Table 1.
Characteristic ultrasound findings in entrapment neuropathies
| CSA | Echogenicity | Vascularity | Other | |
| Median neuropathy at the wrist – CTS | Increased CSA at the distal wrist crease[68] | Reduced median nerve echogenicity proximal to compression site both with[77,78] and without EDX abnormalities in 82.4%[79] | Increased median nerve vascularity, with[65] and without EDX abnormalities in 7.1%[79] | Reduced median nerve mobility (transverse and longitudinal) with[77,80,81] and without EDX abnormalities in 75%[79] |
| Range ULN 8.5-12 mm2[69-73] | In CTS confirmed on EDX:[65] | Reduced transverse sliding of the median nerve correlated with presence of CTS and severity of CTS[82,83] | ||
| Using ULN of 8.5-10 mm2 for the diagnosis of CTS[74] | Loss of fascicular architecture | Sensitivity 95% | Improved mobility post steroid injection[78] | |
| Sensitivity 65%-97%, Specificity 72.7%-98%, PPV 79%-97% | Specificity 71% | Increased Median nerve stiffness - seen with all elastography modalities[51]: | ||
| WFR > 1.4[68] | PPV 94% | Correlates with severity[84,85] | ||
| Comparing 12 cm proximal to the distal wrist crease | Improves post steroid injection[78] | Stiffness decreases after CTS release[86] and steroid injections[87] | ||
| Sensitivity 100% (Specificity NA) | Identify anomalous anatomy: | |||
| US parameters suggested for the presence of CTS with normal EDX[66]: | Bifid median nerve 5%-10%[88-90] | |||
| CSA > 14 mm2 | Persistent median artery 0.5%-3.7%[88,90] | |||
| WFR > 1.8 | Anomalous muscles in the carpal tunnel[91] | |||
| In CTS with a negative US at the distal wrist crease, 25% have CSA enlargement in the distal carpal tunnel. | ||||
| Scanning the whole carpal tunnel increases sensitivity by 15%-20%[75,76] | ||||
| UNE | Increased CSA, maximal at medial epicondyle | Reduced nerve echogenicity and increased nerve hypoechoic fraction[22,98] | Intraneural vascularisation[100,101] | Ulnar nerve subluxation with elbow flexion in 14%-23.1%; dislocation across the medial epicondyle in 8.5%[94] |
| ULN 8-11 mm2, most commonly 10 mm2[92-96] | Hypoechoic fraction increased distal and proximal to the medial epicondyle[98] | Present in 15% | Ulnar nerve dislocation and symptomatic UNE can be found in the context of negative EDX findings | |
| Using the following cut-off values for UNE: | Correlated with axonal loss, atrophy and increased CSA | Highest diagnostic yield is in patients with EDX abnormalities, but unable to localise - diagnosis confirmed by US in 86%[93] | ||
| > 9 mm2 - Sensitivity 84%, Specificity 80%[97] | Alternative locations for entrapment | |||
| > 11 mm2 - Sensitivity 95%, Specificity 71%[95] | Arcade of Struthers[102] | |||
| Note: Maximal CSA may not be at the site of EDX abnormality[98] | Wrist[103] | |||
| Increased ratio of nerve CSA at elbow compared with arm or forearm[95,96,98,99] | Identification of alternate aetiologies[104-112], e.g. trauma, brachial artery occlusion, scar tissue, haematoma, subdermal implants, venepuncture, ganglion cysts, anconeus epitrochlearis muscle | |||
| Upper arm swelling ratio > 2.1[96] | Guide management[113,114] | |||
| Max CSA/ Mid-upper arm CSA | Cubital tunnel – entrapment – benefits surgical decompression | |||
| Sensitivity 60%, Specificity 74% | Epicondylar groove – external compression or stretching – no benefit from surgery | |||
| Forearm swelling ratio > 2.3[96] | ||||
| Max CSA/ Mid-forearm CSA | ||||
| Sensitivity 65%, Specificity 79%, PPV 89.2% | ||||
| Radial neuropathy at the spiral groove or PIN | Increased CSA[16,66,115,116]: | Loss of echogenicity with PIN at level of maximal CSA[117] | Intraneural hypervascularity at site of compression[118] | Loss of fascicular structure with PIN at site of maximal CSA[117] |
| Radial nerve in spiral groove and antecubital fossa – ULN 9-10 mm2 | Provides additional information complementing EDX in 84%[116] | |||
| Posterior interosseous nerve – ULN 1-2 mm2 | ||||
| Superficial radial nerve - ULN 1-3 mm2 | ||||
