Equipment and technique
Currently,
the best assessment of peripheral nerves is achieved with high-frequency linear array probes (8-18 MHz),
using soft tissue software.
Array depth should be as minimal as possible and focal points should be directed to the nerve being studied.
A standard examination will start with axial visualization of the nerve,
followed by proximal and distal following,
finishing with a longitudinal scan,
most of the times at the area of pathology or entrapment.
Dynamic assessment should be performed whenever possible or indicated,
in order to detect nerve instability (e.g.
cubital tunnel syndrome at elbow level,
assessed with forearm in flexion and extension).
Normal nerves
With a fair knowledge from the performing examiner,
US can span almost the entirety of the anatomical course of a peripheral nerve.
The majority of main peripheral nerves can be imaged,
as well as smaller branches when pathological.
Axial images are preferred,
since they provide more information regarding normal anatomy and eventual pathology.
A normal nerve’s image on an axial plane depicts its histology,
presenting itself as a hypoechoic tubular structure with small round or oval hypoechoic areas (axon fascicles),
separated by hyperechoic septa (perineurium),
limited by an outer hyperechoic rim (epineurium),
constituting the normal “honeycomb” pattern (Fig. 1).
On joints with retinacula,
nerves can show a slight compression,
appearing discreetly more hyperechoic.
Occasionally,
a nerve can have a homogeneous and hypoechoic appearance.
On longitudinal planes,
a hypoechogenic structure with alternating hypo and hyperechoic lines (perineurium) will be seen (fascicular or fibrillary pattern) (Fig. 2).
No internal blood flow should be detected with Doppler study on any normal nerve.
It is also important to understand that normal nerves present a proximal-to-distal decreasing calibre,
depending also on its function (motor,
sensory,
both),
temperature,
body mass index,
age and gender.
Since nerve recognition implies knowledge of regional anatomy and topography,
there are important landmarks which should be sought to allow a faster nerve spotting,
as shown on Table 1.
Once identified at any of the sites listed,
tracing of a nerve’s course can be done in a continuous fashion both proximally and distally (Fig. 3 and Fig. 4).
Fig. 1: Normal median nerve seen on an axial plane (yellow dotted circle) at mid-forearm level. The normal hypoechoic tubular structure with small round or oval hypoechoic areas surrounded a hyperechoic rim – “honeycomb” pattern – represents the normal sonographic aspect of nervous structures in cross-section, representing the histological pattern of axon fascicles, perineurium and epineurium.
Fig. 2: Normal median nerve seen on a longitudinal plane (white arrows above nerve) at mid-forearm level. Longitudinal view will show a fascicular or fibrillary pattern, where axon fascicles are seen as hyperechoic bright lines, alternated with hypoechoic areas, corresponding to perineurium and endoneurium.
Table 1: Important landmarks in peripheral nerve recognition.
Fig. 3: Normal nerves seen at wrist level, axial view. A normal median nerve is seen on an axial plane on the left (big yellow dotted perimeter), presenting normal cross-sectional area and echogenicity. Also seen at this level is the ulnar nerve (small yellow dotted perimeter), with a much smaller cross-sectional area, due to having an inferior calibre.
Fig. 4: Normal median nerve on a longitudinal plane (white arrows), reaching the carpal tunnel from the distal forearm. Notice the slight decrease in calibre, but normal, of the nerve as is enters the carpal tunnel (first two arrows from the left).
Compressive neuropathies
Neuropathies due to nerve compression phenomena are very common and often diagnosed clinically and later confirmed with electrophysiological examination.
However,
this approach lacks the assessment of nerve anatomy and surrounding structures.
Since most sites of nerve entrapment are easily accessible with US examination,
it is becoming widely used for this purpose.
Quantifying a pathological nerve with US can be achieved using many criteria,
such as cross-sectional area (CSA),
variability of CSA along the nerve,
echogenicity,
vascularity and nerve mobility.
