MRI of Peripheral Nerves And Entrapment Syndromes

Transcription

1 MRI of Peripheral Nerves And Entrapment Syndromes Christine B. Chung, M.D. Professor of Radiology Musculoskeletal Division UCSD and VA Healthcare System Nerve anatomy Basic classification of nerve injury Basic understanding of advantages of MR protocols Nerve disorders Plan of Attack Nerves Enclosed, Cable-like Bundle of Axons Nerve Anatomy Axon (nerve fiber) is a long, slender projection of a nerve cell or neuron that typically conducts electrical impulses away from the neuron s cell body Axon is one of 2 types of protoplasmic protrusions that extrude from the cell body of the neuron, the other being dendrite Distinguished from dendrites by shape (dendrites taper/ axons maintain constant radius) Length (dendrites are restricted around cell body, axons are much longer) Function (dendrites receive signal, while axons transmit them) Axons make contact with other cells (usually other neurons/ sometimes muscles or glands) at junctions called synapses Individual axons microscopic 1 to 20 microns in diameter but can be several feet in length Axons sheathed in myelin formed by glial cells (Schwann cells in peripheral nervous system/ oligodendrocytes in the CNS) Along the myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly spaced intervals 3 mm External epineurium Internal epineurium Perineurium Endoneurium Axon Myelin μm Nerve Anatomy Nerve Anatomy INTERNAL EPINEURIUM: External epineurium Thick, densely packed collagen fibrils in a lamellar fashion, in longitudinal orientation Internal epineurium 3 mm Perineurium Endoneurium PERINEURIUM: Multiple layers of perineurial cells, basal laminas ( heparan sulfate proteoglycans, fibronectin), interspaced with a lacework of thin collagen fibers. The innermost layer form the BLOOD-NERVE BARRIER 1

2 Standard MR Evaluation External epineurium Internal epineurium Standard High Resolution MR Evaluation Standard High Resolution MR Evaluation Standard High Resolution MR Evaluation 3 mm Perineurium Endoneurium 3T MR Extremity Neurography Protocol High Resolution 3D MRI and Diffusion High resolution imaging (long axis and orthogonal to nerve course) T1w anatomy (fascicular pattern sorrounded by small fat rim) T2w FS pathology (FSE, STIR, SPAIR, 3D T2* ) Abnormalities are recognized by differences in shape, size, and contour of the nerves, as well as by the changes in signal intensity 3T MR Extremity Neurography Protocol High Resolution 3D MRI Technical considerations T2 SPAIR (spectral adiabatic inversion recovery) due to better SNR, better and homogeneous fat suppression, and ability to maintain SAR rate favorability Isotropic submillimeter spin echo type 3D sequences allowing multiplanar isotropic reformats 3T MR Extremity Neurography Protocol Diffusion Fiber Tracking Technical considerations Diffusion-based MRN developed to increase nerve conspicuity by vascular signal suppression and offers potential to quantify SI, apparent diffusion coefficient (ADC), fractional anisotropy (FA) and fiber tracking Several technical challenges (spatial resolution and adequate SNR, as well as ghosting artifacts) Diffusion-based 3D reversed FISP is hybrid sequence optimized using low diffusion moment, and due to steady state nature enables suppression of vascular signal creating nerve-specific images PSIF Reversed Fast Imaging Steady State Precession 3T MR Extremity Neurography Protocol Diffusion Vascular Suppression Chhabra, et al., Skeletal Radiol :

3 UTE MRI with Variable TI TI 80 ms TI 120 ms TI 160 ms TI 200 ms TI 240 ms UTE MRI with Variable TI Contrast Manipulation (TE 10 microseconds) TI 160 ms TI 280 ms Inversion pulse at 280 ms null the signal of the fascicles (neural tissue + perineurium) TI 280 ms TI 320 ms TI 360 ms TI 400 ms TI 500 ms UCSD MSK Imaging Research Group Paolo Felisaz UCSD MSK Imaging Research Group Paolo Felisaz Ultra High Field Nerve Evaluation External epineurium Internal epineurium 11.7 T Gradient echo Nerve Imaging Evaluation Features Size Caliber similar to adjacent vascular structures Decreases proximal to distal Signal Normal signal isointense to muscle on T1- and T2-weighted images Fascicular appearance Uniform in size Course No focal deviation, smooth course Enhancement Normal nerves do not enhance due to preserved blood nerve barrier Perineurium Endoneurium Nerve Imaging Evaluation Features Nerve Imaging Evaluation Features Signal Normal signal isointense to muscle on T1- and T2-weighted images (tibial nerve shown here) Fascicular appearance Uniform in size (in this case, slightly increased SI consistent with mild injury) 3

