Logarithmic distribution of amplitudes of compound muscle action potentials evoked by transcranial magnetic stimulation

Motor evoked potentials were recorded from the first dorsal interosseous muscle on the dominant side evoked by transcranial magnetic stimulation in 20 healthy volunteers. Amplitude data revealed skewness of distribution and variance heterogeneity. Natural logarithmic transformation of amplitude data results in normal distribution and ensures variance homogeneity. There was a negative first-order autocorrelation between consecutive recordings, which indicates that a stimulus-free interval of 5 s in consecutive stimulations is insufficient to ensure independent events. In a variability study, no systematic changes in latency and amplitude during day-to-day examinations or in consecutive examinations were found. There was a significant change in latency data (p = 0.02) during a gradual increase of constant elements in the stimulation procedure, whereas the amplitude data were unchanged. Estimation of random contributors to the total variability in the magnetic stimulation technique was performed. Intersubject differences were the major contributor to the variability of latency data but not to variability of logarithmic amplitude data. The variance estimates of different examiners' contribution to the variability was 0.02 for latency data and 0 for amplitude data. This indicates that staff members familiar with the principles in magnetic stimulation and following a thorough verbal and written instruction in the standardized examination procedure can perform the stimulation procedure without increasing the data variability.

Impaired stretch reflex and joint torque modulation during spastic gait in multiple sclerosis patients

The modulation of the short latency stretch reflex of the soleus muscle during walking was investigated in seven spastic multiple sclerosis (MS) patients and nine healthy control subjects. Ankle joint stretches were applied by a system which can rotate that ankle joint in any phase of the step cycle during treadmill walking. The torque related to the muscle fibres contracting prior to the stretch and the passive tissues around the ankle joint were measured as the "non-reflex torque". At the same time the short latency stretch reflex-mediated EMG response was measured. The findings show that the stretch reflex modulation was impaired in spastic patients during walking. The stretch reflex modulation was quantified by a modulation index of an average 50% (range -5 to 100%) in the patients and 93% (78-100%) in the control subjects (P < 0.05). The passive stiffness of the ankle joint was at the same time increased in the patients (P < 0.05). It is proposed that the impaired modulation of the stretch reflex along with increased ankle joint stiffness contribute to the impaired walking ability in spastic MS patients.

A comparison of clinical and laboratory measures of spasticity

Clinical evaluation of spasticity was performed in lower extremities in 35 ambulatory multiple sclerosis patients and compared with the soleus stretch reflex and the Hoffman reflex. There was no relation between the muscle tone score of dorsiflexion of the foot and the biomechanical/electrophysiological parameters. In contrast, the Achilles tendon reflex score was significantly related to the amplitude (rho = 0.411, P < 0.05) and the slope of the stretch reflex (rho = 0.523, P < 0.01). The clinical examination at the ankle joint revealed 33% normal reflex examinations but only 7% normal muscle tone examinations. In contrast, the number of normal examinations of patellar reflex and muscle tone at the knee joint were similar. It is concluded that the muscle tone score overestimates the amount of spasticity because of changes in the non-reflex properties of the spastic extremity and that a reflex score should be used as a clinical measure of spasticity. In addition, biomechanical/electrophysiological evaluation of spasticity at the ankle joint relates to the over-all total muscle tone and reflex scores of lower extremities in this group of MS patients.

Repetitive magnetic stimulation of cerebral cortex in normal subjects

Changes of excitability in the neuraxis were studied in 11 healthy subjects with repetitive transcranial magnetic stimuli of the motor cortex at frequencies of 1-10 Hz. Following trains of four stimuli, compound muscle action potentials (CMAPs) were obtained from the flexor carpi radialis muscle. At an interstimulation interval (ISI) of 100-180 ms. the amplitudes of the CMAPs evoked by stimuli II and IV were inhibited as compared with the amplitude of the CMAPs evoked by the first stimulus. In contrast, the amplitude of the CMAPs evoked by stimulus III was almost unchanged. Changes in excitability depended on the stimulation intensity. Thus, it was demonstrated that the amplitude of the CMAPs evoked by stimulus III was reduced at low stimulation intensities (100 and 110% of threshold intensity) but normalized at higher stimulation intensities (120 and 130% of threshold intensity). Results of H-reflex studies indicated that excitability changes at the segmental level are not necessarily involved in the inhibition of the CMAPs evoked by stimuli II and IV at an ISI of 100 ms. It is proposed that long acting neurotransmitter modulation can explain part of the inhibition during repetitive magnetic stimulation, and that recruitment of different neurons, due to electric field differences, explains the dependence of excitability on stimulation intensity.

