Effects of sex and age on strength–duration properties

Strength-duration time constant and rheobase measurements: Comparison of the threshold tracking method and a manual procedure

OBJECTIVE

To compare the strength-duration time constant (SDTC) and rheobase measurements obtained by the threshold tracking method (TT) and by a non-automated method (MM).

METHODS

The MM procedure involved measuring, using a routine electrodiagnostic device, the intensity required to evoke a motor response whose amplitude corresponds to 40% of the maximum amplitude for four stimulus duration (1.0, 0.7, 0.5, 0.2 ms), and studying the linear relationship between stimulus charge and stimulus duration (slope = rheobase, intercept on the x-axis = SDTC). Using TT and MM, 30 successive healthy subjects (mean age = 38 years old) underwent a prospective evaluation of SDTC and rheobase of the median nerve motor axons at the wrist. Nerve stimulation and bipolar recording of evoked motor responses were performed with disposable self-adhesive surface electrodes.

RESULTS

The Spearman correlations between the two methods were 0.78 (p < 0.0001) for SDTC and 0.96 (p < 0.0001) for the rheobase. The Bland-Altman analysis did not reveal any systematic bias of MM compared to TT.

CONCLUSIONS

The MM procedure was reliable for strength-duration relationship analysis.

SIGNIFICANCE

We encourage neurophysiologists, who do not have dedicated threshold tracking equipment, not to hesitate to use these simple tools to assess peripheral nerve excitability.

Influence of age and gender in the sensory nerve fibers excitability

OBJECTIVES

To assess the influence of age and gender on sensory nerve axonal excitability parameters.

METHODS

Thirty-three healthy subjects (21 women) were included, with a mean age of 34.6 (range 21-76). Median sensory nerve excitability measurements (index finger) were performed using the TRONDNF nerve excitability protocol of the QTRAC program.

RESULTS

Peak sensory nerve action potential (SNAP) amplitude was significantly higher among women (27.1 vs. 9.2 μV; p = .022), and strength-duration time constant (SDTC) was significantly higher in men (0.7 vs. 0.5; p = .011), not dependent on age. Greater age was negatively correlated with resting I/V slope, not dependent on gender (r = -0.4; p = .024). No other changes in excitability properties with increasing age were found.

CONCLUSIONS

Physiological features like as age and gender do not have a relevant impact on sensory nerve excitability measurements, which can have implications regarding pharmacological treatments.

Effects of topiramate on peripheral nerve excitability

PURPOSE

Antiepileptic drugs are generally used to control the cortical hyperexcitable states. But some of them are also effective on the peripheral nervous system, so they may be used in some states like neuropathic pain. Several recent reports suggest the possible effects of antiepileptic drugs on peripheral nerve excitability. Strength duration time properties gives an indirect idea about the persistent, paranodal sodium (Na) channels and may indirectly reflect the peripheral nerve excitability. Topiramate suppresses the cortical hyperexcitability, but previous studies could not prove a significant effect of topiramate on peripheral nerves. The aim of this study is to investigate the probable nerve excitability changes caused by topiramate.

METHODS

Forty migraine patients and 40 controls were included in the study. Median motor and sensory conduction parameters were recorded. Strength duration properties were also recorded from abductor pollicis longus muscle, with the stimulation of median nerve. The electrophysiological studies were repeated 4 weeks after the initiation of topiramate in the treatment group.

RESULTS

Nerve conduction parameters were not significantly affected by 4-week topiramate treatment. But the strength duration time constant decreased significantly, reflecting a reduction in the excitability. This decrement seemed to be more obvious in those in whom topiramate was also clinically useful.

CONCLUSIONS

The method used demonstrated a probable effect of topiramate on the peripheral nerve excitability.

