Proximally evoked soleus H-reflex to S1 nerve root stimulation in sensory neuronopathies (ganglionopathies)

Bipolar transcutaneous spinal stimulation evokes short-latency reflex responses in human lower limbs alike standard unipolar electrode configuration

Noninvasive electrical stimulation targeting the posterior lumbosacral roots has been applied recently in reflexes studies and as a neuromodulation intervention for modifying spinal cord circuitry after an injury. Here, we characterized short-latency responses evoked by four bipolar electrode configurations placed longitudinally over the spinal column at different vertebral levels from L1 to T9. They were compared with the responses evoked by the standard unipolar (aka monopolar) electrode configuration (cathode at T11/12, anode over the abdominal wall). Short-latency responses were recorded in the rectus femoris, medial hamstrings, tibialis anterior, and soleus muscles, bilaterally, in 11 neurologically intact participants. The response recruitment characteristics (maximal amplitude, motor threshold) and amplitude-matched onset latencies and paired-pulse suppression (35-ms interstimulus interval) were assessed with 1-ms current-controlled pulses at intensities up to 100 mA. The results showed that short-latency responses can be elicited with all bipolar electrode configurations. However, only with the cathode at T11/12 and the anode 10 cm cranially (∼T9), the maximum response amplitudes were statistical equivalent (P < 0.05) in the medial hamstrings, tibialis anterior, and soleus but not the rectus femoris, whereas motor thresholds were not significantly different across all muscles. The onset latency and paired-pulse suppression were also not significantly different across the tested electrode configurations, thereby confirming the reflex nature of the bipolar short-latency responses. We conclude that the bipolar configuration (cathode T11/12, anode ∼T9) produces reflex responses that are ostensibly similar to those evoked by the standard unipolar configuration. This provides an alternative approach for neuromodulation intervention.NEW & NOTEWORTHY Transcutaneous spinal stimulation with the identified bipolar electrode configuration may offer several advantages for neuromodulation interventions over commonly used unipolar configurations: there are no associated abdominal contractions, which improves the participant's comfort; additional dermatomes are not stimulated as when the anode is over the abdominal wall or iliac crest, which may have unwanted effects; and, due to a more localized electrical field, the bipolar configuration offers the possibility of targeting cord segments more selectively.

Rapid neurophysiological screening for sensory ganglionopathy: A novel approach

BACKGROUND AND AIM

Pure sensory neuropathies involving the dorsal root ganglia are commonly referred to as sensory ganglionopathies (SG). Causes of SG can be inherited (as seen in Friedreich's ataxia) or acquired (e.g. immune-mediated or paraneoplastic). Diagnostic criteria for confirming SG have been published and consist of a combination of clinical and neurophysiological parameters. The aim of our study was to develop a neurophysiological method for rapid screening for diagnosis of SG.

METHODS

For each subject we obtained the sensory nerve action potentials (SNAPs) of five nerves (median, ulnar, radial, sural and superficial peroneal) bilaterally. In the presence of an entrapment neuropathy we obtained the SNAP of the medial antebrachial cutaneous nerves bilaterally. We estimated the number of pairs of nerves showing a SNAP asymmetry of >50% (difference of SNAPs/ lower SNAP).

RESULTS

Sixty-eight subjects, 34 patients with SG and 34 age and sex-matched controls, participated in the study. Among all subjects using a receiver operating characteristic (ROC) curve analysis, the area under the curve was 0.984 (95% CI, 0.960-1.000; SE, 0.012; p < .001). In order to detect SG, presence of SNAP asymmetry of >50% in 2 pairs of nerves, not explained by an entrapment neuropathy, shows a sensitivity of 97.1%, a specificity of 94.1%, a positive predictive value of 94.3% and a negative predictive value of 97.0.

CONCLUSION

The number of pairs of nerves showing a SNAP asymmetry of >50% may be used as a novel rapid screening tool of patients with SG.

Clinical uses of H reflexes of upper and lower limb muscles

H reflexes can be recorded from virtually all muscles that have muscle spindles, but reflex reinforcement may be required for the reflex response to be demonstrable. This can allow conduction across proximal nerve segments and most nerve root segments commonly involved by pathology. Stimulus rate is critical in subjects who are at rest. However the reflex attenuation with higher rates is greatly reduced during a background contraction of the test muscle, with only minor changes in latency if any. In addition the contraction ensures that the reflex response occurs in the desired muscle. Reflex latencies should be corrected for height (or limb length) and age. Because the reflex discharge requires a synchronised volley in group Ia afferents, large increases in reflex latency occur rarely with purely sensory lesions. If the H reflex of soleus, quadriceps femoris or flexor carpi radialis is absent at rest but appears during a voluntary contraction at near-normal latency, there is either low central excitability or a predominantly sensory abnormality. With the former H reflexes will be difficult to elicit throughout the body. If H reflexes can be recorded at rest from muscles for which no reflex can normally be demonstrated, there is good evidence for hyperreflexia. In the context of possible ALS, this is an important finding when there is EMG evidence of chronic partial denervation in that muscle.