch of the radial nerve. J Hand Surg 1985; 10:523-526.
NCVs in the Superficial Radial
5. Mackinnon SE, Dellon AL, Huckson DA: Histopathology of compression of the superficial radial nerve in the forearm.] Hand Surg 1986;l la:206-209. 6. Shirali CS, Sandler B: Radial nerve sensory conduction velocity: measurement by antidroinic technique. Arch Phys Med Rehahil 1972;59:457- 460. 7. Spindler HA, Felsenthal G: Sensory conduction in the musculocutaneous nerve. Arch Phys Med Rehabil 1978;59:20-23.
MUSCLE 13 NERVE
January 1990
5
NERVE CONDUCTION STUDIES IN THE SUPERFICIAL RADIAL NERVE ENTRAPMENT SYNDROME HENRY A. SPINDLER, MD, and A. LEE DELLON, MD, FACS
Entrapment of the superficial branch of the radial nerve (SBRN) in the forearm between the tendons of the brachioradialis and extensor carpi radialis is a rarely recognized but definable syndrome. It is frequently misdiagnosed as DeQuervain's extensor tenosynovitis. Dellon and Mackinnon',* have presented a series of 51 patients with this syndrome. Electrodiagnostic studies were done on 19 of their patients with comparison made between the affected and unaffected SBRN. Sixteen of these were reported as abnormal, but three showed no apparent abnormalities. However, only distal latency and amplitude studies were done. In this study we have examined radial sensory conduction in the forearm and cornpared conduction in the contralateral SBRN. Since comparison studies of conduction in adjacent nerves have proven useful in evaluating entrapment syndrome^,^ we have examined the SBRN and lateral antebrachial cutaneous nerve (LACN).
From the Department of Rehabilitation Medicine, University of Maryland School of Medicine and Department of Medicine, Rehabilitation Medicine Section, Franklin Square Hospital (Dr. Spindler), and the Division of Plastic Surgery and Department of Neurologicai Surgery, Johns Hopkins University School of Medicine and the Curtis Hand Center, Union Memorial Hospital (Dr. Dellon). Baltimore, Maryland Address reprint requests to Dr. Spindler at the Franklin Square Medicine Arts Building, 9101 Franklin Square Drive, Baltimore, Maryland, 21237. Accepted for publication September 30, 1988. CCC 0148-639X19010101-05 $04.00 0 1990 John Wiley & Sons, Inc.
NCVs in the Superficial Radial
ANATOMIC CONSIDERATIONS
The radial nerve divides into the superficial and deep branches in the antecubital fossa at the level of the lateral epicondyle. The SBRN then traverses the length of the forearm deep to the brachioradialis. As the nerve travels distally, it passes between the tendons of the brachioradialis and extensor carpi radialis longus at the junction of the middle and lower thirds of the forearm. Just distal to this point, the SBRN pierces the deep fascia and becomes subcutaneous. It then divides into branches supplying the skin over the dorsum of the hand and thumb. Although superficial throughout its course in the forearm, the lateral antebrachial cutaneous nerve parallels the course of the SBRN and, in 75% of patients, a branch of the LACN crosses the SBRN in the distal forearm.4 METHODS
Conduction studies were performed on the SBRN and LACN in the forearms of 30 subjects with no evidence of neurological disease. Subjects ranged in age from 20 to 74 years, with a mean age of 39. Skin temperature at the wrist was maintained greater than 31°C in all patients. In a manner similar to that described by Shirali," the SBRN was stimulated medial to the brachioradialis in the antecubital fossa with surface recording at the base of the thumb. The SBRN was also stimulated 10 cm proximal to the recording electrodes, and conduction velocity in the forearm segment was calculated. Sensory con-
MUSCLE & NERVE
January 1990
1
duction in the LACN was performed as described by Spindler and Fel~enthal.~ The sensory nerve was stimualted at the biceps tendon at the elbow with recording of the sensory nerve action potential 12 cm distally. All latencies were measured from the onset of the stimulus to the onset of the negative phase of the action potential. Peakto-peak sensory nerve action potential amplitudes were recorded. Since the SBRN and LACN are close to each other at the elbow, following stimulation of either nerve at the elbow, recording was made from both nerves distally to look for overlap patterns or spread of the stimulus to the adjacent nerve. RESULTS
In all cases, with stimulation of the radial nerve at the elbow, an evoked response was recorded over the LACN in the forearm indicating that it too was being stimulated. This most likely occurs since the LACN is the more superficial of the two
nerves. In only one subject did LACN stimulation at the elbow evoke a response in the distal radial sensory nerve recording electrodes. This patient was not included in the normal group since it was felt that there might be an unusual plexus formation of the nerves at the wrist (Fig. l ) . Normal radial sensory conduction in the forearm was found to be 61.4 ? 3.1 m/sec, with a range of 54-70 m/secs. This is very similar to the 62.1 5 4.17 reported by Shirali.GThe difference between LACN and SBRN conduction in the same arm was found to be 1.9 t 1.6 mlsec, with a range of 0-7.0 ndsecs. T h e mean difference in conduction between the right and left SBRN in the same subject was found to be 1.8 2 1.6 misec, with a range of 0-7.0 m/secs. A Student’s t-test showed no significant difference in conduction between left and right arms. There appeared to be a slight increase in these means with advancing age, but this is not clear owing to the small number of individuals in each age group. The radial sensory
FIGURE 1. lntraoperative photograph of radial sensory nerve following neurolysis in a patient with radial sensory nerve entrapment in the forearm. The fascia between the brachioradialis muscle (bottom) and the extensor carpi radialis longus (top) has been released to expose the SBRN (center.) Note the small branch from the LACN joining the SBRN (distal to the left.) This anatomic variation may explain the occasional recording of a potential from the SBRN when stimulating the LACN at the elbow.
