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Strength-duration properties of cutaneous afferents in motor neuron disease
Socieiy proceedings
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loss of one or mom of the waveforms occurring after N20 in 45% of cases. This SEP abnormality was not produced by subcoitical infarction. The SEP findings most typical of subcortical infarction (36%) were a reduction in amplitude ipsilateral to infarction and a prolonged response (N20-P45). This also occurmd after a few cortical infarcts (6%). but in each case these were in areas other than that supplied by the middle cerebral artery. Somatosensory evoked potentials, by discriminating between cortical and subcortical infarction in the territory of the middle cerebral artery, may be used to determine the location of stroke and this may aid in determining its likely pathophysiology.
15. Strength-duration properties of cutaneous afferents in motor neuron disease. - Bona Mogyoros, Matthew C. Riernan, David Burke, Hugh Bostock (Prince of Wales Medical Research Institute, Sydney, NSW) Motor neuron disease is usually believed to involve motor neurons exclusively, although mild sensory symptoms occur in lO-25% of patients. It has also been reported that patients with motor neuron disease experience few or no paraesthesiae during ischaemic compression of their limbs. Ischaemic paraesthesiae appear to be related to a voltage-dependent non-inactivating Na+ conductance, expressed more on sensory than motor axons. The present study was undertaken to address the issue of whether the excitability of cutaneous sensory axons is abnormal in motor neuron disease by measuring the strength-duration time constant, refractoriness and supernormality at rest and during a 10 min period of ischaemia. The strength-duration properties of sensory axons were measured at the wrist using a computerized threshold tracking procedure. The threshold current required to produce a CSAP of 30-40% maximum was calculated for 5-10 stimulus durations between 40~s and 1.0 ms. The time constant was determined using Weiss’s formula which relates stimulus charge to stimulus duration. Fifteen patients with motor neuron disease (10 male, 5 female; mean age 61 years, range 45-74 years) were studied. In 12 patients changes in threshold, refractoriness and supernormality were followed during ischaemia for 10 min. The time constant of sensory axons was 544 f 40.3 PCS, significantly shorter than normal (665 f 41 ys). Motor time constant did not differ from the control value. The rheobase for patients’ sensory axons (2.41 f 0.23 mA) was similar to that for normal for motor axons (2.46 f 0.6 mA). The changes in threshold, refractoriness and supemormahty during ischaemia did not differ from control. The shorter time constant of sensory axons suggests that these axons may express less non-inactivating Na+ channels and thereby behave similarly to normal motor axons. 16.
accuracy of high resolution sonography in carpal tunnel syndrome. - R. Ptasnik, R.A.L. Macdonell, 0. Hennessy (Departments of Neurology and Radiology, Austin and Repatriation Medical Centre, Heidelberg, Victoria)
Diagnostic
Carpal tunnel syndrome (CTS) is the most common compressive neuropatby. Nerve conduction studies (NCS) am currently considered the most sensitive diagnostic technique. A new technique, high resolution ultrasonography (SG) is able to measure the surface area of the median nerve in the carpal tunnel. An increase in this has been proposed as a novel diagnostic technique for CTS. Sixty-two wrists in 31 patients referred for electrodiagnosis with a provisional diagnosis of uni- or bilateral carpal tunnel syndrome were also studied using USG. Normal values for USG were obtained from 20 normal volunteers (40 wrists). The mean surface diameter of the median nerve in this group was 10.97 mm*, SD 1.4 mm*. The upper limit of normal was taken as mean * 2 SD (13.77 mm*). Using NCS as the ‘gold standard’, an abnormal USG had a sensitivity of 62% and specificity of 89% in the diagnosis of CTS. Median nerve USG is unlikely to replace NCS as the investigation of choice in CTS. It may be helpful as an ancillary investigation in patients in whom NCS may be difficult to interpret such as
those with continued symptoms of CTS postoperatively superimposed on other neuropathies. 17.
or when CTS is
The myopathic features of the mitochondrial cytopathies. C.M. Sue (Department of Neurology, Westmead Hospital, Sydney, NSW)
Since the introduction of muscle histochemistry and the identitication of the ‘ragged red fibre’ on modified Gomori trichrome staining, a group of neuromuscular disorders referred to as the ‘mitochondrial myopathies’ has been defined. With the advent of molecular biological techniques and sequencing of the mitochondrial genome, many of the mitochondrial disorders have now been associated with defects within mitochondrial DNA (mtDNA). MtDNA defects fail into two main subgroups: gene rearrangements and point mutations. Keams-Sayre syndrome (KSS), chronic progressive external ophthalmoplegia (CPEO) and Pearson’s syndrome are usually sporadic and are associated with mtDNA rearrangements. The myopathic features of KSS and CPEO are distinctive with ptosis, external ophtbalmoplegia and proximal weakness, whereas Pearson’s disease is usually not associated with myopathy. MERRF and MELAS syndrome are maternally transmitted and are usually associated with point mutations in mitochondrial tRNA genes. Myopathic features in these two disorders are usually subtle, with mild proximal weakness or symptoms of muscle fatigue. However, individuals with the same genetic defect may have different clinical presentations and conversely, patients with different genetic defects may have similar clinical manifestations. These differences in the myopathic features of the mitochondrial cytopathies may aid in the diagnosis of these disorders although the mechanisms by which different mtDNA defects result in variable clinical presentations remain enigmatic.