Despite most of them being subjective to the examiner´s interpretation,
the absolute value of CSA is currently regarded as the most relevant and reliable for the diagnosis of focal neuropathy,
with many studies stating reference values.
Commonly used criteria and findings are listed on Table 2.
Table 2: Commonly used criteria and findings for nerve compression.
The basic diagnosis,
however,
is obtained with visualization of an abrupt flattening (notching) of the nerve,
with a proximal fusiform swelling (pseudoneuroma).
Other characteristics also seen include hypoechogenicity of the nerve swelling,
hypervascularity of the compressed nerve due to hyperemia,
and also lack of mobility (Fig. 5,
Fig. 6 and Fig. 7).
US is also capable of identifying extrinsic causes of nerve entrapment like mass lesions (cysts,
tumours,
fluid collections),
osseous abnormalities (e.g.
supracondylar process of the humerus in median nerve neuropathy),
tendinous and muscular alterations,
and anatomical variants (e.g.
bifid median artery in carpal tunnel syndrome).
A comprehensive list of the most common nerve entrapment syndromes can be found on Table 3.
Table 3: Entrapment syndromes and respective compression sites.
Carpal tunnel syndrome (median nerve)
Carpal tunnel syndrome (CTS) is the most common compressive mononeuropathy.
Entrapment of the median nerve at the carpal tunnel usually presents as a classic triad of nerve enlargement proximal to the carpal tunnel,
median nerve flattening and palmar bowing of the flexor retinaculum (Fig. 5,
Fig. 6 and Fig. 7).
US diagnostic accuracy for CTS is variable among studies,
presenting a 77,6-91% sensitivity and an 86,8-93% specificity.
Several sonographic criteria can be used (Table 2) but cross-sectional area (CSA) proximal to the inlet of the carpal tunnel (scaphoid-pisiform level) is the most sensitive and specific imaging diagnostic marker in CTS symptomatic patients.
A cut-off value of CSA >12 mm2 at wrist level has been established as an accurate diagnostic test for CTS by many authors.
Many studies have demonstrated an important correlation between nerve CSA and CTS severity,
showing that a decrease in CSA is associated with improvement of symptom scores and electrodiagnostic parameters.
This makes US particularly helpful in post-operative follow-up of median nerve release.
Most cases of CTS are idiopathic or due to systemic causes (e.g.
diabetes mellitus,
hypothyroidism,
obesity) but a minority of cases is caused by space-occupying lesions and structural abnormalities (e.g.
ganglion cyst,
tendon sheath fibromas,
amyloid deposits),
where US is of the utmost importance.
It is especially useful for the visualization of anatomical variants causative of CTS,
such as a bifid median nerve or a persistent median artery.
Neurosurgeons find a great pre-operative aid from US in these cases,
allowing a better surgical approach.
Dynamic US examination – flexing fingers with the transducer at wrist level - also allows visualization of reduced median nerve slippage during finger and wrist flexion.
Fig. 5: Carpal tunnel syndrome (CTS) – the median nerve is seen axially (yellow dotted perimeter), displaying an enlarged cross-sectional area (14 mm2), due to entrapment and consequent proximal swelling. Also notable is the loss of “honeycomb” pattern (compare with Fig. 3), with hypoechogenicity of the nerve caused by endoneural edema.
Fig. 6: Carpal tunnel syndrome (CTS) – the median nerve seen longitudinally (above white arrows), above the carpal tunnel. Nerve enlargement is also seen, due to nerve swelling above the entrapment site (seen in Fig. 7). The axon fascicles are also poorly defined, with a hypoechoic aspect of the nerve.
Fig. 7: Carpal tunnel syndrome (CTS) – entrapment of the median nerve on a longitudinal plane (first two white arrows from the right). The marked narrowing/notching of the nerve is better visualized using this plane.
Ulnar neuropathy (ulnar nerve)
Entrapment of the ulnar nerve comprises cubital tunnel syndrome,
at elbow level,
and ulnar canal syndrome (Guyon’s canal syndrome),
at wrist level.
