Magnetic Resonance Neurography and Peripheral Nerve Surgery

Fig. 7.1 Normal median nerve within the carpal tunnel. On axial T1w (a), it is isointense to muscle (flexor carpi ulnaris, asterisk) and exhibits a fascicular appearance; on STIR (b), it exhibits mildly high signal intensity (arrows).

7.2 Trauma

Most patients with acute nerve transection do not require MRN imaging. Nevertheless, in patients without nerve transection—which accounts for the majority of serious injuries—it might be difficult to distinguish between those injuries that will and those that will not recover spontaneously and may require surgery. MRN imaging plays an essential role in this subset of patients.

In an attempt to classify the physical and functional state of damaged nerves, Seddon ² introduced the terms neurapraxia, axonotmesis, and neurotmesis. Sunderland ³ refined this classification, based on the recognition that axonotmetic injuries had widely variable prognoses, depending on the degree of connective tissue involvement. These terms and this classification system are useful, because they indicate the pathological status of the nerve, predict the prognosis if the injury is left untreated, and provide a guide to management. MRI can aid in distinguishing between these lesions, providing vital information for management and surgical planning.

With neurapraxia, the nerve is intact, but cannot transmit impulses. With axonotmesis, the axon is damaged or destroyed, but most of the connective tissue framework is maintained. Meanwhile, with neurotmesis, the nerve is disrupted and the connective tissue framework is either totally lost or badly distorted. On MRN imaging, these three classes of injury look different ⁴^, ⁵ ( ▶ Table 7.1). Typical MRN imaging findings in neurapraxic injuries ( ▶ Fig. 7.2) and Sunderland´s first-degree nerve lesions are a focal increase in nerve signal intensity on fluid-sensitive sequences, combined with no signal abnormalities in muscle signal intensity, except for mild atrophy secondary to disuse.

**Table 7.1** MRN findings in traumatic nerve injuries
Seddon classification	Sunderland classification	MRN findings
Neurapraxia	First degree	Nerve: increase SI on FSS Muscle: mild atrophy, no denervation
Axonotmesis	Second degree	Nerve: enlargement, increased SI on FSS Muscle: signs of denervation
	Third degree	Nerve: Acute: enlargement, increased SI on FSS, loss of fascicular appearance Subacute/chronic: neuroma-in-continuity Muscle: signs of denervation
	Fourth degree	Nerve: enlargement, increased SI, loss of fascicular appearance, blockage of axoplasmic flow on DTT Muscle: signs of denervation
Neurotmesis	Fifth degree	Nerve: Acute: gap shows high SI on FSS Chronic: terminal neuroma Muscle: signs of denervation
Abbreviations: DTT, diffusion tensor tractography; FSS: fluid-sensitive sequences; SI, signal intensity.

Fig. 7.2 Neurapraxia. On coronal STIR, note increased signal intensity in the C5 (arrow) and C6 roots (arrowhead) of the brachial plexus.

MRI findings in axonotmetic lesions include neural enlargement and transient increases in nerve signal intensity on T2w and short tau inversion recovery (STIR) images, combined with loss of the normal fascicular appearance, blurring of the perifascicular fat, and signs of muscle denervation (appearing within 24–48 hours); this is followed by muscle volume reduction and fatty atrophy if nerve regeneration does not occur. MRN is further able to distinguish axonotmetic injuries as subclassified by Sunderland. With a type III injury, the endoneurium is disrupted, intrafascicular fibrosis takes place, and a neuroma-in-continuity ( ▶ Fig. 7.3) is formed that appears, on MRN imaging, as a fusiform enlargement with intermediate to high signal intensity in fluid-sensitive images with variable contrast enhancement. With Sunderland type IV injuries, only the epineurium is intact, so MRN images demonstrate lost fascicular appearance, increased nerve signal intensity on T2w and STIR sequences, and blockage of the axoplasmic flow on DTT ( ▶ Fig. 7.4), which is an emerging MRN technique that generates a 3D image of neural tracts, thereby allowing clinicians to assess axonal integrity. ⁶

Fig. 7.3 Neuroma-in-continuity in the elbow, distal to the cubital tunnel. (a) Sagittal STIR and (b) axial proton density (PD) images show fusiform enlargement and a slight increase in signal intensity (arrows).

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