4 MRN Interpretation Pitfalls MRN Interpretation Pitfalls 0 degrees 55 degrees Subject to magic angle phenomenon Kastel, et al., AJNR : Suboptimal fat suppression and image blurring 1.5T STIR MRN Interpretation Pitfalls MR Characterization of Peripheral Nerve Injury Suboptimal fat suppression and image blurring 1.5T STIR compared with 3T 3D STIR SPACE MIP reconstruction Chhabra, et al., AJR : MR Characterization of Peripheral Nerve Injury: Signal Changes Increased in signal Depends on endoneurial fluid and background fat suppression Causes of increased signal Vascular congestion Block of axoplasmic flow, leading to proximal accumulation of endoneurial fluid Wallerian degeneration Traditional Classification of Nerve Injury Neurapraxia Preserved fasciculation, subtle SI change Axonotmesis Absent fasciculation, clear SI change Neurotmesis Nerve transection Chhabra, et al., AJR :

5 Sunderland Classification Neurapraxia Preserved Fasciculation- Subtle SI Change I and II degree injuries correspond to neurapraxia and axonotmesis III degree injury involves endoneurium IV degree injury involves perineurium MRI appearance typically shows neuroma in contintuity Regenerating nerve sprouts unable to grow down disrupted fascicles tangle in conjunction with perifascicular and intrafascicular fibrosis to form neuroma (seen in continuity with proximal and distal uninvolved nerve segments) V degree injury involves epineurium MRI appearance of complete nerve discontinuity Injury involves only the myelin sheath around the axon with resultant transient functional loss and associated with excellent prognosis MRI shows abnormal T2 hyperintensity and mild enlargement of the nerve Milder MR appearance seen in stretch injury, nerve entrapment (tunnel syndromes), compression from space-occupying lesions T2 hyperintensity of peroneal nerve at fibular tunnel Chhabra, et al., Skeletal Radiol : Neurapraxia Preserved Fasciculation- Subtle SI Change PD FS FSE TE=41 3D CUBE TE=92 3D CUBE TE=92 Preservation of fasciculation, increased SI on CUBE EMG shows slowed conduction of CPN at fibular head Axonotmesis Absent Fasciculation Clear SI Change Axon suffers complete rupture resulting in wallerian degeneration of distal segment, however supporting structures including perineurium and epineurium remain intact Prognosis good, but axonal regeneration is 1mm per day MRI findings include effacement, enlargement or disruption of individual fascicles Similar appearance with moderate to severe nerve entrapment Fascicular appearance of tibial nerve preserved, though mildly increased SI (small arrow) Fascicular appearance of common peroneal obliterated (large arrows) Chhabra, et al., Skeletal Radiol : Neurotmesis Nerve Transection Most severe type of injury and refers to complete severance of the nerve Functional loss complete and without surgical intervention no recovery anticipated MRI appearance acutely shows nerve discontinuity with fluid filled gap and granulation tissue Fibrosis at injury site typically seen in subacute and chronic stages as strandy hypointense soft tissue within the nerve gap on T2-weighted images Transection right femoral nerve with end bulb neuroma Neuroma Any nerve lacerated, avulsed or traumatized may form a neuroma Classified into 2 basic types: NIC Neuroma in Continuity Involves all degrees of nerve injury, from normal to neurotmesis, coexisting within scarred nerve Proximal injured nerve fascicles sprout in attempt to unite Due to lattice disruption, disorganization, regeneration, hypertrophy of nerve fascicles, and associated fibrosis, proximal and distal nerve fibers at site of injury may fail to appose End-bulb neuroma Occur anywhere a nerve is completely divided and unopposed by another neural tissue Chhabra, et al., AJNR :