Cubital tunnel syndrome (CUTS) is the second most common entrapment mononeuropathy after CTS.
It is mostly caused by entrapment of the ulnar nerve between the olecranon and the medial epicondyle at the humero-ulnar arcade.
Ulnar canal syndrome is less common,
with the occurrence of ganglion cysts being a common cause for the entrapment.
The most common US feature of CUTS is increased CSA of the ulnar nerve (cut-off value of >10 mm2) with a sensitivity of >80%,
although the value is still not consensual.
If electromyography is combined with US,
diagnostic sensitivity can be upgraded from 78% to 98%.
However,
US stand-alone sensitivity for diagnosing cubital tunnel syndrome is still very low when compared to CTS.
It is also noteworthy the fact that a normal ulnar nerve has already a slight enlargement at the cubital tunnel (normal CSA of 6,8 mm2).
As with CTS,
CSA is significantly correlated with the severity of nerve entrapment.
Other criterion fairly used is the elbow-to-arm swelling ratio.
Dynamic US examination of the ulnar nerve at the cubital tunnel is very important and it should be done first with extension of the forearm,
followed by flexion.
Subluxation and dislocation of the ulnar nerve can be diagnosed with US across the medial epicondyle with flexion.
Subluxation happens when the ulnar nerve moves towards the tip of the medial epicondyle and dislocation is a complete relocation over the epicondyle (Fig. 8 and Fig. 9).
Although normal individuals may have subluxation/dislocation of the nerve,
a repetitive displacement can cause nerve microtrauma and symptomatic entrapment.
Other extrinsic causes such as snapping of the medial head of the triceps,
ganglia,
osteophytes or tumours can also be imaged.
Fig. 8: Normal ulnar nerve seen at elbow level with extended forearm (yellow dotted perimeter). The nerve runs its course normally inside the cubital tunnel, located posteriorly in relation to the medial epicondyle.
Fig. 9: Normal ulnar nerve seen at elbow level with flexed forearm (yellow dotted perimeter). In comparison with Fig. 8, the nerve has shifted its location from the cubital tunnel, completely relocating itself over the medial epicondyle to a more anteromedial position (ulnar nerve subluxation). With arm extension, the nerve returns to the cubital tunnel.
Less prevalent compressive neuropathies
Other compressive mononeuropathies are harder to image through US due to the relatively small size of the nerves.
Some are described below:
- Spiral groove syndrome (radial nerve): it’s the most common radial neuropathy,
occurring at the spiral groove of the humerus,
where the nerve courses.
The nerve is susceptible to compression after prolonged inmobilization (e.g.
“saturday night palsy”),
humeral shaft fracture,
muscular hypertrophy or infarction from vasculitis.
US is usually helpful for localizing scar tissue,
fracture fragments or for muscular tissue evaluation.
- Ulnar canal syndrome (Guyon’s syndrome): neuropathy of the ulnar nerve at the Guyon’s canal at wrist level is rare and is mostly caused by work or sport-related pressure lesions.
Other causes include ganglia,
heterotopic ossification or foreign bodies at the Guyon’s canal.
- Peroneal neuropathy (peroneal nerve): entrapment of the peroneal nerve is the most common entrapment syndrome in the lower limb.
Compression at the level of the fibular head may occur due to many conditions such as trauma,
immobilization or mass lesions.
Ganglion cysts,
intraneural ganglia and neurofibromas are some of the major causes of peroneal nerve compression,
easily diagnosed with US.
- Morton’s metatarsalgia (interdigital nerves): normally occurs at the second or third plantar interdigital nerves,
with the formation of a neuroma-like enlargement due to impingement of the nerve,
with ischemia and compression.
- Supinator tunnel syndrome (posterior interosseous nerve): this syndrome results from repetitive overuse of the forearm,
causing thickening or hypertrophy of anatomical structures such as the supinator muscle and the extensor carpi radialis brevis muscle,
causing entrapment of the posterior interosseous nerve (deep branch of the radial nerve).