Strength-duration time constant in peripheral nerve: no abnormality in multiple sclerosis

Objectives. To investigate the properties of the strength-duration time constant (SDTC) in multiple sclerosis (MS) patients. Methods. The SDTC and rheobase in 16 MS patients and 19 healthy controls were obtained following stimulation of the right median nerve at the wrist. Results. SDTC and rheobase values were 408.3 ± 60.0 μs and 4.0 ± 1.8 mA in MS patients, versus 408.0 ± 62.4 μs and 3.8 ± 2.1 mA in controls. The differences were not significant in SDTC or rheobase values between the patients and controls (P = 0.988 for SDTC and P = 0.722 for rheobase). Conclusion. Our study showed no abnormality in relapsing remitting MS patients in terms of SDTC, which gives some indirect information about peripheral Na⁺ channel function. This may indicate that alterations in the Na⁺ channel pattern in central nervous system (CNS) couldnot be shown in the peripheral nervous system (PNS) in the MS patients by SDTC. The opinion that MS can be a kind of channelopathy might be proven by performing other axonal excitability tests or SDTC in progressive forms of MS.

Nerve excitability properties in early preclinical diabetic neuropathy

Diabetic polyneuropathy can be easily diagnosed when the nerve conduction studies are affected. Strength Duration Time (SDTc) reflects nerve excitability properties and was previously used several times to demonstrate the excitability properties of the nerves in the existence of electrophysiologically developed diabetic polyneuropathy. But as we all know, diabetic patients may experience neuropathic symptoms even though their routine nerve conduction studies are normal. SDTc may be useful in this early stages of developing neuropathy. In this study we aimed to evaluate the SDTc properties of diabetic patients in this early preclinic stage. Recently SDTc was commonly studied in the upper extremities but most of the diabetic neuropathies are predominant in the lower extremities. So here we also studied both upper and lower extremities to demonstrate a possible difference.

Sensory and motor stimulation thresholds of the ulnar nerve from electric and magnetic field stimuli: implications to gradient coil operation

Rapidly changing magnetic fields from gradient coils induce electric fields in the individual being imaged, which can potentially result in peripheral nerve stimulation (PNS). This is a safety concern in MRI. Nerves exposed to either electric fields or time-varying magnetic fields are presumed to display equivalent stimulation threshold characteristics. This assumption has motivated the use of electric stimulation literature to be applied to gradient field safety standards. The consistency of peripheral nerve stimulation thresholds were compared by measuring chronaxie times for electric and magnetic stimulation for both motor and sensory fibers in the ulnar nerve for a group of healthy volunteers. Thresholds were determined with both electromyography and also by having the subjects report stimulation onset. Chronaxie times measured between motor and sensory fibers were statistically different. However, this difference does not account for the substantial discrepancy reported between measured electric and magnetic stimulation chronaxie times. We further establish that sensation threshold as defined perceptually by the subject volunteer is adequate as a simple and reliable measurement tool. Based on these observations, significant adjustments may need to be made to nerve parameters taken from the electric field stimulation literature prior to applying them directly to gradient induced stimulation in MRI.

Effects of colchicine on strength-duration properties of sensory and motor axons

OBJECTIVE

The strength-duration time constant (SDTC) is a measure of axonal excitability and it can provide information about Na(+) channel function. In this study, we sought to examine the changes in the SDTCs of motor and sensory fibers of the median nerve in patients taking colchicine, which affects axoplasmic flow and may result in axonal neuropathy.

METHODS AND RESULTS

The SDTCs of motor and sensory fibers of 29 patients who had been taking colchicine were measured following stimulation of the right median nerve at the wrist. The results were compared with ten healthy age-matched subjects. No significant differences were found between the groups.

CONCLUSIONS

The lack of any effect on the SDTC by colchicine might have been due to the fact that axonal degeneration caused by colchicine affects the Na(+)-K(+) ATP pump or that it affects internodal channels other than nodal channels.

Effects of age on excitability properties in human motor axons

OBJECTIVE

The threshold tracking technique is a new approach to non-invasively assess biophysical properties of axonal membrane in human subjects. The aim of this study was to evaluate the effects of age and gender on excitability properties of human motor axons.

METHODS

Computerized threshold tracking was used to measure multiple excitability indices in median motor axons of 93 normal subjects (50 men; age, 20-86 years).

RESULTS

Regression analyses showed that the higher age was associated with longer strength-duration time constant (p=0.03), smaller threshold changes in depolarizing threshold electrotonus (p=0.02), smaller supernormality (p=0.01), and steeper slope of the current-threshold relationship for hyperpolarizing currents (p<0.001). There were slight sex differences in rheobase, threshold electrotonus, supernormality, late subnormality, and current-threshold slope, though they were significant only in the subgroup with age <50 years.