6 Neuroma in Continuity Distinguished from neurogenic tumors by presence of surrounding scarring, lack of a split fat or target sign and absence of abnormal enhancement Neuroma in continuity CPN (large arrow), normal tibial nerve (medium arrow), cutaneous nerves (small arrows) Secondary Findings Nerve Injury Muscle signal changes Shown to represent shift between intra- and extracompartmental fluid components and do not reflect real edema Edema-like SI may manifest as early as 24 hours after onset of neuropathy Subacute changes, edemalike SI and minimal fatty replacement, weeks to months Chronic changes, fatty replacement and atrophy months to years after injury Chhabra, et al., AJNR : Bendszus, et al., AJNR : Peripheral Nerve Disorders Injury or entrapment Suprascapular and Axillary nerves Compressive Suprascapular and Axillary nerves Intraneural ganglion Inflammatory/dysmyelinating Radiation Superior Inferior Pop v Fib Tib Theories of Pathogenesis Degenerative Mucoid degeneration of the epineurium leads to cyst formation Articular Fluid dissects through the epineurium from the joint Tumoral Tumor within the epineurium involutes with cystic degeneration Spinner RJ, Scheithauer BW, Amrami KK, et al. Neurosurg 2009;65:A115-A124 Synovial Theory of Pathogenesis Cyst fluid dissects from a degenerative synovial joint along an articular nerve branch Fluid follows path of least resistance Pressure fluxes further alter cyst dimensions, configurations, and directionality >> waxingwaning clinical symptoms guide.com 6

7 Synovial Theory of Pathogenesis Imaging Findings Popliteus Denervation Popliteus changes Edema (denervation change) Tibial nerve Articular branch Spinner RJ, Herbert-Blouin MN, Maniker AH, et al. Skeletal Radiol 2009;38: Imaging Findings J Sign Tibial nerve Posterior to anterior hook of articular branch Imaging Findings Signet Ring Eccentric displacement of nerve fascicles by cyst Spinner RJ, Herbert-Blouin MN, Maniker AH, et al. Skeletal Radiol 2009;38: Spinner RJ, Herbert-Blouin MN, Maniker AH, et al. Skeletal Radiol 2009;38: Imaging Findings Clock Face Pathology Tibial nerve articular branch 9 o clock Signet ring sign 4-5 o clock Transverse limb sign 12-2 o clock Spinner RJ, Herbert-Blouin MN, Maniker AH, et al. Skeletal Radiol 2009;38: Entrapment Syndromes Upper Extremity Brachial Plexus at the Thoracic Outlet Entrapment at the Shoulder Musculocutaneous Nerve Axillary Nerve Suprascapular Nerve Entrapment at the Arm and Elbow Radial Nerve (Arm) Ulnar Nerve (Elbow) Radial Nerve and Posterior Interosseous Nerve (Elbow) Median Nerve and Anterior Interosseous Nerve (Elbow) Entrapment at the Wrist Ulnar nerve Median Nerve Palmar Cutaneous Branch of the Median Nerve Superficial Branch of the Radial Nerve 7

8 Entrapment Shoulder: Suprascapular Nerve Entrapment Shoulder: Suprascapular Nerve C5 root C6 root Ant scalene m. C C7 root C8 root T1 root From upper trunk of brachial plexus, C5 and C6 nerve roots Motor fibers to the supraspinatus and infraspinatus muscles Sensory fibers to the glenohumeral and A-C joints Entrapment Shoulder: Suprascapular Nerve Entrapment Shoulder: Suprascapular Nerve H Suprascapular a. Transverse ligament H S Suprascapular n. Suprascapular n. S UCSD MSK Imaging Research Group Niyata Chitrapazt Spinoglenoid notch perilabral cyst with secondary findings of infraspinatus edema Shoulder: Suprascapular Nerve Entrapment Thickened Transverse Scapular Ligament Shoulder: Suprascapular Nerve Entrapment Post-Traumatic Scar Transverse scapular ligament Spinoglenoid ligament Coracoclavicular ligament injury with fibrous scar and tethered suprascapular nerve 8

9 Shoulder: Suprascapular Nerve Entrapment Post-Traumatic Scar Shoulder: Axillary Nerve Axillary a. Coracoid Subscapularis m. Terminal branch of posterior cord of brachial plexus, contributions from C5 and C6 Courses along anterior surface of subscapularis muscle, then sharp turn posteriorly to travel along the inferior glenohumeral joint and then enters the quadrilateral space Axillary nerve can be injured by trauma following anterior shoulder dislocation Axillary nerve can be compressed in the quadrilateral space Coracoclavicular ligament injury with fibrous scar and tethered suprascapular nerve Axillary n. UCSD MSK Imaging Research Group Niyata Chitrapazt Axillary Nerve Anterior Dislocation Traction Phenomenon Shoulder: Axillary Nerve Deltoid H H G G Teres Minor Axillary n. Axillary n. Edema deltoid and teres minor post reduction follow-up UCSD MSK Imaging Research Group Niyata Chitrapazt Shoulder: Axillary Nerve Quadilateral Space H Axillary n. Long head of triceps m. Teres minor m. Teres major m. Superior: Teres Minor Inferior: Teres Major Medial: Long head triceps Lateral: Humerus UCSD MSK Imaging Research Group Niyata Chitrapazt Axillary Nerve Quadilateral Space Syndrome (Entrapment) Fibrous Band 9