US guided infiltration can be performed for diagnostic purposes.
Polyneuropathies
In recent years,
further research has been made in the field of polyneuropathies,
especially regarding the utility and applications of US in its evaluation.
It is common to find clinical and electrophysiological overlapping of findings in axonal and demyelinating polyneuropathies.
Many authors have shown that US can accurately discriminate between the two types of neuropathies,
and also between their different subtypes.
Firstly,
peripheral nerve enlargement tends to occur more and be greater in demyelinating neuropathies,
whereas axonal neuropathies seldom do so.
Demyelinating polyneuropathies like Charcot-Marie-Tooth Disease (CMT),
chronic inflammatory demyelinating polyneuropathy (CIDP) are some of the many polyneuropathies that can present enlargement of peripheral nerves (Table 4).
Moreover,
different subtypes of CMT,
such as CMT2 (an axonal type),
will have no significant peripheral nerve enlargement.
CIDP’s nerve enlargement derives from its histopathological “onion-bulb” appearance,
which is due to multiple episodes of demyelination and remyelination.
What’s more,
different patterns of nerve enlargement also provide useful information.
Both in CMT and CIDP,
peripheral nerve enlargement occurs mainly at non-entrapment anatomical sites,
whereas in hereditary neuropathy with liability to pressure palsies (HNLPP) it can be seen specifically at the entrapment sites.
Most polyneuropathies mentioned before present nerve enlargement in a multifocal manner,
and studies are being directed to establish US scores and protocols for diagnosing diseases such as CIDP,
multifocal motor neuropathy (MMN) and multifocal acquired demyelinating sensory and motor neuropathy (MADSAM).
Guillain-Barré syndrome has been associated with specific nerve enlargement of median and ulnar nerves (Fig. 10).
However,
wider prospective cohort studies are still needed to prove real sonographic diagnostic relevance.
Regarding leprosy,
US is helpful demonstrating peripheral nerve enlargement due to edema – the hallmark of lepromatous neuropathy –,
with loss of fascicular architecture and increased endoneurium vascularity,
as well as hypervascularity.
Table 4: Polyneuropathies that can present peripheral nerve enlargement.
Fig. 10: Guillain-Barré syndrome with multifocal nerve enlargement. The median nerve is focally enlarged, with an expanded cross-sectional area. However, the normal echogenicity of the nerve is preserved in this case.
Tumours and other mass lesions
Ultrasound has also shown its value in adequately demonstrating several nervous sheath tumours.
The most common types are schwannomas and neurofibromas,
which are benign lesions.
Malignant peripheral nerve sheath tumours (MPNSTs) can also be easily diagnosed.
Differentiation between schwannomas and neurofibromas relies on the nerve course location.
Both appear as well-defined round masses with a hyperechoic rim in the course of a nerve.
Schwannomas involve the nerve eccentrically,
allowing the integrity and continuity of nerve fascicles,
which are displaced due to the mass effect of the tumour.
Neurofibromas,
on the other hand,
will abruptly sever nerve continuity,
appearing centrally in a disruptive manner of the normal fascicular architecture (Fig. 11 and Fig. 12).
Schwannomas will also show hypervascularization,
which neurofibromas usually do not.
MPNSTs share many of the characteristics of neurofibromas,
but tend to be larger in size with ill-defined borders.
Another nervous tumour readily diagnosed with US is lipofibromatous hamartoma (which arises from fatty tissue between axons) with a characteristic fusiform enlargement of the nerve and a hyperechoic pattern caused by fat.
Ganglion cysts are the second most common cause of peripheral mononeuropathy after entrapment phenomena.
They appear as hypoechoic structures with or without internal septa.
Most arise from the superior tibio-fibular joint – affecting the peroneal or tibial nerves – but they may also affect the tibial nerve at the ankle or the ulnar nerve at the cubital tunnel.
Whenever a neuropathy’s etiology remains unexplained,
a careful sonographic screening examination should be performed,
thoroughly searching for tumour lesions.
