Most common anomaly is a cross-over of “median-to-ulnar” fibers, the Martin-Gruber anastomosis (MGA). (M  U) Occurs in 15-30% of the population Unilateral or bilateral. Involves only motor fibers; sensory fibers are spared. Cross-over usually occurs in the mid-forearm, either directly from the main trunk of the median or from its branches, mostly AIN. Crossed over fibers run with the distal ulnar nerve to innervate any of the ulnar muscles: - hypothenar muscles (ADM), - first dorsal interosseous muscle (FDI), - thenar muscles (Adductor pollicis, FPB), or - a combination. Three Types Type I – Cross-over fibers from Median to Ulnar supply the “hypothenar muscles (ADM).” Differential diagnosis of this pattern: 1. Excessive stimulation of the Ulnar nerve at the ‘Wrist’ with co-stimulation of the Median nerve 2. Submaximal Stimulation of the Ulnar nerve at the ‘Elbow’, 3.Conduction block of the Ulnar nerve b/w the ‘W’ and ‘E’sites, or 4.MGA with crossing fibers innervating the ADM. -10% drop in the ulnar CMAP ‘BE’ to ‘W’ is normal secondary to temporal dispersion. ULNAR NERVE STIMULATION (Recorded From ADM) Increase CMAPs at wrist. Decrease CMAPs at elbow. Conduction Block. MEDIAN NERVE STIMULATION (APB) NORMAL MEDIAN NERVE STIMULATION (ADM) Elbow: CMAP PRESENT AT ELBOW Wrist: No Response Type II (most common pattern) – Crossover fibers Median to Ulnar supply the “thenar muscles” (adductor pollicis and the flexor pollicis brevis). During routine median studies, when M-to-U cross-over innervates one of the ulnar innervated thenar muscles (AP/FPB). Ulnar motor studies, recording ADM are normal. CMAP is higher when median nerve is stimulated at the ‘Elbow’ than stimulation at ‘Wrist’ This is opposite to the normal. So Stimulate ulnar nerve at ‘W’ and ‘E’ while recording the thenar. Normally, ulnar stimulation at the ‘W’, recording thenar, evokes a thenar CMAP, with an initial positive deflection. This CMAP reflects the normal ulnar-innervated muscles in the thenar eminence. If no MGA is present, subsequent stimulation of the ulnar nerve at the ‘E’ site will evoke a CMAP potential with the same amplitude. If an MGA is present, CMAP will be substantially lower when ulnar nerve is stimulated at the ‘E’ site than at the W. Distal median motor latency is prolonged when stimulated at ’Wrist’. All median nerve fibers stimulated at ‘W’ must travel through the carpal tunnel and therefore are delayed. However, when the median nerve is stim. at the antecubital fossa, most fibers travel through the carpal tunnel as usual, but some median fibers bypass the carpal tunnel by traveling through the anastomosis and innerv. ulnar muscles. Because these fibers bypass the carpal tunnel, they arrive in the hand much sooner than the median fibers that are delayed through the carpal tunnel. When they depolarize their ulnar-innerv. muscles, a positive deflection is seen at the thenar, indicating that a

1. Anomalous
Innervation
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Anomalous = deviating from what is
standard or expected.
Faizan Abdullah
30/10/23

2. Anomalous Innervations
• Median To Ulnar-MGA
• Ulnar to Median-Marinacci
• Riche Cannieu Anastomosis
• Superficial Radial to DUC
• Accessory Deep Peroneal
• Tibial to Peroneal Nerve
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3. MARTIN GRUBER ANASTOMOSIS
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4. MARTIN-GRUBERANASTOMOSIS (MGA)
• Most common anomaly is a cross-over of “median-to-ulnar” fibers,
the Martin-Gruber anastomosis (MGA). (M  U)
• Occurs in 15-30% of the population
• Unilateral or bilateral.
• Involves only motor fibers; sensory fibers are spared.
• Cross-over usually occurs in the mid-forearm, either directly from the
main trunk of the median or from its branches, mostly AIN.
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10. Motor NCS of Median Nerve
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13. 10/30/2023
MEDIAN
ULNAR

