High-resolution ultrasonography (HRUS) is a new tool to investigate the peripheral and spinal nerves(1). It serves as a diagnostic tool in neurological diseases in adults(2). The introduction of US probes with high frequencies (greater than 12–15 MHz) has played a significant role in the ultrasound diagnostics of the peripheral nerves(3).
Normal peripheral nerves have a typical sonographic appearance, demonstrating multiple hypoechoic bands representing fascicular bundles surrounded by a hyperechoic rim(1). High-resolution ultrasonography provides a cost-efficient and accurate imaging modality in the diagnosis of peripheral nerve lesions(4). The variability in cross-sectional area (CSA) measurements is helpful in investigating pathologies of the peripheral nerves(3).
The cross-sectional area (CSA) tends to be symmetrical in both lower limbs, and it is larger in the lower limb motor nerves than in the sensory nerves at similar sites(5). Recent imaging techniques allow for the assessment of anatomical characteristics of extremities exposed to a traumatic injury, greatly enhancing the quality of patient care, and help in optimizing clinical outcomes(6).
The examination of the peripheral nerves by US imaging is non-invasive and easily tolerated by patients(7). High-resolution ultrasonography is helpful in assessing the morphology of the peripheral nerves(5). It has added to the diagnosis and treatment decisions among mononeuropathies, and assisted in the diagnosis of peripheral nerve tumors, hereditary neuropathy, and dysimmune neuropathy(8,9).
High-resolution ultrasonography can detect changes in the peripheral nerves caused by a number of disease processes including trauma, infection, inflammation, and benign and malignant tumors, in a cost-effective manner(10).
The method is able to detect acute and chronic changes in the nerves caused by compression neuropathies(11). Chronic nerve compression causes disruption of the paranodal junctions and axonal domains required for proper peripheral nerve function(12).
Ultrasound evaluation can lead to early diagnosis of nerve injuries and hence facilitate prompt treatment(13). The ability of the clinical evaluation and electrodiagnostic studies to determine the extent of nerve damage within the first 6 weeks after trauma is limited(14). The availability of CT and MR neurography is limited and the associated costs are high(15). On the other hand, ultrasonography is a cost-efficient, portable, and dynamic modality(15).
Not many studies have been done in the past to determine the reference values of the cross-sectional area of the normal sciatic nerve. Thus, the present study seeks to obtain high-resolution ultrasonography images of the normal sciatic nerve, and on that basus assess possible relationships between the cross-sectional area and the patient’s age, height, weight, and body mass index (BMI)(1).
Two hundred subjects of both genders, above 18 years of age, and without any history of peripheral neuropathy or trauma to the lower limb, were studied by high-resolution ultrasonography.
Subjects with no history of peripheral neuropathy or trauma to the lower limb.
All patients with peripheral neuropathy due to:
trauma involving a lower extremity and/or lumbar plexus injury,
hypothyroidism,
diabetes mellitus,
pregnancy,
alcohol,
drug-induced.
After taking the informed written consent from each patient, detailed clinical history was recorded, and general physical and local examination was carried out, and high-resolution ultrasonography of the sciatic nerve was performed in both lower limbs.
High-resolution sonography was performed using Philips Affiniti 50 unit with a linear transducer with a frequency of 5–18 MHz (Fig. 1). The depth, gain, and dynamic range were adjusted appropriately for the optimal differentiation between the nerves and other soft tissue structures. The ultrasound images were obtained by placing the transducer perpendicular to the normal sciatic nerve at two levels on both lower limbs. The images were obtained with the subject in prone position. The pressure of the transducer on the skin was kept to a minimum to reduce as far as possible the deformation of underlying structures. A few studies have demonstrated the use of standard imaging as well as write-zoom magnification methods for the measurement of the CSA. In the present study, we used only standard imaging.
The cross-sectional areas of the sciatic nerve were measured at the following locations. Level I was located 1 cm above the bifurcation of the sciatic nerve into the tibial and common peroneal nerves. Level II was located 4 cm above the bifurcation of the sciatic nerve into the tibial and common peroneal nerves (Fig. 2, Fig. 3, Fig. 4). At each site, the cross-sectional area of the sciatic nerve was obtained by tracing the nerve just inside its hyperechoic rim. Three measurements were taken at each site, with the transducer repositioned. The mean value was used for each level (Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12).
The age, gender, height, weight and body mass index obtained for each subject were documented, and then correlation coefficients were calculated by statistically correlating these parameters with the cross-sectional area of the sciatic nerve at both levels.
The data was analyzed using SPSS 19.5 software. The
The mean cross-sectional area of the normal sciatic nerves located 4 cm above the bifurcation of the sciatic nerve into the tibial and common peroneal nerves (Level I) was 0.512 cm2 in the right lower limb, and 0.514 cm2 in the left lower limb.
