Sural nerve abnormalities in sacral perineural (Tarlov) cysts

Sirs: Perineural cysts (PCs) are formed by ectasia of the perineural space of spinal nerve roots that occur at or distal to the dorsal root ganglion, most frequently at the lumbosacral level. We investigated electrophysiologically the sural nerves of 11 patients with sacral PCs, selected among the outpatients referred to our EMG laboratory in the last two years. We found sural sensory nerve action potential (SNAP) abnormalities in 5 (3 males and 2 females, aged 40–66 years). These five patients underwent a complete clinical evaluation and neurophysiological investigation. The main clinical and radiological data are summarized in Table 1. Common causes of acquired peripheral neuropathies were excluded by means of laboratory investigations. A clinical history was undertaken in order to rule out familial neuromuscular diseases or major spinal traumas. The presence of PCs was documented in all these cases by MRI scans of the lumbosacral spine. The cysts were bilateral in 3 cases. One patient (case 3) was asymptomatic; three patients (cases 1, 2 and 4) had been referred to our EMG laboratory for low back pain; one patient (case 5) complained of bilateral plantar pain and dysaesthesia. Sensory nerve conduction studies were performed on all the patients on the sural nerve, with surface recording electrodes in bipolar montage below the lateral malleolus and stimulating 14 cm proximally. The latency was measured at the negative peak and amplitude was measured peak to peak. SNAP amplitude was considered abnormal if it was lower than 7 μV, and sensory nerve conduction velocity (NCV) of the sural nerve was considered abnormal if it was lower than 36 m/s. Peroneal and tibial nerve motor conduction studies, including F waves, were performed according to standard methods [6]. Electroneurographic studies were also conducted on superficial peroneal, saphenous, median and ulnar nerves in the 3 patients in whom bilateral sural SNAP abnormalities were found (cases 1, 3 and 5). Nerve conduction studies were performed at skin temperature of 36°C, kept constant by means of an infrared lamp and a skin temperature sensor. The main electrophysiological findings are summarized in Table 1. Sural SNAP abnormalities were found on the same side as the cyst when the latter was unilateral, and bilaterally in patients with bilateral cysts. Motor nerve conduction studies were normal in all subjects. Saphenous and superficial peroneal SNAPs and upper limb nerve conduction studies were all normal in the 3 patients tested. Our findings show that PCs can in some cases be associated with reduced sural SNAP amplitude. The electroneurographic studies were directed to the sural nerve because it is composed of fibres originating mainly from the S1 and S2 roots [8], which are the most frequently affected by PCs. The concordance between the cyst side and the side of the reduced SNAP was extremely high: patients with unilateral cysts showed SNAP abnormalities only on the cyst side, and patients with bilateral cysts showed abnormalities on both sides. The sensory NCVs were slightly reduced in cases 1, 2 and 3, possibly owing to the loss of large-diameter sensory fibres. In patients 1, 3 and 5 the bilateral integrity of saphenous and superficial peroneal SNAPs showed that the amplitude reduction of sural nerve SNAPs was not due to length-dependent damage as in polyneuropathies [3]. Moreover, the clinical picture and laboratory investigations were not compatible with the diagnosis of polyneuropathy. Perineural cysts are symptomatic in 20 % – 25 % of cases, most frequently manifesting with low back pain and sciatica [4, 11]. Our patients complained mainly of senLETTER TO THE EDITORS