Normal threshold values for a monofilament sensory test in sural and radial cutaneous nerves in Indian and Nepali volunteers.

The monofilament test (MFT) is a reliable method to assess sensory nerve function in leprosy and other neuropathies. Assessment of the radial cutaneous and sural nerves, in addition to nerves usually tested, can help improve diagnosis and monitoring of nerve function impairment (NFI). To enable the detection of impairments in leprosy patients, it is essential to know the monofilament threshold of these two nerves in normal subjects. The radial cutaneous, sural, ulnar, median and posterior tibial nerves of 245 volunteers were tested. All nerves were tested at three sites on both left and right sides. Normal monofilament thresholds were calculated per test-site and per nerve. We assessed 490 radial cutaneous and 482 sural nerves. The normal monofilament was 2g (Filament Index Number (FIN) 4·31) for the radial cutaneous and 4g (FIN 4·56) for the sural nerve, although heavy manual laborers demonstrated a threshold of 10g (FIN 5·07) for the sural nerve. For median and ulnar nerves, the 200mg (FIN 3·61) filament was confirmed as normal while the 4g (FIN 4·56) filament was normal for the posterior tibial. Age and occupation have an effect on the mean touch sensitivity but do not affect the normal threshold for the radial cutaneous and sural nerves. The normal thresholds for the radial cutaneous and sural nerves are determined as the 2g (FIN 4·31) and the 4g (FIN 4·56) filaments, respectively. The addition of the radial cutaneous and sural nerve to sensory nerve assessment may improve the diagnosis of patients with impaired sensory nerve function.