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Biomedical subjects

T C Britton

Publications and source records attributed to T C Britton.

49 records · Page 3Linked to original sources
PubMed1991

New observations on the normal auditory startle reflex in man.

The latency and pattern of muscle recruitment in the startle response elicited by unexpected auditory stimulation was determined in 12 healthy subjects. Reflex EMG activity was recorded first in orbicularis oculi. This was of similar latency to the normal auditory blink reflex and, unlike the generalized startle response, persisted despite the frequent presentation of the test stimulus. It is argued that this early latency activity in orbicularis oculi represents a normal auditory blink reflex and is not part of the generalized auditory startle reflex. With the exception of this early latency activity in orbicularis oculi, the relative latencies of both cranial and distal muscles in the auditory startle response increased with the distance of their respective segmental innervations from the caudal brainstem. Thus the earliest EMG activity was recorded in sternocleidomastoid. The recruitment of caudal muscles was relatively slow and the latencies of the intrinsic hand muscles were disproportionately long. The pattern of recruitment of cranial muscles suggests a brainstem origin for the normal startle response. Studies on the auditory startle reflex in animals are reviewed in the light of this finding.

Acoustic Stimulation↗
PubMed1991

The hyperekplexias and their relationship to the normal startle reflex.

The startle response to unexpected auditory and somaesthetic stimulation was studied in 8 patients with hereditary or symptomatic hyperekplexia. It was abnormal in its resistance to habituation and in its exaggerated motor response. Both noise and taps to the face and head elicited a normal early blink response, separate from the subsequent true startle reflex. The earliest reflex EMG activity recorded after the blink was in sternocleidomastoid. EMG activity in masseter, and trunk and limb muscles followed later. This pattern of muscle recruitment suggests a brainstem origin for the abnormal startle responses. In addition, the abnormal startle responses exhibited disproportionately long latencies to the intrinsic hand and foot muscles and relatively slow recruitment of caudal muscles. The pattern of muscle recruitment was similar between patients, irrespective of the absolute latency of the response, and regardless of whether stimulation was auditory or somaesthetic. This suggests that auditory and somaesthetic afferents converge on a common brainstem efferent system, and that this system forms the final common pathway for abnormal startle responses of differing latency. The characteristics of this efferent system differ from those previously described in brainstem reticular reflex myoclonus, but are similar to those described in the normal auditory startle reflex in man. This suggests that the abnormal startle response in hyperekplexia, and the normal startle reflex represent pathological and physiological activity in the same brainstem efferent system.

Acoustic Stimulation↗
PubMed1991

Central motor pathways in patients with mirror movements.

Central motor pathways were investigated in three patients with congenital mirror movements using magnetic motor cortex stimulation. Response thresholds, amplitudes and latencies were normal. The projection of the corticomotoneuronal pathways was assessed by placing the coil over the vertex and comparing the size of responses in the first dorsal interosseous (FDI) muscles evoked by anticlockwise and clockwise [corrected] coil currents. In normal subjects, right FDI responses are larger with anticlockwise currents than with clockwise [corrected] currents at the same stimulation strength and vice versa. In two out of three patients with congenital mirror movements, this sensitivity of response amplitude to coil current direction was reversed. The third patient with congenital mirror movements and a fourth patient with acquired mirror movements had responses which were normally sensitive to current direction. These findings support the hypothesis that some cases of congenital mirror movements may be due to abnormal projection of corticomotoneuronal pathways.

Adult↗
PubMed1991

Magnetic stimuli applied over motor and visual cortex: influence of coil position and field polarity on motor responses, phosphenes, and eye movements.