18.
Ongoing EMG reduces maximal CMAP. - J.L. Taylor, J.E. Butler, G.M. Allen, S.C. Gandevia (Prince of Wales Medical Research Institute, Sydney, NSW)
Motor evoked potentials (MEPs) elicited by high intensity transcranial magnetic stimulation grow with background muscle activity. However, with very strong contractions (75-1008 MVC), MEPs in adductor pollicis are smaller than with contractions of 25-50% MVC. To assess whether this decrease is a peripheral phenomenon, supramaximal electrical stimuli were delivered to the ulnar nerve at the wrist during rest and during brief (2-3 s) contractions of 25,50,75 and 100% MVC. Compound muscle action potentials (CMAPs) were recorded with surface electrodes over adductor pollicis. Area and amplitude of CMAPs were averaged for each contraction strength for each subject. At 100% MVC, CMAP area decreased to 69 i 4% (mean + SD; n = 4) and peak-to-peak amplitude to 75 f 9% of the size during rest. The initial peak of the biphasic potential decreased to 55% of rest amplitude whereas the subsequent trough did not change significantly. Average CMAP size declined linearly with increasing contraction strength (area, r = 0.97; amplitude, r = 0.99). Factors likely to contribute to the reduced CMAP amplitude are nerve and muscle fibre refractoriness and the effect of continuing repolarization of motor units activated prior to the stimulus. 19.
Localisation of the frontal eye field (FEF) using transcranial magnetic stimulation (TMS). - G.W. Thickbroom, R. Stell, F.L. Mastaglia (ANRI, Queen Elizabeth II Medical Centre,
Perth, WA 6009) The anatomical location and surface markings of the human frontal eye field (FEF) have not been clearly defined. To provide more reliable surface coordinates for the FEF, we investigated the region of cortex over which transcranial magnetic stimulation (TMS) could delay saccadic eye movement. In 5 healthy right handed males (22-51 years of age), focal TMS (Magstim 200, 5 cm fig-8 coil) was delivered during saccade reaction time and the delay in saccade onset plotted as a func-
62P
loss of one or mom of the waveforms occurring after N20 in 45% of cases. This SEP abnormality was not produced by subcoitical infarction. The SEP findings most typical of subcortical infarction (36%) were a reduction in amplitude ipsilateral to infarction and a prolonged response (N20-P45). This also occurmd after a few cortical infarcts (6%). but in each case these were in areas other than that supplied by the middle cerebral artery. Somatosensory evoked potentials, by discriminating between cortical and subcortical infarction in the territory of the middle cerebral artery, may be used to determine the location of stroke and this may aid in determining its likely pathophysiology.
15. Strength-duration properties of cutaneous afferents in motor neuron disease. - Bona Mogyoros, Matthew C. Riernan, David Burke, Hugh Bostock (Prince of Wales Medical Research Institute, Sydney, NSW) Motor neuron disease is usually believed to involve motor neurons exclusively, although mild sensory symptoms occur in lO-25% of patients. It has also been reported that patients with motor neuron disease experience few or no paraesthesiae during ischaemic compression of their limbs. Ischaemic paraesthesiae appear to be related to a voltage-dependent non-inactivating Na+ conductance, expressed more on sensory than motor axons. The present study was undertaken to address the issue of whether the excitability of cutaneous sensory axons is abnormal in motor neuron disease by measuring the strength-duration time constant, refractoriness and supernormality at rest and during a 10 min period of ischaemia. The strength-duration properties of sensory axons were measured at the wrist using a computerized threshold tracking procedure. The threshold current required to produce a CSAP of 30-40% maximum was calculated for 5-10 stimulus durations between 40~s and 1.0 ms. The time constant was determined using Weiss’s formula which relates stimulus charge to stimulus duration. Fifteen patients with motor neuron disease (10 male, 5 female; mean age 61 years, range 45-74 years) were studied. In 12 patients changes in threshold, refractoriness and supernormality were followed during ischaemia for 10 min. The time constant of sensory axons was 544 f 40.3 PCS, significantly shorter than normal (665 f 41 ys). Motor time constant did not differ from the control value. The rheobase for patients’ sensory axons (2.41 f 0.23 mA) was similar to that for normal for motor axons (2.46 f 0.6 mA). The changes in threshold, refractoriness and supemormahty during ischaemia did not differ from control. The shorter time constant of sensory axons suggests that these axons may express less non-inactivating Na+ channels and thereby behave similarly to normal motor axons. 16.