Motor pathway function in normoalbuminuric IDDM patients

Central motor pathways were studied in 17 normoalbuminuric insulin-dependent diabetic (IDDM) patients who had been diabetic for more than 20 years, and compared with findings in 17 age-, sex-, and height-matched control subjects. The central motor conduction time was calculated from recordings of the compound muscle action potentials of the abductor pollicis brevis muscle after single transcranial and spinal root magnetic stimulation. The central motor conduction time from motor cortex to cervical spinal roots was 9.8 +/- 1.65 ms in diabetic patients and 10.1 +/- 1.48 ms in control subjects. In diabetic patients with neuropathy the central motor conduction time was 9.5 +/- 1.76 ms vs 10.1 +/- 1.56 ms in patients without neuropathy. The excitability of the motor pathways was studied by paired transcranial magnetic stimulation at interstimulation intervals of 30-1000 ms. In normal control subjects, an early facilitation of the amplitude of the compound muscle action potential at an interstimulation interval of 30 ms was found, while no facilitation was present in diabetic patients. In addition the compound muscle action potential latencies were prolonged at interstimulation intervals of 30-50 ms in diabetic patients. The changes of excitability did not correlate with the presence of peripheral neuropathy, metabolic control or diabetes duration. It is concluded that long-term normoalbuminuric IDDM patients have imparied excitability but normal central conduction time of the motor pathways.

A new high-frequency magnetic stimulator with an oil-cooled coil

A new high frequency magnetic stimulator, Labmag, is described which may be used in clinical neurophysiology and for the treatment of spasticity in multiple sclerosis. In order to avoid heating of the coil during repetitive stimulation, an oil/air cooling system was employed. The magnetic stimulus waveform is a half cosine with a rise time of 200 microseconds and a pulse width of 400 microseconds induced by alternating capacitor voltage. The Labmag product was compared with a commercially available high frequency stimulator, MagPro. The electric fields induced by Labmag were half the size of those induced by MagPro at identical stimulus intensities. Using a leaky integrator to simulate the effects on neurons, only minor differences in integrated electric fields were observed. There were no changes of the electric fields in relation to coil geometry. A minor difference in electric fields (1.0 +/- 0.8%) was observed when the polarity of the capacitor voltage alternated. Compound muscle action potentials (CMAPs) evoked by transcranial magnetic stimulation were recorded from the first dorsal interosseous muscle (FDI) in eight subjects. Threshold intensities of CMAPs evoked by Labmag (58 +/- 7%) were significantly smaller (p < 0.05) than threshold intensities obtained with MagPro (63 +/- 8%). Peak-to-peak amplitudes, P-Pamps, of CMAPs were significantly smaller after single stimulations with Labmag at stimulation intensities of 130 and 140% of threshold intensity.

A Danish family with limb-girdle muscular dystrophy with autosomal dominant inheritance

We describe a Danish family of five generations showing a slowly progressive autosomal dominant limb-girdle myopathy with onset in early childhood. Three patients from two generations were examined and showed weakness predominantly of the proximal muscles of upper and lower extremities. Early contractures of wrist, tight heel cords, metacarpophalangeal and interphalangeal joints were present. Walking ability was partly preserved. Serum creatine kinase activity was normal or slightly elevated. EMG and biopsy showed mildly myopathic changes. Magnetic resonance images of the thigh showed severe atrophy of all compartments but with musculus semimembranosus, sartorius, and gracilis relatively preserved. This myopathy has a clinical picture similar to the Bethlem myopathy.