2
NCVs in the Superficial Radial
MUSCLE & NERVE
January 1990
nerve action potential amplitude with elbow stimulation was never less than half that with wrist stimulation, and the amplitude with elbow stimulation was never less than half of the opposite side’s. Detailed results are presented in Table 1.
sory nerve in the forearm in August 1984. She was last seen in December 1984, at which time the numbness, tingling, and pain had cleared in both hands. In February, 1984 a 36-year-old female fell from a ladder, twisting her left upper extremity, causing hyperpronation of the forearm. She was treated conservatively, but continued to complain of aching and a burning sensation over the dorsal-radial aspect of the left hand. On physical examination there was diminished sensation over the dorsum of the radial aspect of the left hand. There was also a positive Tinel’s sign over the left radial sensory nerve in the forearm. There appeared to be pain-reduced effort when measuring left grip strength. Block of the radial sensory nerve was carried out with Xylocaine and caused a decrease in the patient’s discomfort with a resultant increase in her grip strength. Electromyography was performed on the left upper extremity and was normal. Radial, median, ulnar, and musculocutaneous sensory and motor conduction studies were performed. Left radial sensory conduction in the forearm was normal at 58 mhec but was significantly slower than right radial sensory conduction at 68 m/sec and left LACN sensory conduction at 67 m/sec. T h e clinical and electrical studies were felt to be compatible with a localized entrapment of the left superficial radial nerve in the forearm. In July 1984 the patient underwent neurolysis of the left radial sensory nerve in the Case 2.
CASE REPORTS Case 1. In November 1982 a 41-year-old female bookkeeper was seen complaining of pain and numbness in the hands, more on the left than right. There was a positive Tinel’s sign over the left radial sensory nerve. She was treated conservatively without improvement. In November 1983 a diagnostic nerve block of the left radial nerve gave symptomatic improvement, and the patient underwent neurolysis of the left radial sensory nerve. Following this, there was definite improvement in her symptoms. Electrodiagnostic studies were not performed prior to surgery on the left. In February 1984 the patient was seen for electrodiagnostic studies because of increased symptoms in the right hand. Median and ulnar motor and sensory conduction studies were normal. Right superficial radial sensory conduction in the forearm was slow at 53 mlsec, whereas it was normal on the left at 61 m/sec. Right radial sensory conduction velocity from the wrist to the base of the first web space was normal, as were all amplitudes. Musculocutaneous conduction studies were not done. Because of the lack of improvement with conservative treatment on the right, the patient underwent neurolysis of the right radial sen-
Table 1. Normal radial and lateral antebrachial cutaneous sensory conduction velocity and amplitude. Velocity Nerve Radial Radial Radial LACN LACN-Radial Difference-same side
Segment
Mean velocity t SD (m/sec)
Range
Wrist-1st web space Elbow-wrist Right-left difference Elbow-wrist
564 2 4 0 614t31 18k16 617t29
48-67 54-70 0-7 0 56-70
1 9 t 1 6
0-7 0
Amplitude Nerve Radial Radial Radial Amplttude with elbow Stimulation Amplitude with wrist stimulation Amplitude with elbow Stim (smaller of L or R) Amplitude with elbow Stim (larger of L o r R) ~~~
~~
NCVs in the Superficial Radial
Segment
Mean Amplitude 2 SD (kV)
Range
Wrist- 1st web space Elbow-1st web space
305 t 5 9 1 9 2 -C 6 4
20-50 10-30
0.5-1 .O 0.5- 1 .O ~
~~
MUSCLE & NERVE
~
January 1990
3
forearm. Postoperatively there was loss of pain and improvement in sensation. Case 3.