accuracy of high resolution sonography in carpal tunnel syndrome. - R. Ptasnik, R.A.L. Macdonell, 0. Hennessy (Departments of Neurology and Radiology, Austin and Repatriation Medical Centre, Heidelberg, Victoria)
Diagnostic
Carpal tunnel syndrome (CTS) is the most common compressive neuropatby. Nerve conduction studies (NCS) am currently considered the most sensitive diagnostic technique. A new technique, high resolution ultrasonography (SG) is able to measure the surface area of the median nerve in the carpal tunnel. An increase in this has been proposed as a novel diagnostic technique for CTS. Sixty-two wrists in 31 patients referred for electrodiagnosis with a provisional diagnosis of uni- or bilateral carpal tunnel syndrome were also studied using USG. Normal values for USG were obtained from 20 normal volunteers (40 wrists). The mean surface diameter of the median nerve in this group was 10.97 mm*, SD 1.4 mm*. The upper limit of normal was taken as mean * 2 SD (13.77 mm*). Using NCS as the ‘gold standard’, an abnormal USG had a sensitivity of 62% and specificity of 89% in the diagnosis of CTS. Median nerve USG is unlikely to replace NCS as the investigation of choice in CTS. It may be helpful as an ancillary investigation in patients in whom NCS may be difficult to interpret such as
those with continued symptoms of CTS postoperatively superimposed on other neuropathies. 17.
or when CTS is
The myopathic features of the mitochondrial cytopathies. C.M. Sue (Department of Neurology, Westmead Hospital, Sydney, NSW)
Since the introduction of muscle histochemistry and the identitication of the ‘ragged red fibre’ on modified Gomori trichrome staining, a group of neuromuscular disorders referred to as the ‘mitochondrial myopathies’ has been defined. With the advent of molecular biological techniques and sequencing of the mitochondrial genome, many of the mitochondrial disorders have now been associated with defects within mitochondrial DNA (mtDNA). MtDNA defects fail into two main subgroups: gene rearrangements and point mutations. Keams-Sayre syndrome (KSS), chronic progressive external ophthalmoplegia (CPEO) and Pearson’s syndrome are usually sporadic and are associated with mtDNA rearrangements. The myopathic features of KSS and CPEO are distinctive with ptosis, external ophtbalmoplegia and proximal weakness, whereas Pearson’s disease is usually not associated with myopathy. MERRF and MELAS syndrome are maternally transmitted and are usually associated with point mutations in mitochondrial tRNA genes. Myopathic features in these two disorders are usually subtle, with mild proximal weakness or symptoms of muscle fatigue. However, individuals with the same genetic defect may have different clinical presentations and conversely, patients with different genetic defects may have similar clinical manifestations. These differences in the myopathic features of the mitochondrial cytopathies may aid in the diagnosis of these disorders although the mechanisms by which different mtDNA defects result in variable clinical presentations remain enigmatic.
18.
Ongoing EMG reduces maximal CMAP. - J.L. Taylor, J.E. Butler, G.M. Allen, S.C. Gandevia (Prince of Wales Medical Research Institute, Sydney, NSW)
Motor evoked potentials (MEPs) elicited by high intensity transcranial magnetic stimulation grow with background muscle activity. However, with very strong contractions (75-1008 MVC), MEPs in adductor pollicis are smaller than with contractions of 25-50% MVC. To assess whether this decrease is a peripheral phenomenon, supramaximal electrical stimuli were delivered to the ulnar nerve at the wrist during rest and during brief (2-3 s) contractions of 25,50,75 and 100% MVC. Compound muscle action potentials (CMAPs) were recorded with surface electrodes over adductor pollicis. Area and amplitude of CMAPs were averaged for each contraction strength for each subject. At 100% MVC, CMAP area decreased to 69 i 4% (mean + SD; n = 4) and peak-to-peak amplitude to 75 f 9% of the size during rest. The initial peak of the biphasic potential decreased to 55% of rest amplitude whereas the subsequent trough did not change significantly. Average CMAP size declined linearly with increasing contraction strength (area, r = 0.97; amplitude, r = 0.99). Factors likely to contribute to the reduced CMAP amplitude are nerve and muscle fibre refractoriness and the effect of continuing repolarization of motor units activated prior to the stimulus. 19.
Localisation of the frontal eye field (FEF) using transcranial magnetic stimulation (TMS). - G.W. Thickbroom, R. Stell, F.L. Mastaglia (ANRI, Queen Elizabeth II Medical Centre,
Perth, WA 6009) The anatomical location and surface markings of the human frontal eye field (FEF) have not been clearly defined. To provide more reliable surface coordinates for the FEF, we investigated the region of cortex over which transcranial magnetic stimulation (TMS) could delay saccadic eye movement. In 5 healthy right handed males (22-51 years of age), focal TMS (Magstim 200, 5 cm fig-8 coil) was delivered during saccade reaction time and the delay in saccade onset plotted as a func-