[Significance of distal motor latency after carpal tunnel operation]

Motor distal latency (MDL) is one of the most important parameters in the electrodiagnosis of carpal tunnel syndrome (CTS). In a retrospective study of 1816 open surgical decompressions for CTS, a total of 612 postoperative motor nerve conduction measurements on 485 hands could be evaluated. In patients with good or satisfactory results after carpal tunnel release, an average improvement of MDL of 1.0 ms after 9-13 days, and of 2.2 ms after 1 year and longer was found. The individual motor nerve conduction improvement was in close correlation with the extent of preoperative prolongation of the MDL. Whereas relief of symptoms can be noted almost immediately, prolonged latencies often do not return to normal, even when the study is done a year later. Of the 10 patients with persisting symptoms, four had a new postoperative impairment of MDL, and also four had a marked improvement, whereas it remained unchanged in two. Patients with severe recurrent CTS presented in 11 out of 31 cases with an improved MDL from 0.3 ms to 3.0 ms in comparison to the initial preoperative evaluation; in 6 hands MDL was unchanged, further prolongation up to 2.0 ms was seen in 6 cases and marked worsening with new loss of motor response had to be noted in 8 hands at repeat electrodiagnosis. In 33 cases of postoperative reflex sympathetic dystrophy, an improvement of MDL from 0.3 ms to 3.3 ms (mean 1.7 ms) was observed in 22 hands and dissolution of a preexisting motor conduction block in three others, whereas two remained unchanged (without motor response) and an electrophysiological impairment was found only in six hands. Three of them presented with a further prolongation of MDL from 0.3 to 1.0 ms and a new loss of response was noted in the remaining three. In conclusion, postoperative motor nerve conduction studies may assess a favorable course following carpal tunnel release. However, they are often not helpful when surgical results are unsatisfactory, and indication for repeat surgical decompression should be based merely on clinical symptoms.