A 65-year-old female was seen on August
27, 1986 because of hand paresthesias. The patient had previously been seen in August 1981 with the finding of a right carpal tunnel syndrome. She eventually underwent bilateral carpal tunnel release. The patient had been doing well but recently had begun complaining of recurrent pain in her right wrist and thumb. She also complained of tingling in the radial distribution of her right hand. On sensory examination there was decreased sensation over the radial aspect of the right hand as well as over the palmar aspect of digits 1,2,and 3 bilaterally. Grip strength was decreased on the right, probably owing to pain. Electromyography performed on both upper extremities was normal. Ulnar, median, radial, and musculocutaneous sensory and motor conduction studies were performed. Right radial sensory conduction in the forearm was normal at 56 misec but was significantly slower than in the LACN at 67 m/sec and the contralateral SBRN at 68 mlsecs. All other values were normal. The clinical and electrical findings were felt to be indicative of entrapment of the SBRN in the forearm, and release of the nerve alleviated the symptoms. DISCUSSION
Dellon and Mackinnon’x2have proposed that the SBRN may become entrapped in the forearm at the level of its passage between the brachioradialis and extensor carpi radialis longus tendons, where the nerve transits from deep to superficial. They feel that this is a true entrapment neuropathy and may be treated by release of the fascia and neurolysis where indicated. Pathological evidence of entrapment of the SBRN has been presented in support of this t h e ~ i s .In ~ Dellon and Mackinnon’s series, patients presented with pain, numbness, or paresthesias over the dorsal-radial aspect of the hand, usually aggravated by ulnar flexion of the wrist. A history of trauma was common, and there was frequently job-related activity requiring pronation-supination andlor hyperpronation of the forearm. Patients in their series had a positive Tinel’s sign in the area of the brachioradialis tendon insertion, and in 98% there was altered sensation in the dorsal-radial aspect of the hand as compared with the opposite hand.
4
NCVs in the Superficial Radial
On electrodiagnostic evalution Dellon and MackinnonLS2 reported absent or prolonged distal radial sensory latency or decrease in the amplitude of the evoked response, comparing side to side. Three of 19, or 16%, of the clinically affected nerves showed no electrical abnormalities. However, only the distal radial sensory nerve was examined. As demonstrated in case 1, distal conduction may be normal while conduction in the forearm segment is impaired. This is the expected finding in an entrapment neuropathy, i.e., segmental conduction velocity slowing. In more advanced cases, slowing of conduction in the distal portion of the nerve would not be unexpected, nor would complete loss of conduction. In clinically mild cases, one would also suspect that normal conduction might sometimes be seen, as in other entrapment neuropathies. Cases 2 and 3 illustrate that comparison studies may increase the diagnostic yield when individual studies appear to be “normal.” Comparison of LACN to SBRN conduction may be helpful when bilateral entrapment is present o r when there is a unilateral amputation. Side-to-side or wrist-to-elbow amplitude comparison might also be expected to be helpful, but these were normal in the patients presented. We have previously noted, in one of our control patients, that stimulation of the LACN at the elbow illicited an evoked response from the recording electrodes over the radial nerve at the wrist. Anatomic studies4 have shown that in 75% of patients a branch of the LACN crosses the SBRN in the distal forearm, and there may be overlap of the dermal innervation (Fig. 1). As we have seen, stimulation of the SBRN in all patients illicited a response from the LACN-recording electrodes, most likely owing to the superficial postition of the LACN as compared with the SBRN. It is therefore possible that in some of the previously reported “normal” radial sensory conduction studies in clinically involved patients, the examiner may have actually been recording from a branch of the LACN at the wrist rather than the SBRN. We therefore recommend that during all examinations for possible entrapment of the radial nerve in the forearm, the musculocutaneous nerve at the elbow also be stimulated. If a response is recordable from the radial sensory pickup electrodes while stimulating the musculocutaneous nerve, radial sensory conduction studies should be interpreted with great caution.
MUSCLE & NERVE
January 1990
REFERENCES
1. Dellon AL, Mackinnon SE: Entrapment of the radial sensory nerve in the forearm.] Hand Surg 1986;11:199-205. 2. Dellon AL, Mackinnon SE: Radial sensory nerve entrapment. Arch Neurol 1986;43:833-835. 3. Felsenthal G : Median and ulnar distal motor and sensory latencies in the same normal subject. Arch Phys Med Rehahd 1977;58:297-302. 4. Mackinnon SE, Dellon AL: The overlap pattern of' the lateral antebrachial cutaneous nerve and the superficial branch of the radial nerve. J Hand Surg 1985; 10:523-526.
NCVs in the Superficial Radial
5. Mackinnon SE, Dellon AL, Huckson DA: Histopathology of compression of the superficial radial nerve in the forearm.] Hand Surg 1986;l la:206-209. 6. Shirali CS, Sandler B: Radial nerve sensory conduction velocity: measurement by antidroinic technique. Arch Phys Med Rehahil 1972;59:457- 460. 7. Spindler HA, Felsenthal G: Sensory conduction in the musculocutaneous nerve. Arch Phys Med Rehabil 1978;59:20-23.
MUSCLE 13 NERVE
January 1990
5