The mean cross-sectional area of the normal sciatic nerve 1 cm above the bifurcation of the sciatic nerve into the tibial and common peroneal nerves (Level II) was 0.391 cm2 in the right lower limb and 0.390 cm2 in the left lower limb. Women had smaller cross-sectional areas of the normal sciatic nerves than men in both measuring sites (Tab. 1). However, no correlation was observed between the cross-sectional area and the age of the subjects (
Cross-sectional area of the sciatic nerve at two levels and its relationship with gender
Gender | No. of cases | Lower limb level 1 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
Male | 108 | 0.560 | 0.074 |
|
0.562 | 0.071 |
|
Female | 94 | 0.456 | 0.051 | 0.458 | 0.051 |
Gender | No. of cases | Lower limb level 2 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
Male | 108 | 0.428 | 0.060 |
|
0.424 | 0.061 |
|
Female | 94 | 0.348 | 0.047 | 0.351 | 0.046 |
Cross-sectional area of the sciatic nerve at two levels and its relationship with age
Age group (years) | No. of cases | Lower limb level 1 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
18–30 | 43 | 0.501 | 0.062 |
|
0.503 | 0.078 |
|
31–50 | 82 | 0.512 | 0.055 | 0.515 | 0.061 | ||
>50 | 77 | 0.528 | 0.093 | 0.532 | 0.092 |
Age group (years) | No. of cases | Lower limb level 2 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
18–30 | 43 | 0.388 | 0.056 |
|
0.385 | 0.059 |
|
31–50 | 82 | 0.396 | 0.041 | 0.395 | 0.072 | ||
>50 | 77 | 0.403 | 0.073 | 0.403 | 0.071 |
Cross-sectional area of the sciatic nerve at two levels and its relationship with height
Height (cm) | No. of cases | Lower limb level 1 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
<165 | 74 | 0.430 | 0.032 |
|
0.434 | 0.032 |
|
166-175 | 64 | 0.513 | 0.023 | 0.513 | 0.025 | ||
>175 | 64 | 0.605 | 0.055 | 0.607 | 0.055 |
Height (cm) | No. of cases | Lower limb level 2 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
<165 | 74 | 0.322 | 0.032 |
|
0.324 | 0.029 |
|
166–175 | 64 | 0.404 | 0.030 | 0.403 | 0.035 | ||
>175 | 64 | 0.457 | 0.047 | 0.453 | 0.046 |
Cross-sectional area of the sciatic nerve at two levels and its relationship with body weight
Weight (kg) | No. of cases | Lower limb level 1 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
<60 | 61 | 0.426 | 0.025 |
|
0.429 | 0.027 |
|
61–70 | 64 | 0.498 | 0.032 | 0.499 | 0.030 | ||
>70 | 77 | 0.591 | 0.064 | 0.593 | 0.064 |
Weight (kg) | No. of cases | Lower limb level 2 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
<60 | 61 | 0.321 | 0.027 |
|
0.323 | 0.021 |
|
61–70 | 64 | 0.386 | 0.040 | 0.387 | 0.041 | ||
>70 | 77 | 0.449 | 0.051 | 0.445 | 0.054 |
Cross-sectional area of the sciatic nerve at two levels and its relationship with BMI
Body mass index | No. of cases | Lower limb level 1 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
19.5–22.5 | 80 | 0.453 | 0.047 |
|
0.458 | 0.053 |
|
22.6–24.5 | 85 | 0.532 | 0.052 | 0.531 | 0.051 | ||
>24.5 | 37 | 0.593 | 0.105 | 0.596 | 0.101 |
Body mass index | No. of cases | Lower limb level 2 mean CSA (cm2) | |||||
---|---|---|---|---|---|---|---|
Right |
|
Left |
|
||||
Mean | SD | Mean | SD | ||||
19.5–22.5 | 80 | 0.343 | 0.041 |
|
0.344 | 0.040 |
|
22.6–24.5 | 85 | 0.415 | 0.046 | 0.413 | 0.048 | ||
>24.5 | 37 | 0.437 | 0.089 | 0.433 | 0.085 |
The sciatic nerve is the thickest nerve and the largest branch of the sacral plexus. It has the root values of L4, L5, S1, S2, S3. It has two parts: tibial and common peroneal. The tibial part is formed by the ventral divisions, and the common peroneal part by the dorsal divisions of the anterior primary rami of L4, L5, S1, S2, S3(16).
Ricci
Chen
In the present study, the cross-sectional area of the sciatic nerve showed a positive correlation with the height and weight of the patients. The cross-sectional area of the sciatic nerve was found to be higher in men than in women. No significant relationship was established with the age of the subjects. The results of the present assessment are consistent with the findings of the study conducted by Chen
High-resolution ultrasonography is an effective imaging technique that complements electrophysiological and other neuroimaging studies(4). Lee
Qrimli
Kowalska
In the present study, high-resolution ultrasonography (5–18 MHz) was used to measure the cross-sectional area (CSA) of the normal sciatic nerve. It was clear that the CSA of the sciatic nerve was a more consistent measurement than the diameter. The results highlight the basic clinical applications of high-resolution ultrasonography for the future diagnosis, treatment, and prognostic evaluation of peripheral neuropathies.
A limitation of the study is that the measurement of the cross-sectional area of the sciatic nerve was done only at two levels.
High-resolution ultrasonography allows direct imaging of the peripheral nerves including the sciatic nerve. It is a preferred technique, since it is easily accessible, noninvasive, and associated with a shorter examination time and lower costs. The proximal part of the normal sciatic nerve has a greater cross-sectional area, which decreases as the nerve courses distally. The reference values of the cross-sectional area of the sciatic nerve can facilitate the analysis of abnormal nerve conditions including peripheral neuropathies.