| Common peroneal neuropathy at fibular head | Increased CSA at fibular head | Loss of echogenicity at the site of maximal compression, with and without a normal CSA[121,124] | NA | Sensitivity for diagnosis[125]: 90% (95%CI, 79.7%- 97.3%) |
| ULN 8-14 mm2, commonly 11 mm2[16,119-122] | US localisation[120]: | |||
| Using the following cut-off values for the common peroneal nerve at the fibular head: | Fibular head 55% | |||
| > 8 mm2 - Sensitivity 90%, Specificity 69%[120] | Above fibular head 71% | |||
| > 11.5 mm2 - Sensitivity 89%, Specificity 99%[121] | Addition of US to EDX allowed for localisation in 88%, compared with 65.5% EDX alone[120] | |||
| Suggested cut-offs: | Alternate aetiology[16,126,127]: | |||
| 8 mm2 if abnormal EDX[120] | Ganglion cysts (18% of cases) lipomas, scar tissue after knee surgery, abnormal biceps femoris anatomy | |||
| 11 mm2 for normal EDX[120] | Guides requirement for surgical intervention[128] | |||
| CSA may be normal if conduction block, but no axonal loss, on EDX[123] | ||||
| LCFN - MP | Increased CSA at anterior superior iliac spine[129,130] | Intraneural hypoechogenicity[129] | NA | Anatomical variants in 20%[131] |
| 4-5 mm2[129,130] | LFC nerve closer to/lateral to the Anterior superior iliac spine in patients with idiopathic MP[132,133] | |||
| ULN 5 mm2 – Sensitivity 95.7%, Specificity 95.5%, PPV 95.7%[130] | Neuroma[131] | |||
| Increased nerve diameter | ||||
| 4 mm[129] | ||||
| Tibial neuropathy - TTS or Proximal tibial neuropathy | Increased CSA at or distal to the tarsal tunnel (cf. median nerve) can be seen in the[134]: | Intraneural and fascicular hypoechogenicity[135] | NA | Identification of underlying aetiology of TTS |
| Tibial nerve, Medial plantar and Lateral plantar nerves | Ultrasound identifies 60-80%[136,137] | |||
| Using the following cut-off values for the tibial nerve within the tarsal tunnel[134]: | Preoperative US assessment identifies[136,138-140]: Ganglia (7-58%), Talocalcaneal coalition +/- ganglia, Varicose veins (18- 6%), Accessory soleus and flexor digitorum muscles (6%) | |||
| > 19 mm2 - Sensitivity 61%, Specificity 88% | Multiple sites of compression frequently seen in the tarsal tunnel and more distal or proximal[137] | |||
| Within tunnel-to-proximal tunnel CSA ratio > 1[134] | Identification of underlying aetiology in proximal tibial neuropathies[141-143]: Baker’s cysts, intraneural ganglion cysts, deep vein thrombosis, popliteal artery aneurysms, soleal sling | |||
| Measured within the tarsal tunnel, proximal and inferior to the medial malleolus, and 4 cm proximal at the upper border of the medial malleolus | ||||
| Sensitivity 74%, Specificity 100% | ||||
| Best to compare CSA to contralateral side | ||||
| Neurogenic thoracic outlet syndrome | Enlarged CSA of lower trunk of the brachial plexus[144] | Hypoechogenicity of the lower trunk of brachial plexus[144] | NA | Alternate aetiologies[144-147]: |
| Affected - 32.5 mm2 (range: 20-50 mm2) | Loss of fascicular structure[144] | Cervical rib | ||
| Controls – 16.7 mm2 (range: 9-23 mm2) | Hyperechoic fibromuscular band (wedge shaped) adjacent to the middle scalene muscle – indenting the lower trunk of the brachial plexus (sickle shaped)[144] | Anomalous muscular insertion (scalene muscle between subclavian artery and brachial plexus) | ||
| “Wedge-sickle” sign | Brachial plexus branching variants | |||
| Sensitivity 95%, PPV 82.6% | Subclavian arterial occlusion with dynamic movement | |||
| Sonographic Tinel’s sign[144] | ||||
| Sensitivity of 55% (95% CI: 31.53%–76.94%) | ||||
| PPV of 100% (95% CI: 71.51%–100.00%) |
CTS: Carpal tunnel syndrome; CSA: Cross-sectional area; ULN: Upper limit of normal; WFR: Wrist to forearm ratio; US: Ultrasound; EDX: Electrodiagnostics; PPV: Positive predictive value; UNE: Ulnar neuropathy at the elbow; PIN: Posterior interosseous syndrome; NA: Not available; LFCN: Lateral femoral cutaneous neuropathy; MP: Meralgia paraesthetica; TTS: Tarsal tunnel syndrome; LFC: Lateral femoral cutaneous; CI: Confidence interval.