Fig. 11: Neurofibroma of the ulnar nerve above elbow level (red oval perimeter), seen on an axial plane. Notice the hypoechoic structure and the lack of axon fascicles seen in cross-section, typical for this lesion.
Fig. 12: Neurofibroma of the ulnar nerve above elbow level (red oval perimeter), seen longitudinally. The same lesion seen in Fig. 11, but in this case better characterised on a longitudinal plane, showing a central disruptive round/oval lesion along the course, with an abrupt interruption of nerve continuity with no fascicular pattern seen in or around the neurofibroma.
Traumatic nerve injury
Peripheral nerve trauma can result from traction,
contusion,
penetrating trauma or be iatrogenic.
Its assessment is important because different degrees and types of lesion imply different treatment strategies.
Usually severe nerve injuries (neurotmesis) require surgical intervention,
whereas axonotmesis and neurapraxia may or may not.
US is nowadays useful in assessing the continuity of the nerve and can detect less serious nervous injuries such as neurapraxia,
by visualizing nerve swelling with a hypoechoic appearance.
Other lesions such as neuromas-in-continuity or stump neuromas in severed nerves are readily detected using an ultrasound examination and cannot be diagnosed with electrophysiological exams.
Once again,
post-operative lesions which affect nerves,
such as scars,
callus formation,
bone fragments,
hematomas or foreign bodies,
can also be seen with US.
Neurapraxia and axonotmesis can have similar sonographic appearance,
whereas neurotmesis can be easily seen by detecting complete discontinuity of the nerve.
This differentiation is not always achieved with electromyography.
Moreover,
in traumatic incidents resulting in intra-neural metallic foreign bodies,
US is superior to MRI,
since the latter cannot be performed (Fig. 13 and Fig. 14).
Fig. 13: A carpenter’s work-related accident, where a screw entered the patient’s arm above the elbow. Prior to surgery, the surgeon requested ultrasound examination to see what anatomical structures were affected by the foreign body, especially if the ulnar nerve was afected (see Fig. 14).
Fig. 14: The same case referred in Fig. 13. The screw is seen lodged at the elbow but the ulnar nerve remains untouched (yellow dotted oval perimeter). The surgeon acquires the information that no nerve damaged was sustained.
US: a perioperative must-have
When it comes to surgery on peripheral nerves,
both orthopaedic surgeons and neurosurgeons find a great aid in US.
Firstly,
surgical planning with US prior to incision is helpful for locating nervous structures,
especially when they are not palpable or are located within deeper tissues.
Traumatic lesions causing neurotmesis also find great pre-operative help in US,
since the distance between both ends of a severed nerve is of the utmost importance for the correct surgical approach (neurorrhaphy or nerve transplant).
What’s more,
anatomical variants such as bifid median nerve or persistent median artery,
can also change surgical approach.
Intraoperatively,
US is also able to identify surface regions of a nervous tumour,
correctly assessing the displacement of the nervous fibres,
thus decreasing the risk of iatrogenic nerve injury during surgical resections or even simple percutaneous nervous tumour biopsies.
Also,
intraoperative US can further help locating smaller lesions in more difficult anatomical locations.
Last but not least,
post-operative follow up benefits from sonographic assessment,
since it can verify continuity of operated nerves,
cicatricial changes,
or any anatomical change due to surgical intervention (Fig. 15).
Fig. 15: Median nerve after surgery. A neurorrhaphy of the median nerve was performed due to previous trauma with section of the nerve. A post-operative sonographic examination was performed, showing a longitudinal plane of the median nerve, with a hypoechoic round lesion at the site of the suture (pseudoneuroma), with loss of normal fascicular pattern.
US guided nerve intervention
Anaesthetic nerve blockage sonographically guided has already a defined and important role.
US-guided administration of regional anaesthesia is a common procedure amongst anaesthesiologists and physicians of many areas of expertise.
Pre-surgical nerve blockage as well as chronic pain and muscle spasticity management are some of the various applications of dynamic sonographic examinations.