10 Shoulder: Axillary Nerve Quadilateral Space Syndrome (Compression) Osteophyte Shoulder: Axillary Nerve Posterior Instability with Traction Phenomenon Prominent osteophytes can compress axillary nerve POLPSA with edema teres minor Shoulder: Axillary Nerve Posterior Instability with Traction Phenomenon 3% incidence of isolated teres minor atrophy Associated findings Rotator cuff 47/61 Posterior labral tear 31/61 GHJ OA 29/61 Possible relationship to traction or rotator cuff injury during translational episode Sofka,, et al., Skeletal Radiol 2004,33(9): Ulnar Nerve At the midhumerus level, it pierces the intermuscular septum and enters the posterior compartment. The ulnar nerve may pass under the arcade of Struthers and follows a groove in the medial head of the triceps muscle, arriving at the cubital tunnel. Cubital tunnel is formed by Anterior: medial epicondyle Lateral: olecranon Roof: arcuate ligament Travels between the humeral and ulnar heads of FCU muscle and enters the anterior compartment of forearm after piercing flexor pronator aponeurosis Ulnar Nerve Ulnar Nerve Cubital Tunnel Thickened Osborne s Ligament (Arcuate Ligament) Posterior humerus, groove of triceps, proximal to cubital tunnel Abnormal caliber, SI and loss of fasciculation 10

11 Ulnar Nerve Cubital Tunnel Mass Effect from Anconeus Epitrochlearis/ Hypertrophied Medial Head Triceps Valgus Extension Overload With existing valgus instability, elbow slightly subluxed in valgus position as flexion occurs Valgus Extension Overload and Posteromedial Impingement Distal to elbow bifurcates to Posterior Interosseous Nerve (PIN) and Superficial Radial Nerve (SRN) Ulnar Nerve Injury Common Flexor Tendon Injury Terminal branch posterior cord brachial plexus SRN superficial to supinator PIN courses between heads of supinator 5 potential sites of compression of PIN: Proximal edge of superficial portion of supinator arcade of Fröhse (1) Fibrous bands of tissue anterior to radiocapitellar joint between brachialis and brachioradialis (2) Recurrent radial vessels aka leash of Henry (3) Edge of extensor carpi radialis brevis (ECRB) (4) Distal edge of supinator muscle (5) 11

12 ECRB Ext. Dig Brachioradialis /ECRL ECU Supinator Extensor carpi radialis brevis Extensor digitorum communis Extensor carpi ulnaris Extensor digiti minimi Supinator Abductor pollicis longus Extensor pollicis longus Extensor pollicis brevis, Extensor indicis proprius Ganglion at level of Arcade of Frohse T1 PIN T1 FS Post Gado Thickened ECRB Tendon Lipoma Edema ECRB, ED and Supinator ECRB Ext. Dig Supinator ECRB Ext. Dig Supinator ECU Resistant Tennis elbow Motor neuropathy Deep forearm pain Weakness /loss extension of all digits and decreased wrist extension Median Nerve Crosses elbow, passes deep to bicipital aponeurosis Dives into antecubital fossa, medial to both biceps tendon and brachial artery, volar to the brachialis Then passes between 2 heads of pronator teres, deep to the humeral head and superficial to the ulnar head Median Nerve After this, nerve travels deep to the proximal fibrous arch of the flexor digitorum superficialis (FDS) Continues through forearm between the FDS and FDP Continues distally through carpal tunnel 12

13 Median and Anterior Interosseous Nerve AIN courses off main trunk of the median nerve approximately 4 cm distal to medial epicondyle Supply: deep muscles of the forearm (flexor pollicis longus (FPL), FDP to the index and middle fingers, and the pronator quadratus) Median and Anterior Interosseous Nerve FCR FDS FDP Median AIN FPL FDP Median and Anterior Interosseous Nerve Kiloh Nevin Syndrome R Compression of anterior interosseous nerve in proximal forearm, purely motor Motor weakness of FPL, FDP of index and middle finger and Pronator Quadratus Unable to form a O with thumb and index finger Imaging of elbow and forearm: courses over FDP and interosseous membrane Causes: Direct trauma (most common), surgery, cast pressure, bulky tendinous origin of ulnar head of pronator teres, mass, accessory muscle FPL R Pronator Quadratus Upper Extremity Nerve Entrapment Take Home Points Nerve structure MR appearance Site specific anatomy Manifestations and Distribution of Findings H Suprascapular n. S 13