The most common types of nerve disorders, entrapment neuropathies, tend to show changes that are the inverse of the changes seen on muscle imaging of chronic neuro-muscular disease. Although chronically diseased muscles become atrophic and hyperechoic, chronically compressed nerves tend to become enlarged and hypoechoic. In both cases, the changes are clearly of diagnostic usefulness. For example, currently, there are more than 250 published studies of nerve ultrasonography in carpal tunnel syndrome showing that measured nerve enlargement and loss of echo-genicity are hallmark diagnostic features of the disorder. However, it is unclear if these changes are useful as surrogate markers in nerve disease. This is because there is a structural limit to how much a compressed nerve can enlarge; with increasingly severe compression and axonal loss, nerves may have less swelling than in less pronounced cases, and there is a floor effect of how much echogenicity a nerve can lose. This suggests that there may be an inverted U-shaped relationship between nerve enlargement and severity of compression neuropathy and possibly a similar relationship with loss of echogenicity; as such, they may have some limitations as surrogate markers of severity of compression.
The cause of increased size and loss of echogenicity of compressed nerves is not well understood, but, in part, this may result from increased vascularity of the nerve in carpal tunnel syndrome, a finding that can be demonstrated with color flow Doppler (
). Excess blood flow would tend to engorge the nerve and, because blood is much less echoic than tissue, would reduce nerve echogenicity.70–73
Both ultrasound-guided steroid injections and surgical treatment of carpal tunnel syndrome lead to prompt and sustained reductions in nerve size as measured by cross-sectional area.70–72,74,75
When Cartwright and colleagues70
looked at median nerve vascularity in conjunction with nerve size, both decreased in tandem in patients with carpal tunnel syndrome after ultrasound-guided steroid injections around the median nerve. Nerve echogenicity, measured by examiner rating, showed a similar time course of improvement. As such, there may be a role of ultrasound in gauging treatment effects in carpal tunnel syndrome because several measures demonstrate responsiveness to therapeutic intervention in these studies.
Fig. 6 (A) Cross-sectional image of the wrist showing a markedly enlarged and somewhat flattened median nerve. The cross-sectional area of the nerve (green tracing) is 31 mm2, which is about 4 times the normal size. Note that the nerve is hypoechoic, particularly (more ...)
Is Quantitative Ultrasonography Useful in Infectious Neuropathy?
If carpal tunnel syndrome is the most common treatable focal neuropathy, it seems likely that leprosy may be the most common curable generalized polyneuropathy, at least from a global perspective.76
Leprosy's worldwide prevalence and the potential for ultrasonography to significantly affect its management warrant its inclusion in this review. Two major categories of leprosy include lepromatous, which is a diffuse infection of skin and nerves caused by reduced cell-mediated immunity in the disorder, and tuberculoid, in which there is an active immunologic response to the disease, restricting its distribution to relatively few areas of skin and nerve.76
In up to 25% of patients undergoing antibiotic therapy, there are treatment reactions, usually in lepromatous forms of the disease.76
These reactions can be mild but can also be associated with significant inflammatory impairment of nerve function. Increased blood flow in nerves, detected by color Doppler ultrasonography, provides evidence of such treatment reactions.77
Ultrasound measurements showing increased nerve size are a sensitive indicator of the presence of neuropathy in leprosy.78,79
This is not surprising because it is well known that nerves are often palpably enlarged in leprosy, particularly in areas where they are superficial and in tissues that are typically cooler than core body temperature, for example, the ulnar nerve at the elbow, the tibial nerve at the ankle, the fibular nerve at the fibular head, and the greater auricular nerve.76
Another study of leprosy, at least one of semiquantitative impact, is the use of vaso-motor blood flow studies in the distribution of the ulnar nerve. Wilder-Smith and colleagues80
showed that color Doppler measurements of blood flow in the ulnar artery by ultrasonography were sensitive and specific in identifying small fiber autonomic dysfunction in 12 patients with leprosy compared with 20 healthy controls. The study is of interest because it demonstrates not only the diagnostic utility of ultra-sound in this disorder, in which small fiber dysfunction is significant in terms of tissue injury and digit loss, but also the potential of ultrasound to diagnose other disorders of small fibers controlling the autonomic nervous system.
Leprosy is an example of a disorder in which nerve ultrasonography may prove to be a useful technique for screening, diagnosis, and assessment of the extent of the disease. However, of even greater importance, leprosy is treatable, and, as such, it can help establish the responsiveness of ultrasound measures of nerve disease in this disorder to antibiotic therapy. Because a percentage of treated patients develop treatment reactions, it is also likely that ultrasonography can contribute to their recognition and management. In this context, leprosy could prove to be a model disease for studying how a low-cost portable imaging technology can alter the diagnosis, treatment, and management of nerve disease prevalent in populations with limited access to sophisticated electrophysiologic testing.