Test yourself: weakness and wasting of forearm
Test yourself: weakness and wasting of forearm
Ayano Tachibana 0 1
Nikhil Kotnis 0 1
0 Sheffield Teaching Hospitals NHS Foundation Trust, Northern General Hospital , Herries Road, Sheffield S5 7AU , UK
1 Ayano Tachibana
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Discussion
Posterior interosseous nerve (PIN) syndrome is an uncommon
neuropathic condition that occurs because of compression of the
deep branch of the radial nerve within the radial tunnel. It is also
referred to as supinator syndrome or deep radial nerve syndrome.
At the level of the elbow joint, the radial nerve divides into
the superficial sensory branch and deep motor branch. The
deep branch runs distally along the radial tunnel and then
enters the supinator muscle. At the point where the nerve exits
from the posterior aspect of the supinator muscle, it is referred
to as the posterior interosseous nerve (PIN), which then
courses distally along the dorsal aspect of interosseous
membrane. However, some authors use the term PIN
interchangeably with the deep branch of the radial nerve at the point of
bifurcation. The PIN supplies most of the extensor muscles in
the forearm, with the most common branching pattern to the
extensor digitorum, extensor carpi ulnaris, extensor digiti
minimi, abductor pollicis longus, extensor pollicis brevis,
extensor pollicis longus and the extensor indicis muscles [1–3].
The most common site of compression is at the most distal
point of the radial tunnel along the proximal margin of the
supinator muscle, which is a either a membranous or
tendinous band called the arcade of Frohse, the latter believed to
form a fibrous arcade in response to repeated rotary movement
of the forearm [1–3].
Radial nerve compression can result in either PIN syndrome
or radial tunnel syndrome. It is not known why some patients
develop one syndrome over the other. PIN syndrome describes
clinical features of wasting and weakness of the extensor
forearm, whereas radial tunnel syndrome is associated with a
burning sensation along the lateral aspect of the elbow [2, 4].
This is a case of PIN syndrome, which was correlated with
clinical and intra-operative findings. On clinical examination
the patient had significant visible wasting of her forearm
muscles in addition to the clinical history provided.
The MRI scan showed denervation oedema within the
forearm extensor compartment on fluid sensitive sequence (SPAIR)
and fatty atrophy of the extensor muscles on the T1 W sequence.
A small rounded soft tissue abnormality was seen just proximal
to the supinator origin, which demonstrated low T1 W and high
T2 W signal characteristics (Figs. 1, 2, 3, 4). Dilatation of a
nerve was suspected based on the MRI findings but no extrinsic
cause for compression was identified. The literature suggests
that a structural cause for PIN syndrome can be difficult to
demonstrate on MRI. If the abnormal segment of radial nerve
can be depicted, it may show high signal intensity on
fluidsensitive sequences; however the actual compressive structure
may not be visualised [3].
An ultrasound scan was subsequently organised that
demonstrated a significantly dilated deep branch of the radial nerve just
proximal to the origin of the supinator with marked reduction in
calibre as the nerve passed into the supinator muscle belly
(Figs. 5, 6, 7). Appearances thus suggested pre-stenotic dilatation
of the nerve due to entrapment in the region of the Arcade of
Frohse. This segment of the nerve was very dilated in
comparison with the normal nerve calibre more proximally at the level of
the lateral intermuscular septum of the antecubital fossa (Figs. 6
and 7). Case reports of visualised abnormalities of PIN
compression on ultrasound have been previously described [5, 6].
The US and MRI findings were confirmed intra-operatively
where a very tight arcade of Frohse was found with dilatation of
the deep branch of the radial nerve proximal to this. The arcade
was surgically released together with other potential constricting
structures such as the leash of Henry and extensor carpi radialis
brevis (ECRB) fascia. Three months following the operation,
the patient has regained some degree of finger extension;
however residual weakness unfortunately still remains, which is
unsurprising given the extent of muscle atrophy at presentation.
Compliance with ethical standards
Conflict of interest
interest.
The authors declare that they have no conflict of
Thomas SJ , Yakin DE , Parry BR , Lubahn JD . The anatomical relationship between the posterior interosseous nerve and the supinator muscle . J Hand Surg [Am] . 2000 ; 25 : 936 - 41 .
Miller TT , Reinus WR . Nerve entrapment syndromes of the elbow, forearm and wrist . AJR Am J Roentgenol . 2010 ; 195 ( 3 ): 585 - 94 .
Peripheral neuropathies of the median, radial, and ulnar nerves: MR imaging features . Radiographics . 2006 ; 26 : 1267 - 87 .
Tsai P , Steinberg DR . Median and radial nerve compression about the elbow . Instr Course Lect . 2008 ; 57 : 177 - 85 .
Sonographic diagnosi (...truncated)