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ULNAR

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16. MARTIN – GRUBERANASTOMOSIS
• Crossed over fibers run with the distal ulnar nerve to innervate any of
the ulnar muscles:
- hypothenar muscles (ADM),
- first dorsal interosseous muscle (FDI),
- thenar muscles (Adductor pollicis, FPB), or
- a combination.
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17. TYPES
• Three Types
• Type I – Cross-over fibers from Median to Ulnar
supply the “hypothenar muscles (ADM).”
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MEDIAN N.
ULNAR N.
ADM

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21. Differential diagnosis of this pattern:
1. Excessive stimulation of the Ulnar nerve at the ‘Wrist’ with co-
stimulation of the Median nerve
2. Submaximal Stimulation of the Ulnar nerve at the ‘Elbow’,
3.Conduction block of the Ulnar nerve b/w the ‘W’ and ‘E’sites, or
4.MGA with crossing fibers innervating the ADM.
-10% drop in the ulnar CMAP ‘BE’ to ‘W’ is normal secondary to
temporal dispersion.
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22. 10/30/2023

23. ULNAR NERVE STIMULATION (Recorded From ADM)
•Increase CMAPs at wrist.
•Decrease CMAPs at elbow.
•Conduction Block.
MEDIAN NERVE STIMULATION (APB)
•NORMAL
MEDIAN NERVE STIMULATION (ADM)
•Elbow: CMAP PRESENT AT ELBOW
•Wrist: No Response
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MEDIAN N.
ULNAR N.
ADM

24. • Type II (most common pattern) –
• Crossover fibers Median to Ulnar supply the “thenar
muscles” (adductor pollicis and the flexor pollicis brevis).
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MEDIAN N.
ULNAR N.

27. Routine Median Study: Increased CMAP Proximally
• During routine median studies, when M-to-U cross-over innervates
one of the ulnar innervated thenar muscles (AP/FPB).
• Ulnar motor studies, recording ADM are normal.
• CMAP is higher when median nerve is stimulated at the ‘Elbow’ than
stimulation at ‘Wrist’
• This is opposite to the normal.
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29. • So Stimulate ulnar nerve at ‘W’ and ‘E’ while recording the
thenar.
• Normally, ulnar stimulation at the ‘W’, recording thenar,
evokes a thenar CMAP, with an initial positive deflection. This
CMAP reflects the normal ulnar-innervated muscles in the
thenar eminence.
• If no MGA is present, subsequent stimulation of the ulnar
nerve at the ‘E’ site will evoke a CMAP potential with the
same amplitude.
• If an MGA is present, CMAP will be substantially lower when
ulnar nerve is stimulated at the ‘E’ site than at the W.
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30. TYPE II WITH CTS
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31. MGA-CTS:
• Distal median motor latency is prolonged when stimulated at ’Wrist’.
• All median nerve fibers stimulated at ‘W’ must travel through the carpal
tunnel and therefore are delayed.
• However, when the median nerve is stim. at the antecubital fossa, most fibers
travel through the carpal tunnel as usual, but some median fibers bypass the
carpal tunnel by traveling through the anastomosis and innerv. ulnar muscles.
• Because these fibers bypass the carpal tunnel, they arrive in the hand much
sooner than the median fibers that are delayed through the carpal tunnel.
• When they depolarize their ulnar-innerv. muscles, a positive deflection is
seen at the thenar, indicating that a depolarization has occurred at a distance.
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32. MGA-CTS: Positive Proximal Dip and Factitiously Fast
ConductionVelocity Proximally
• Clues to an MGA with CTS are
- a positive deflection with median nerve stim. at the antecubit. fossa record.
the thenar muscles, and often.
- a surprisingly fast CV of the median nerve in the forearm.
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33. • Type III – Combination of type 1 and 2
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