Normal nerve conduction with an abnormal ultrasound: A separate group of CTS?

Abstract

sensory nduction the most C. Ultrasound of the peripheral nerves is a helpful adjunct to nerve conduction for characterizing peripheral nerve disorders. It is particularly useful in the study of chronic nerve entrapments (Joy et al., 2011) and has contributed in the etiological assignment of nerve entrapments, by identifying structures directly responsible for entrapment, such as ganglion cysts (Claes et al., 2013). However, studies suggest that in chronic nerve entrapments such as carpal tunnel syndrome (CTS), nerve morphological features, as seen by imaging detects less change at a later stage than nerve functional tests of nerve conduction. We recently observed two cases of CTS demonstrating considerable change of median nerve size in ultrasound, were nerve conduction was (repeatedly) normal. A 41-year-old Chinese male suffered nocturnal/early morning predominant numbness (maximum middle finger) in the right hand provoked by certain arm movements in particular holding the steering wheel of the car. There was improvement with hand flick. Nerve conduction, was normal on three separate occasions over the previous 1 year (Table 1). Clinical symptoms did not change over time. EMG of hand and arm muscles was normal. After further two normal nerve conductions, ultrasound was performed and showed maximal median nerve cross sectional area of 0.13 cm (n < 0.09) at the carpal tunnel entry with increased median nerve blood flow. A 38-year-old Asian female had been operated on right CTS 5 years earlier and presented with 3 years of increasing intermittent left hand numbness provoked by hand flexion and extension with some relief on arm/hand massaging movements. Nerve conduction was normal at the onset of symptoms and when repeated 3 years later (Table 1). Clinical symptoms did not appreciably change over the 3 years. The median nerve cross sectional area at carpal tunnel entry performed after the second nerve conduction study was 0.22 cm (n < 0.09) and at exit 0.20 cm (normal < 0.12) (Fig. 1). These two cases show what we term ‘‘pattern reversal’’ in CTS by showing clearly abnormal nerve morphology in the presence of (repeatedly) normal nerve conduction. The two cases described here, are quite different from the recently described subclinical median nerve enlargement in normal subjects where an increase in median nerve cross sectional area was accompanied by proportionate changes in nerve conduction abnormality (Su et al., 2013). We speculate that this ‘‘pattern reversal’’ may result from ischemia–reperfusion injury of the synovial connective tissue surrounding the median nerve, with irritation of the nerve involving mainly the connective tissue inside the nerve. It may be that axons are either more resistant to the injurious process or able to functionally compensate for longer time periods in certain types of patient or injury. Similar considerations have been used to explain the dissociation of subjective symptoms in CTS with nerve pathology (Hirata et al., 2005).