Therimadasamy et al. [1] investigated the premotor potential from the median–motor conduction studies, especially from the thenar muscles like others [4–6]. In normal subjects using the Preston and Logigian’s [2] method (Fig. 2), they observed a small negative potential (usually in the range of 10–80 µV) to precede the median 2L CMAP.

Hasegawa et al. [7] suggested that the premotor potential from the 2L muscle (PMP–2L) might represent potentials arising from the antidromically activated digital sensory branches. As a result of this, Therimadasamy et al. [1] investigated the further advantage to the 2L–INT recording technique, which can simultaneously provide information concerning both motor–and sensory–median nerve fibers that have crossed the carpal tunnel, as well as from the ulnar–motor fibers. They compared the PMP-2L potential (which they called “2-LUMP”) with other standard nerve conduction techniques, such as sensory orthodromic nerve conduction to the wrist stimulating digit III (middle finger) and conventional motor conduction technique to the abductor pollicis brevis (APB) muscle. They concluded that in the CTS, the use of PMP–2L and other 2L–INT-associated parameters could replace the recording of a conventional median–sensory conduction to the digits and motor studies of the median and ulnar nerves. The authors found some limitations of PMP–2L in CTS complicated by additional ulnar neuropathy or polyneuropathy because the study was designed to test the practicability of the PMP–2L in healthy subjects and those with uncomplicated carpal tunnel.

Patients affected by CTS were electrodiagnostically graded with the standard scale of Bland [8]: normal (grade 0), very mild (grade 1: abnormal 2L–INT DIFF test only, i.e., >0.6 ms), mild (grade 2: abnormal sensory orthodromic median nerve conduction, i.e., <40 m/s), moderate (grade 3: abnormal sensory orthodromic potential but preserved and prolonged distal CMAP latency to the abductor pollicis brevis muscle, i.e., >3.9 ms and <6.5 ms), severe (grade 4: sensory orthodromic potential absent and prolonged distal CMAP latency to the abductor pollicis brevis muscle, i.e., >3.9 ms and <6.5 ms), very severe (grade 5: sensory orthodromic potential absent and prolonged distal CMAP latency to the abductor pollicis brevis muscle, i.e., >6.5 ms), and extremely severe (grade 6: sensory and motor potentials both absent and unrecordable). The authors developed a scale for the PMP–2L potential, similar to that of Bland [9] as follows: normal (grade 0), very mild (grade 1: abnormal ulnar interossei motor to the PMP–2L sensory latency comparison only, i.e., ≥0.5 ms), mild (grade 2: abnormal premotor potential latency, i.e., >2.7 ms, or motor 2L–INT latency difference >0.8 ms), moderate (grade 3: abnormal PMP–2L latency >3.2 ms or 2L–INT latency difference >2.5 ms), severe (grade 4: absent sensory PMP–2L potential or 2L–INT latency difference >2.5 ms), very severe (grade 5: absent sensory PMP–2L potential or 2L–INT latency difference >4.0 ms), and extremely severe (grade 6: sensory and motor potentials both absent and unrecordable). They examined whether motor and sensory data obtained with a 2L–INT recording could reduce the number of tests used to support a diagnosis of CTS by comparing data derived from the 2L–INT recording with data from the recording techniques commonly used for the electrodiagnosis of CTS. Sensitivity was calculated as the proportion of people with CTS having a positive test result, and specificity was the proportion of healthy people having a negative test result. The authors found the ulnar interossei motor to the PMP–2L sensory latency comparison to have the highest sensitivity (79 %) and specificity (94 %), the PMP–2L sensory peak latency to have high sensitivity (77 %) and specificity (94 %), and the 2L–INT LAT difference to have high sensitivity (75 %) and specificity (94 %).