Considerations about the multidimensional evaluation of a stab wound tibial neuropathy: a case report

Mononeuropathy is a damage or a dysfunction of a single peripheral nerve, resulting in muscle weakness, numbness, tingling or pain in the innervated area. Nerve trauma is a common cause of acquired mononeuropathy and a major source of disability among younger people^(1,2). A multidimensional evaluation through ultrasound examination (US) and neurophysiological examination can provide a meaningful contribution in the diagnostic and therapeutic approach of traumatic mononeuropathies^(3,4). The role of a comprehensive assessment of the peripheral nerve pathology should be highlighted in clinical practice, especially when multiple sources of potential nerve injuries are present.

We report the case of a 33-year-old man who came to our attention after being a victim of a stabbing attack, which had occurred three months earlier, resulting in multiple knife wounds. No clinical reports or imaging investigation had been recorded at the time of the event. His past medical history was significant for alcohol and cocaine abuse five years before. He presented at least four lesions at the posterior face of the left thigh and the left leg, subsequently developing severe gait impairment with abolished left foot plantar flexion and chronic intermittent pain radiating from the injured thigh to the foot plantar region. Physical examination showed hyposthenia in the left leg: the Medical Research Council Scale for Muscle Strength was 2/5 in foot and toe plantar flexion, in foot internal rotation and supination. Triceps surae reflex was absent. No other neurological symptoms and signs were reported and no left anterior leg muscles were involved. The rest of the physical examination was unremarkable. No abnormalities were found in blood tests. A left lower tibial nerve injury was suspected, and a neurophysiological examination was performed.

Nerve conduction study showed absent motor response after left tibial nerve proximal and distal stimulation (Tab. 1), while left peroneal nerve conduction recorded in the fibular nerve territory, as well as sensitive response of the left sural nerve were normal (Tab. 2). Needle electromyography showed significant hallmarks of denervation without recruitment patterns of motor units of the lower muscles innervated by the left tibial nerve at rest and during voluntary contraction. The muscles of the left anterior leg showed normal activity. (Tab. 3). All the electrical activity recorded in the right lower limb was within normal limits. A severe neuropathy of the left tibial nerve was diagnosed.

US was performed for the assessment of the suspected traumatic tibial nerve lesion in the patient. The left tibial nerve was detected just below the popliteal fossa and then antidromically followed all along its course up to the lower division of the sciatic nerve trunk. Surprisingly, no signs of lesion were detected at this level. A hypoechoic swelling of the sciatic nerve, one cm in length, was instead detected more proximally, twenty cm below the left gluteal fold secondary to another cutaneous injury. The swelling was followed by a second three cm-long hypoechoic swelling similar to the one described above. Between them, apparently untouched nerve fascicles with a significant reduction of the trunk cross-sectional area were detected. Even more proximally, the nerve structure appeared normal (Fig. 1). These findings were found to be consistent with a partial lesion of the sciatic nerve fibers, without a complete structural interruption of the trunk: between the two traumatic neuromas, nerve fibers appeared to be spared. Considering neurophysiological results together with US findings, we concluded for a traumatic injury to the tibial fibers of the left sciatic nerve located in the median third of the thigh, with peroneal fibers apparently spared. A neurotmetic nature of the lesion was suspected with a high degree of confidence. The nerve lesion was found to be significantly more proximal than expected. The patient was immediately enlisted for a reconstructive procedure of direct nerve repair, tailored to the site and the degree of the unexpected lesion and scheduled for elective surgery.

Fig. 1.

The sciatic nerve arises from the lumbosacral plexus and descends posteriorly in the thigh generating two main terminal nerves, the common peroneal nerve and the tibial nerve, which are already well-defined inside the sciatic nerve trunk before the division⁽⁵⁾. Traumatic injuries to the tibial division are rather rare lesions among acquired sciatic neuropathy and they are usually located below the distal thigh downstream of the tibial nerve origin⁽⁶⁾.

Neurotmesis is a complete physiological and anatomical transection of both axons and connective tissue of a peripheral nerve⁽³⁾. Without a surgical approach, regeneration of the nerve is unlikely⁽²⁾ and early surgical interventions often result in the best outcome^(3,7), for these reasons neurotmetic lesion requires a rapid identification of site and degree of the injury⁽⁸⁾.

Neurophysiological studies provide functional information with significant efficiency, but anatomical and morphological detailing often requires neuroimaging assessment⁽⁹⁾.

Magnetic Resonance Neurography and US have emerged as the most comprehensive imaging modalities in the assessment of peripheral nerve pathology and their applications are mostly based on clinical experience and their availability^(10,11).

US of the peripheral nerve system requires a linear multifrequency transducer with frequencies up to 15 MHz⁽¹²⁾. Higher-frequency transducers, up to 17 to 18 MHz, can be useful for scanning the nerve in the ankle area and more distally. Frequency may need to be lowered for patients of larger body habitus⁽¹³⁾. On US, nerves appear more echogenic than muscles and less echogenic than tendons and in a longitudinal view, a nerve appears as a hypoechoic stripe (nerve fascicles) partitioned by hyperechoic lines (perineurium)⁽¹⁴⁾. This imaging technique can assess structural abnormalities of nerve trunk thanks to a very high spatial resolution and a high sensitivity especially for detecting focal neuropathic injuries⁽¹⁵⁾. US has also a pivotal role in estimating the degree of lesions together with physical examination, clinical expertise and neurophysiological findings⁽¹⁶⁾, and in diagnosing many types of peripheral neuropathies in asymptomatic stages through elastography, an US application capable of evaluating the biological tissue stiffness⁽¹⁷⁾. In brief, US of the peripheral nervous system is an essential diagnostic contribution and helps guide treatment decisions without side effects and with a high level of tolerance⁽¹⁸⁾.

Regarding the case, relying only on the clinical presentation of our patient and neurophysiological findings together with the presence of multiple scars in the left leg close to the nerve path, we suspected severe neuropathy of the left tibial nerve. Three months had passed from the event, so a neurotmetic damage had likely occurred. Imaging investigations appeared to be necessary to confirm the diagnosis and to plan the future therapeutic approach. Those considerations led us to perform US of the peripheral nerve as a first choice. US allowed us an easy and immediate access to the entire course of the nerve with identification of the unexpected site of the injury on the sciatic trunk and confirmation of the lesion degree.

In conclusion, a multidimensional diagnostic approach including US should be part of the clinical practice for the assessment of traumatic peripheral nerve pathology, especially when a neurotmetic damage is suspected and a surgical intervention is potentially required. Based on our experience, when a traumatic lesion with a tibial neuropathy-like presentation occurs consequently to multifocal cutaneous wounds of the lower limbs, the involvement of the sciatic nerve should be excluded. US could also be considered as an appropriate choice for this neuroimaging request.