Standardization of facilitation of compound muscle action potentials using a modified myometer during magnetic stimulation in healthy volunteers

In 20 normal subjects motor evoked potentials (MEPs) from first dorsal interosseous (FDI) were obtained following transcranial magnetic stimulation during relaxation and voluntary contraction of 2-20% of maximum. A new method to standardize facilitation of MEP using surface EMG and a modified myometer was used. The MEP amplitudes increased significantly at contraction levels of 2%, 10% and 15% of maximum. The latencies decreased significantly at contraction levels of 2%, 5% and 15%. Coefficients of variation of the amplitude decreased from 6.4% during relaxation to approximately 2% during facilitation. In contrast, coefficients of variation of take-off latencies (T-lat) increased significantly from 1.7% during relaxation to approximately 3% during facilitation. The variability of amplitudes and latencies was mainly due to inter-individual variation. The observations in the present study indicate that the actual contraction level should be precisely defined during transcranial magnetic stimulation.

Inability of insulin to maintain normal nerve function during high-frequency stimulation in diabetic rat tail nerves

The effect of insulin on the response to long-term high-frequency stimulation (HFS = 143 Hz for 20 min) was studied in mixed tail nerves of acute streptozotocin-induced diabetic rats. In consecutive tests, untreated diabetic rats showed a significant decrease in nerve conduction velocity (NCV) and peak-to-peak amplitude (P-Pamp) and the depression of the P-Pamp during HFS was augmented. In contrast, NCV and P-Pamp in the insulin-treated rats were unchanged from the prediabetic state, but the depression of the P-Pamp during HFS reached the same degree as in untreated rats. This implies that although insulin treatment of acute experimental diabetes is able to preserve a normal NCV and P-Pamp in the resting state, it is unable to preserve normal nerve function under stress produced by HFS. Monitoring of the axon membrane functional capacity may have clinical implications in the control of peripheral neuropathies.

Reduced capability of transmitting high frequency impulses in tail nerves of diabetic rats

The effect of long-term (40 min) high frequency stimulation (143 Hz) of sensory-motor tail nerves was studied in normal and in streptozotocin-diabetic rats. The study comprised a 6-week period, repeating the test at 2-week intervals. In the initial (prediabetic) study, single experiments showed a mild depression of the peak-to-peak amplitude during high frequency stimulation, which reversed completely during a subsequent rest period. In normal rats, the amplitude depression was unchanged in repeated tests over a 6-week period. Diabetic rats showed a greater amplitude depression during high frequency stimulation. The difference was statistically significant after 2 weeks, but showed no further change at subsequent tests. The greater decrease in the amplitude in diabetic rats may reflect a depression of the axon membrane function, which may be the functional correlate to the inhibition of the Na/K ATPase activity in diabetes, described by others. Monitoring of the axon membrane functional capacity may have clinical implications in the control of peripheral neuropathies.

Haemoptysis caused by aneurysm of saphenous bypass graft to a coronary artery

In a 51-year-old man, coronary artery bypass surgery for severe angina pectoris was followed by protracted mediastinal infection. After recovery the patient had several haemoptyses. Angiography revealed aneurysm of a graft near its distal anastomosis. Upper left lobectomy and ligation of the graft were necessitated by bleeding from the aneurysm into a segmental bronchus. Angina pectoris recurred but was successfully treated with verapamil.

Inflatable splints: do they cause tissue ischaemia?

The effect of increasing pneumatic splint pressure on cutaneous oxygen tension measured transcutaneously (TcPO2) was investigated in 12 subjects. The mean initial TcPO2 was 70.6 mmHg. TcPO2 decreased linearly on increasing the pressure within the splint. TcPO2 became zero at a mean splint pressure of 28 mmHg. Second, three accepted methods used by the ambulance crew to assess inflation pressures of the splint were investigated and found to be unreliable. In the light of these findings the risk of ischaemic complications as a result of applying pneumatic splints to a fractured limb is discussed. We advocate a recommended maximum splint pressure of 15 mmHg and that the splint should be manufactured with a security blow-off valve set at 15 mmHg.