Anterior interosseous nerve syndrome: fascicular motor lesions of median nerve trunk

Abstract

Objective: We sought to determine lesion sites and spatial lesion patterns in spontaneous anterior interosseous nerve syndrome (AINS) with high-resolution magnetic resonance neurography (MRN).

Methods: In 20 patients with AINS and 20 age- and sex-matched controls, MRN of median nerve fascicles was performed at 3T with large longitudinal anatomical coverage (upper arm/elbow/forearm): 135 contiguous axial slices (T2-weighted: echo time/repetition time 52/7,020 ms, time of acquisition: 15 minutes 48 seconds, in-plane resolution: 0.25 × 0.25 mm). Lesion classification was performed by visual inspection and by quantitative analysis of normalized T2 signal after segmentation of median nerve voxels.

Results: In all patients and no controls, T2 lesions of individual fascicles were observed within upper arm median nerve trunk and strictly followed a somatotopic/internal topography: affected were those motor fascicles that will form the anterior interosseous nerve further distally while other fascicles were spared. Predominant lesion focus was at a mean distance of 14.6 ± 5.4 cm proximal to the humeroradial joint. Discriminative power of quantitative T2 signal analysis and of qualitative lesion rating was high, with 100% sensitivity and 100% specificity (p < 0.0001). Fascicular T2 lesion patterns were rated as multifocal (n = 17), monofocal (n = 2), or indeterminate (n = 1) by 2 independent observers with strong agreement (kappa = 0.83).

Conclusion: It has been difficult to prove the existence of fascicular/partial nerve lesions in spontaneous neuropathies using clinical and electrophysiologic findings. With MRN, fascicular lesions with strict somatotopic organization were observed in upper arm median nerve trunks of patients with AINS. Our data strongly support that AINS in the majority of cases is not a surgically treatable entrapment neuropathy but a multifocal mononeuropathy selectively involving, within the main trunk of the median nerve, the motor fascicles that continue distally to form the anterior interosseous nerve.

Figure 1. Fascicular lesions of median nerve at upper arm level in anterior interosseous nerve syndrome

Spatial (red circles, upper left) and statistical (red box-and-whisker plot, upper left) distributions of predominant individual lesion sites and array of individual median nerve cross-sectional magnetic resonance neurography images (upper right) for the patient group (n = 20, numbered in accordance with table e-1). Localization of individual lesion sites is given as distance in cm from humeroradial joint space (at 0 cm). Differences in individual arm lengths were not corrected for. In all patients, strongly increased T2 signal (bright) was found within a group of fascicles whereas other fascicles were spared, indicated by dashed (lesion) and solid (normal fascicles) white arrows in patient 1. Nerve T2 signal was normal in controls (lower right showing 5 representative out of 20 control subjects).

Figure 2. Somatotopy of fascicular T2 median nerve lesion on individual level, group level, and atlas

On the left, the T2-weighted source image of the median nerve of patient 15 is shown for the site of predominant lesion focus (17.1 cm proximal to humeroradial joint). Anatomical orientation is given by labeling ventral/dorsal/medial/lateral contours. In the middle, a spatial map of the patient group mean normalized T2 signal is shown. This map was rendered after segmentation and intersubject image registration. On the right is a somatotopic/topographic internal map of fascicles of the median nerve trunk. This schematic drawing was obtained by Jabaley et al. from tracing extraneural median nerve branches from distally to intraneural proximal fascicles within the median nerve trunk on 20-μm-thick cuts after intraneural microsurgical dissection and histologic photographing (modified from Jabaley et al. with permission). On this map, the red fascicles (“ai”: anterior-interosseous) are in close spatial arrangement with the T2 lesion focus on individual (left) and group level (middle). This cross-sectional lesion area is at the dorsal and lateral/radial aspect of the median nerve at upper arm level with a mean distance of 14.6 ± 5.4 cm proximal to the humeroradial joint space.

Figure 3. Monofocality and multifocality as 2 principal lesion patterns of anterior interosseous nerve syndrome

Representative monofocal (left) and multifocal (middle) lesion patterns are compared to one representative control subject (right). In each column, contiguous longitudinal high-resolution imaging slices are displayed. Numbers on the left of patient images indicate slice position relative to the predominant lesion focus (at 9.2 cm in patient 2 and 22.2 cm in patient 8). The monofocal and multifocal lesion pattern both follow the internal topography/somatotopy of anterior interosseous motor fascicles. These fascicles will form the anterior interosseous nerve further distally but at lesion site are located within epineurium of median nerve trunk. The anatomical localization of the predominant lesion focus is shown on the metric scale to the left of the schematic drawing of bony landmarks. Differences in individual arm lengths were not corrected for.

Figure 4. T2 signal analysis of median nerve trunk fascicles at upper arm level

The upper graph (red) visualizes ordered percentile ranks of normalized T2 signal of each individual fascicular median nerve T2 lesion of the patient group (n = 20, red circles). The lower graph (green circles) visualizes percentile ranks of median nerve normalized T2 signal of the control group (n = 20). Perfect discriminative power to separate patients with anterior interosseous nerve syndrome from controls was observed at a cutoff value of ≥1.7 of normalized T2 signal. The complete separation of both distributions corresponds to 100% sensitivity and 100% specificity. Manual segmentation of lesioned fascicles (patients) and normal median nerves of healthy controls was performed as indicated by red and green regions of interest on the right side of the diagram.

Figure 5. Fascicular torsion of median nerve motor fascicles as rare cause of anterior interosseous nerve syndrome

Monofocal T2 lesion on high-resolution magnetic resonance (MR) neurography of patient 1 guided targeted surgical epineurotomy of the median nerve trunk at upper arm level. The fascicular lesion focus (MR neurography fascicular T2 lesion) was at 9.2 cm proximal to the humeroradial joint space (left, red circle). Surgical exploration precisely at this level revealed subtle increase in median nerve trunk caliber before dissection of its epineurium (before epineurotomy, middle). Fascicular torsion of the anterior interosseous fascicles within the median nerve trunk was found after epineurotomy (asterisk, right). This constriction was carefully mobilized so that eventual detorsion could be performed, followed by satisfactory clinical recovery. Intraoperative images provided by Dr. Thomas Dombert, Dossenheim-Heidelberg, Germany.