CONCLUSIONS

Aging may increase persistent sodium currents, inward rectification, and possibly, outward potassium currents. The combination of changes raises the possibility of slight membrane depolarization in elderly people. For the sex-related differences, further studies will be required with the evaluation of sex hormonal effects.

SIGNIFICANCE

Age-related effects on excitability properties are subtle, but should be taken into consideration in the clinical application of nerve excitability testing, particularly in elderly subjects.

Strength-duration properties in diabetic polyneuropathy

We sought to examine the changes in the strength-duration time constant (SDTC) of the median nerve in diabetic polyneuropathy. The SDTC is a measure of axonal excitability and depends on the biophysical properties of the axonal membrane. It may provide some information about Na(+) channel functioning. Forty-nine diabetic patients (30 men, 19 women; mean age, 54.6+/-9.4 years) and 15 age-matched healthy subjects (11 men, 4 women; mean age, 53.1+/-7.6 years) participated in the study. SDTC and rheobase values were 384.5+/-97.9 micros and 4.6+/-2.4 mA in patients and 313.8+/-46.7 micros and 6.0+/-2.6 mA in controls. The SDTC was found to be significantly different between the two groups (P=.009 for SDTC and P=.066 for rheobase). Fasting glucose and HbA1c levels of patients were 206.6+/-88.7 mg/dL and 8.8+/-2.4%, and these were not correlated with SDTC or rheobase (P<.05). We suggest that longer SDTC, indicating increased axonal excitability may develop in patients with polyneuropathy associated with uncontrolled diabetes.

Effects of botulinum toxin on strength-duration properties

Axonal excitability studies have been used in several diseases to investigate the underlying pathophysiology. The threshold tracking technique was developed to measure noninvasively several indices of axonal excitability, such as strength-duration properties. This study investigated the possible effects of botulinum toxin on strength-duration time constant (SDTC) in patients with the symptoms and signs of botulism. The clinical and electrophysiological findings of 13 patients who were admitted to the authors' clinic with botulism signs and symptoms were evaluated in a 5-day period after exposure to the toxin prospectively. After routine diagnostic electroneuromyographic examinations and electromyogram with repetitive nerve stimulation at 20-50 Hz, SDTC was studied. The results were compared with 13 age- and sex-matched healthy volunteers. The SDTCs were 381 +/- 60 micros and 471 +/- 84 micros in patients and controls, respectively. There was a statistical difference between the two groups (p = .003, Mann Whitney U test). These findings suggest a possible effect of botulinum toxin, known to be effective at neuromuscular junction, on Na(+)/K(+) pump activity, and Na(+) or K(+) conductance.

The effect of hyperventilation on strength–duration properties in diabetic polyneuropathy

OBJECTIVE

Hyperventilation and ischaemia increase axonal excitability by changing Na+ conductance in healthy subjects. However, the changes in excitability during and after ischaemia in diabetic patients are less than in healthy controls. This is known as ischaemic resistance. In this study, we investigated the effects of hyperventilation for 20 min on strength-duration time constant (SDTC) of motor axons of the median nerve of diabetic patients with polyneuropathy to determine whether diabetics are less affected by hyperventilation, a form of resistance similar to the ischaemic resistance of diabetics.

METHODS

The SDTC of 14 diabetic patients with polyneuropathy and 10 healthy volunteers were measured following stimulation of right median nerve at the wrist prior to and after hyperventilation for 20 min.

RESULTS

There was a significant increase in the SDTC in control subjects, but no significant change in the SDTC for patients with diabetic polyneuropathy. The score of the clinical response (paraesthesiae and carpopedal spasm) to hyperventilation of controls was also significantly greater in the controls than the patients.

CONCLUSION

Hyperventilation for 20 min has little influence on SDTC in patients with diabetic polyneuropathy.

SIGNIFICANCE

The 'resistance' of diabetic nerve is not confined to ischaemia but involves other manoeuvres that can alter axonal excitability.