Tarsal Tunnel Syndrome: Ultrasound of the Posterior Tibial Nerve
Tarsal tunnel syndrome is classified among compressive neuropathies of the lower limb and is associated with entrapment of the posterior tibial nerve. Ultrasound examination in this situation is used for direct visualization of the nerve and identification of potential causes of compression, including ganglion cysts and mass lesions (according to Beltran LS et al. Entrapment neuropathies III: lower limb. Semin Musculoskelet Radiol. 2010).
General Ultrasound Findings in Peripheral Nerve Injury
In compressive and traumatic neuropathies, ultrasound assessment includes: changes in echo texture and nerve caliber, presence of hypoechoic spindle-shaped thickening, formation of neuromas, and presence of intraneural hyperemia on color Doppler imaging. According to the source, transection neuromas are visualized as heterogeneous, predominantly hypoechoic lesions in continuity with the corresponding nerve.
What to Look for in the Tarsal Tunnel
Ultrasound allows identification of causes of nerve entrapment: ganglion cysts and peripheral nerve sheath tumors, whose sonographic characteristics are described in specialized literature (Reynolds Jr. DL, Jacobson JA et al. AJR Am J Roentgenol. 2004). Specific diagnostic thresholds for cross-sectional area (CSA) of the posterior tibial nerve in tarsal tunnel syndrome are not provided in the available source fragments [to be clarified].
Role of Doppler
By analogy with assessment of other compressive neuropathies, intraneural vascularization should be evaluated in the zone of maximum nerve thickening. Neural hyperemia has low sensitivity but high specificity as a sign of nerve entrapment (according to Griffith JF et al., Diagnostic Ultrasound: Musculoskeletal, 2025, as applied to CTS). Extrapolation of these parameters to the tarsal tunnel requires confirmation [to be clarified].
Frequently asked questions
What is the ultrasound CSA threshold for the posterior tibial nerve in tarsal tunnel syndrome?
In the available source fragments, a specific numerical CSA threshold for the posterior tibial nerve is not provided [to be clarified]. The CSA thresholds presented in the source refer to the median nerve in carpal tunnel syndrome.
What causes of compression does ultrasound detect in the tarsal tunnel?
Ganglion cysts and peripheral nerve sheath tumors, as well as post-traumatic changes and neuromas, visualized as heterogeneous predominantly hypoechoic lesions in continuity with the nerve.
Should Doppler assessment be performed when neuropathy is suspected?
Yes. Intraneural hyperemia should be evaluated in the zone of maximum nerve thickening. This finding has low sensitivity but high specificity (according to the source as applied to carpal tunnel syndrome).
What does a neuroma look like on ultrasound?
As a heterogeneous, predominantly hypoechoic lesion that maintains continuity with the corresponding nerve (transection neuroma).