Posterior interosseous neuropathy: Supinator syndrome vs fascicular radial neuropathy

Abstract

Objective: To investigate the spatial pattern of lesion dispersion in posterior interosseous neuropathy syndrome (PINS) by high-resolution magnetic resonance neurography.

Methods: This prospective study was approved by the local ethics committee and written informed consent was obtained from all patients. In 19 patients with PINS and 20 healthy controls, a standardized magnetic resonance neurography protocol at 3-tesla was performed with coverage of the upper arm and elbow (T2-weighted fat-saturated: echo time/repetition time 52/7,020 milliseconds, in-plane resolution 0.27 × 0.27 mm²). Lesion classification of the radial nerve trunk and its deep branch (which becomes the posterior interosseous nerve) was performed by visual rating and additional quantitative analysis of normalized T2 signal of radial nerve voxels.

Results: Of 19 patients with PINS, only 3 (16%) had a focal neuropathy at the entry of the radial nerve deep branch into the supinator muscle at elbow/forearm level. The other 16 (84%) had proximal radial nerve lesions at the upper arm level with a predominant lesion focus 8.3 ± 4.6 cm proximal to the humeroradial joint. Most of these lesions (75%) followed a specific somatotopic pattern, involving only those fascicles that would form the posterior interosseous nerve more distally.

Conclusions: PINS is not necessarily caused by focal compression at the supinator muscle but is instead frequently a consequence of partial fascicular lesions of the radial nerve trunk at the upper arm level. Neuroimaging should be considered as a complementary diagnostic method in PINS.

Figure 1. Two distinct lesion patterns in posterior interosseous nerve syndrome

(A) Anatomical scheme of the radial nerve with dashed lines indicating the level of images. (B) Magnetic resonance neurography findings of a healthy control with normal radial nerve T2-weighted signal. (C, D) The first, expected pattern of findings for a patient presenting with posterior interosseous neuropathy syndrome: proximally at the upper arm level, a normal appearance and signal of the radial nerve, while after bifurcation and at entry into the supinator muscle, the nerve shows severely increased T2-weighted signal of the deep branch (arrow), in this case with little caliber increase. (D) A patient with the second lesion pattern: a proximal fascicular lesion, involving only a portion of the radial nerve trunk at the upper arm level (arrows).

Figure 2. Posterior interosseous nerve fascicular lesion appearance and localization

(A) Spatial of predominant individual lesion sites distributions (blue circles for pattern 1, red circles for pattern 2). (B) Array of individual radial nerve cross-sectional MRN images for the patient group with proximal lesions (pattern 2, n = 16, numbered in accordance with the table). (C) Proximal MRN cross-sections for patients with only distal lesions (pattern 1, n = 3). Localization of individual lesion sites is given as distance in centimeters from the humeroradial joint space (at 0 cm). Differences in individual arm lengths are not corrected for. In 12 of 16 patients with proximal lesions, strongly increased T2 signal (bright) was found not in the entire cross-section but only within a group of fascicles whereas other fascicles were spared. (D) Nerve T2 signal was normal in controls (showing 6 representatives of 20 controls).

Figure 3. Somatotopy of posterior interosseous nerve fascicular lesions on individual level and group level compared to atlas

(A) The T2-weighted source image of the radial nerve of patient 7 is shown for the site of predominant lesion focus (9.6 cm proximal to the humeroradial joint). Anatomical orientation is given by labeling ventral/dorsal/medial/lateral contours. (B) Spatial map of the patient group mean normalized T2 signal. This cross-sectional lesion area is at the dorsal/posterior and medial aspect of the radial nerve at the upper arm level with a mean distance of 8.3 cm proximal to the humeroradial joint space. The map was rendered after segmentation and intersubject image registration. (C) Somatotopic/topographic internal map of fascicles of the radial nerve trunk, obtained ex vivo as schematic drawing (modified from Sunderland S. The intraneural topography of the radial, median and ulnar nerves. Brain 1945;68:243–299, by permission of Oxford University Press). On this map, the fascicles identified as posterior interosseous fascicles (black filled dots with red background) are in spatial arrangement with the T2 lesion focus on individual (A) and group level (B). Note that lesion focus appears larger on the averaged map compared to historical map partly because of swelling of involved fascicles present in patients but not in specimens used by Sunderland.

Figure 4. Lesion extension into the brachial plexus in posterior interosseous neuropathy syndrome

Representative brachial plexus imaging findings for patient 19 are displayed. (A) At trunk level, superior (s), medial (m), and inferior (i) trunk appear normal, without increased T2 signal. (B, C) At cord levels, the posterior cord (pc; from which the radial nerve arises more distally) is severely thickened and shows high T2 signal (marked by *) as typical imaging findings of selective plexopathy. The lesion extends continuously through the posterior cord into the radial nerve. Lateral (lc) and medial cord (mc) are inconspicuous.