Diagnostic ultrasound ( PDFDrive )

CHAPTER 23 Overview of Musculoskeletal Ultrasound Techniques and Applications 865
A
B
C
FIG. 23.18 Normal Median Nerve. (A) In long axis, the
nerve demonstrates a fascicular appearance (arrows), although
coarser than a tendon. The hypoechoic fascicles are distinguishable
from the intervening echogenic epineurium. (B) In short
axis, in the forearm, the median nerve shows a honeycomb
appearance (arrows). (C) In short axis, at the level of the
carpal tunnel, the echogenic tendons (arrows) are seen adjacent
to the median nerve (arrowheads).
of collagenous and adipose components, with small blood vessels
and lymphatics. On short-axis view, peripheral nerves appear
ovoid or round and have punctate internal hypoechoic fat
representing nerve fascicles within the echogenic epineurium.
Dynamic maneuvers including lexion and extension of the imaged
region should show no substantial motion of a peripheral nerve,
as distinct from tendons.
Assessment of peripheral nerves using ultrasound is performed
primarily in the short-axis plane. he nerve is evaluated at a
known anatomic location and followed proximally and distally
as needed. Anisotropy is less problematic for peripheral nerves
than for tendons. 23 Longitudinal scanning is helpful for providing
an overview and illustrating relative caliber changes of peripheral
nerves detected on transverse imaging. Caution should be
exercised in primary interpretation of longitudinal sonographic
images of peripheral nerves because of the potential to scan in
a plane, which is not parallel to the nerve with potential artifactual
changes in caliber and echogenicity.
Nerve dysfunction can result from compression by masses
arising from or adjacent to the nerve, entrapment within ibroosseous
tunnels (such as the carpal tunnel in the case of median
nerve compression at the wrist), or subluxation from ibro-osseous
tunnels. Masses include neurogenic tumors, neuroibromas and
schwannomas, considered more fully later. Sot tissue ganglia,
usually appearing anechoic on ultrasound, may also cause nerve
compression. 45-47 In the setting of nerve compression by any
cause, the indings detectible on ultrasound relate to echotextural
and caliber changes. Local compression causes venous congestion,
which can lead to intraneural edema. Chronic compression can
ultimately lead to intraneural ibrosis. hese alterations in
intraneural composition lead to loss of normal hyperechogenicity
of the interfascicular epineural tissue, causing an overall
hypoechoic appearance of the nerve in addition to poor deinition
or disappearance of the normal fascicular pattern. 43
One of the most common clinical forms of entrapment
neuropathy is carpal tunnel syndrome. In this condition,
compression and lattening of the nerve occurs within the carpal
tunnel, at the palmar aspect of the wrist, and the nerve is typically
swollen and expanded just proximal to the carpal tunnel. he
sonographic diagnosis of carpal tunnel syndrome is made by
measuring the cross-sectional area of the median nerve at the
level of the pronator quadratus and comparing this to the crosssectional
area of the median nerve in the carpal tunnel at the
level of the pisiform. A diference of more than 2 mm 2 between
the two measurements is highly associated with carpal tunnel
syndrome 48 (Fig. 23.19). Variable cut-of values for the diagnosis
of carpal tunnel syndrome based on single cross-sectional
measurements of the median nerve in the carpal tunnel have
been reported in the literature, in the 9- to 11-mm 2 range. 49-51
Bowing and thickening of the overlying lexor retinaculum can
also be observed. 51
Symptoms may also arise from dynamic nerve subluxation
from ibro-osseous tunnels. One example of this is at the cubital
tunnel, at the medial posterior aspect of the elbow. he ulnar
nerve normally passes through this ibro-osseous tunnel at the
posterior aspect of the humerus and is stabilized by an overlying
retinaculum. his retinaculum normally passes between the
olecranon and the medial epicondyle of the humerus. Developmental
or posttraumatic deiciency of this retinaculum can allow
the ulnar nerve to dynamically subluxate out of the cubital tunnel