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Two
relevant variants in the innervation of the upper and
lower extremities will be discussed. One is the median-to-ulnar
communication, also known as a crossover, and the second
is the accessory peroneal nerve.
Crossovers
In this instance a few fibers
of the median nerve separate from its anterior interosseous
branch in the upper third of the forearm, and cross over
to join the ulnar nerve.
Crossovers are found in 15 to 30
percent of otherwise normal individuals.
Three types can be seen either separately or in combination:
1- The hypothenar type, where the crossing fibers end
up in the hypothenar muscle group.
2 - The first dorsal interosseous type, where the crossing
fibers end up in the first dorsal interosseous muscle.
3 - The thenar type where the crossing fibers go to
the ulnar muscles in the thenar group.
In the hypothenar type,
routine conductions reveal the following: when the ulnar
nerve is stimulated at the wrist, the ADM motor amplitude
is much greater than that obtained from stimulation
at the elbow (more than 2 mv difference between both
responses). This is due to the fact that at the wrist,
the nerve contains, in addition to the fibers it already
had at the elbow, fibers which have crossed from the
median nerve. To differentiate this from an ulnar nerve
conduction block at the elbow the following is done:
the median nerve is stimulated at the elbow while recording
the ADM. If a response is obtained it indicates that
there has been a crossover of some median fibers to
the ADM since normally, elbow stimulation of the median
nerve should not evoke any motor response in the ADM.
In the first dorsal interosseous
type, routine conductions do not reveal the cross-over.
When the first dorsal interosseous muscle is studied
however, the findings are similar to those with the
ADM in the hypothenar type. When the ulnar nerve is
stimulated at the wrist the first dorsal interosseous
motor amplitude is much greater than that obtained when
the nerve is stimulated at the elbow (more than the
normal 2 mv drop). With median nerve stimulation, the
following changes are seen: normally if no crossover
is present, a response can be recorded from the first
dorsal interosseous on median wrist and elbow stimulation
(wrist greater than elbow). When a crossover is present
the response from the median elbow stimulation is much
greater than that from wrist stimulation because median
fibers that will cross to the first dorsal interosseous
are still part of the median nerve at this level.
In the thenar presentation,
routine studies will reveal the following: when the
median nerve is stimulated at the elbow, the APB amplitude
is higher than when the nerve is stimulated at the wrist.
When the ulnar nerve is stimulated, the APB evoked response
is much higher from the wrist than from elbow stimulation.
Normally one would obtain a response from the APB from
both wrist and elbow stimulation of the ulnar nerve
because of volume conduction to the APB from other ulnar
innervated muscles in the thenar area. These responses
from wrist and elbow stimulation however are usually
roughly equal in size. When a crossover is present,
the elbow stimulation results in a much lower amplitude
than from the wrist because at that level (elbow) the
crossover has not taken place yet.
Although median-to-ulnar crossovers
do not significantly complicate routine upper extremity
workups, they can render the interpretation of nerve
conduction studies difficult in the presence of a carpal
tunnel or an ulnar neuropathy.
Accessory Peroneal Nerve
Although the EDB is usually
innervated only by the deep peroneal nerve, occasionally
(in about one third of the population) it derives additional
innervation from an accessory peroneal nerve, a branch
of the superficial peroneal, which curves around the lateral
malleolus and usually supplies the lateral portion of
the EDB.
When the peroneal nerve is stimulated at the ankle, the
response obtained has a lower amplitude than that obtained
from the knee. This difference is due to the fact that
at the ankle only the deep peroneal nerve is stimulated
but at the knee both deep and superficial branches (the
latter with its accessory peroneal branch) are stimulated.
If the accessory peroneal nerve
is stimulated behind the lateral malleolus, a response
is obtained from the EDB, with an amplitude that is
equal to the difference between the amplitudes of the
responses obtained from knee and ankle stimulation.
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