Diagnostic and Interventional Imaging (2012) 93, 398—400 LETTER / Neurology Axonotmesis of the sciatic nerve a,∗ b c c M. Ohana , S. Quijano-Roy , F. Colas , C. Lebreton , c c C. Vallée , R.-Y. Carlier a Radiology Department, Nouvel Hôpital Civil, Strasbourg University Hospitals, 1, place de l’Hôpital, 67000 Strasbourg, France b Paediatrics Department, Raymond-Poincaré Hospital, 104, boulevard Raymond-Poincaré, 92380 Garches, France c Radiology Department, Raymond-Poincaré Hospital, 104, boulevard Raymond-Poincaré, 92380 Garches, France Case report KEYWORDS Peripheral nerve; We report the case of an eight-year old girl who was admitted for aftercare and rehabilita- MRI; tion one month after a serious head injury that required a four-day stay in intensive care. Axonotmesis Initial investigations did not show any evidence of post-traumatic injury. During her admission, she developed significant pain in the left buttock radiating to the lower limb associated with a sensorimotor deficit. These disabling pains persisted at rest. The clinical examination revealed that the patient had great difficulty walking, presenting a limp, a tender point on palpation of the left buttock radiating to the thigh and the leg along a posterolateral course, with hyperaesthesia in the whole area. Extension of the leg and both flexion and extension of the foot were impossible; hip flexion was normal. Hypoaesthesia was noted on the inside of the left leg and foot. The left patellar and Achilles reflexes were absent. Vital signs were normal. First-line magnetic resonance imaging (MRI) of the lumbar spine did not reveal any abnormalities. An MRI of the pelvis and lower limbs was then carried out and this highlighted involve- ment of the sciatic nerve along its whole extra-perineal course (Fig. 1a), with an overall increase in its calibre (Fig. 1b), a clear and homogenous high T2 signal intensity and a loss of its fascicular structure compared to the contralateral side (Fig. 1c). The nerve remained uninterrupted along its whole course. ∗ Corresponding author. E-mail address: [email protected] (M. Ohana). 2211-5684/$ — see front matter © 2012 Éditions françaises de radiologie. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.diii.2012.03.001 Axonotmesis of the sciatic nerve 399 Figure 1. MRI of the pelvis and lower limbs: a: coronal STIR; b: axial T1; c: axial T2 SPAIR. Clear and continuous high-signal intensity of the left sciatic nerve along its extra-perineal course (a), with an increased calibre compared to the contralateral side (b, right arrow). On T2-weighted SPAIR sequences (c), the normal fascicular appearance visible on the right (small right arrows) is not found on the injured side (curved arrow). surrounded by a structure of connective tissue called the On electroneuromyography, the territory of the left sci- endoneurium. Several axons group as a nerve fascicle, which atic nerve showed marked signs of acute denervation: the is enclosed in a further layer of connective tissue, the per- tibialis anterior, semimembranosus and semitendinosus mus- ineurium. A variable number of fascicles (up to a hundred) cles were affected, showing an increased insertion activity, are grouped together to form the nerve trunk, encased positive waves and fibrillations. The tensor fascia latae by the epineurium. The Seddon classification is the sim- muscle (innervated by the superior gluteal nerve — L5), the plest scale available, distinguishing three levels of pathology gluteus maximus muscle (inferior gluteal nerve — S1), the depending on the depth of the post-traumatic injury: psoas muscle (femoral nerve — L2/L3) and paravertebral • neurapraxia: the trauma causes destruction of the myelin muscles (branches L5/S1) were intact, with no spontaneous sheath, without affecting the axons or causing rupture of activity and normal voluntary contractions. the surrounding connective tissue. This local conduction The diagnosis of post-traumatic nerve injury also known blockage resolves fully in less than twelve weeks; as high-grade axonotmesis of the sciatic nerve, secondary • axonotmesis: the trauma causes destruction of the to a trauma located at the notch, as suggested by the imag- myelin sheath and downstream wallerian degeneration. ing results was confirmed by the electroneuromyogram and The encapsulating connective tissue (endoneurium) the patient’s clinical evolution. Final clinical outcome was is preserved, thus serving as a guide for proximodistal slowly favourable over several months. axonal regrowth. Recovery is slow (1 mm/day) and usually complete; Discussion • neurotmesis: there is a full section of the nerve, with Post-traumatic nerve injuries can be categorised using Sed- disrupted continuity in all the layers and downstream don’s clinical classification [1], based on the histological wallerian degeneration. Surgical intervention to re- anatomy of the nerve fibre (Fig. 2). The axon is the base establish continuity is required, or nerve regrowth will unit of a peripheral nerve; it can be myelinated and is end up forming a proximal neuroma. 400 M. Ohana et al. Figure 2. Schematic anatomy of the peripheral nerve. The MRI appearance of peripheral nerves is concor- common in animal models (nerve crush in mice) but have dant with their anatomy [2]. On T1-weighted sequences, rarely been reported in humans. the nerve appears isointense compared to the muscle, sometimes demonstrating a peripheral halo of high- Disclosure of interest signal intensity corresponding to the epineurium fat. On T2-weighted sequences, the nerve appears with a moder- The authors declare that they have no conflicts of interest ately high signal due to endoneurial fluid; high-resolution concerning this article. acquisitions can demonstrate its fascicular ultrastruc- ture. Under experimental conditions, wallerian degeneration References can be visible on MRI from 48 hours onwards [3]. It appears as a clear high signal on T2-weighted sequences, a loss of [1] Durandeau A, Fabre T. Lésions traumatiques des nerfs the normal fascicular structure and an overall increase of périphériques (plexus brachial exclu), Encycl Med Chir (Appareil the nerve calibre [4]. It is therefore theoretically possible Locomoteur). Éditions Scientifiques et Médicales Elsevier SAS. 15-003-A-10 (2000). on MRI to distinguish neurapraxia (absence of imaging abnor- [2] Moser T, Kremer S, Holl N. Imagerie du nerf périphérique : malities), from axonotmesis (signs of wallerian degeneration anatomie, techniques d’explorations et principales pathologies. with preservation of nerve continuity) and neurotmesis J Radiol 2009;90(10):1448. (total loss of nerve continuity with signs of wallerian degen- [3] Bendszus M, Wessig C, Solymosi L, Reiners K, Koltzenburg eration below the injury). In practice, the distinction is not M. MRI of peripheral nerve degeneration and regeneration: always that clear and final diagnosis will depend above all correlation with electrophysiology and histology. Exp Neurol on clinical and electroneuromyographic findings. 2004;188:171—7. Our case is very demonstrative, as we depict nicely the [4] Filler AG, Maravilla KR, Tsuruda JS. MR neurography and muscle high-signal intensity on T2-weighted images and the hyper- MR imaging for image diagnosis of disorders affecting the periph- trophy of the nerve along its course. These observations are eral nerves and musculature. Neurol Clin 2004;22:643—82..