Transcranial magnetic stimulation was performed over the motor and visual cortex with the Novametrix 12-cm diameter stimulation coil. The influence of coil position on the size of electromyographic responses and on the intensity and position of phosphenes in the binocular visual field was studied. Furthermore, it was investigated whether stimulation over the visual cortex or over the frontal eye field evoked or disturbed eye movements. Coil position was recorded by constructing grids on the skull surface using extracranial bony landmarks, and was then related individually to underlying cerebral sulci by analysis of magnetic resonance images of the brain. Motor responses. Excitatory effects were maximal when the coil windings in the middle of the coil ring lay over the particular motor representation field of area 4. The response size depended on the direction of the coil currents passing over the motor cortex. For example, coil currents passing over the lateral part of the precentral gyrus from the front and transversely to the central sulcus elicited maximal responses in hand muscles. With the currents passing over the same area in the opposite direction, response amplitudes were much smaller, suggesting activation of different inputs to cortical motoneurons. Phosphenes. Fields of fixed, white and unstructured phosphens occurred in the lower half of the visual field when the coil center was placed about 7 cm anterior to the inion on the inion-nasion line. Counterclockwise or clockwise coil currents elicited phosphenes within the left or right lower quadrant of the binocular visual field, respectively, which could be attributed to an activation of the right or left primary visual cortex (area 17). The 'cortical' phosphenes moved with voluntary eye movements, but not during caloric and optokinetic nystagmus. Phosphenes resulting from an excitation of the optic nerve rather than the retina could be evoked by stimulation over frontal parts of the skull. Eye movements. The application of single magnetic field pulses over the frontal eye field or over the visual cortex did not elicit eye movements except for small vertical eye movements as part of a magnetically elicited blink.

Adolescent↗
PubMed1990

Clinical use of the magnetic stimulator in the investigation of peripheral conduction time.

The application of rapidly changing magnetic fields (magnetic stimulation) over the neck or lower back elicits EMG responses in the muscles of the arm or leg respectively. Such responses have stable onset latencies but their amplitudes vary depending on the position of the coil over the neck or lower back. Supramaximal responses could not be obtained. Comparison of onset latencies with estimates of peripheral conduction time using a conventional F-wave technique suggest that the site of excitation of the motor axons is about 1.3 msec conduction time distal to the cervical motoneurons and 3 msec distal to the lumbosacral motoneurons. Response configuration after paravertebral magnetic stimulation was similar to that of the standard electrically evoked M-wave in the small hand muscles but not in lower limb muscles. Responses in lower limb muscles after paravertebral magnetic stimulation may consist of additional F-wave and H-reflex components. The possible clinical role of paravertebral magnetic stimulation in the investigation of peripheral and central motor pathways is discussed in the light of these findings.

Adult↗
PubMed1989

Investigation of unilateral facial weakness: magnetic stimulation of the proximal facial nerve and of the face-associated motor cortex.

Twenty-four patients with unilateral facial weakness of various aetiologies were investigated using a magnetic stimulator to stimulate the proximal segment of the facial nerve directly (short latency response) and also to activate the facial motoneurons bilaterally via corticonuclear pathways by placing the stimulating coil over the motor cortex (long latency responses). Electromyographic recordings were taken from both mentalis muscles using concentric needle electrodes. Seventeen patients were investigated at various times after onset of idiopathic facial palsy (Bell's palsy). In the acute stage (less than 5 days after onset) short and long latency responses on the paretic side were abnormal, being absent in all but one patient, in whom the short latency response was delayed. These abnormal responses were the earliest neurographic correlate for nerve conduction block. In 4 out of 9 patients seen up to 30 days after onset of palsy, trans-synaptically evoked long latency responses were absent. In patients examined more than 2 months after onset, long latency responses could always be obtained and, in 5 of 8 patients, short latency responses could also be elicited, indicating a return of the direct excitability of the nerve. Five patients with cerebral hemisphere lesions causing mild unilateral facial weakness had absent long latency responses when stimulating over the affected hemisphere, but normal bilateral long latency responses following stimulation over the unaffected cerebral hemisphere; short latency responses were normal. Magnetic stimulation of the brain and of the facial nerve can differentiate between central and peripheral causes of unilateral facial weakness and may prove useful in the early assessment of the degree of conduction block in Bell's palsy.

Adolescent↗
PubMed1989

Safety and efficacy of acetarsol suppositories in unresponsive proctitis.

In a prospective open study, 10 patients with intractable proctitis were treated with acetarsol suppositories (250 mg b.d. for 4 weeks) and were monitored clinically, biochemically and toxicologically. Proctitis resolved symptomatically and sigmoidoscopically within 2 weeks in nine patients; one patient was unaffected. The only side-effect was of transient thrombocytosis in a single patient. Maximal blood and urinary arsenic concentrations occurred after 1 week's treatment with a total inorganic arsenic in the hazardous range in six patients; subsequent concentrations fell despite continuing therapy and at 4 weeks potentially hazardous values persisted in only two patients. Continued renal excretion and diminished absorption across an improved rectal mucosa is thought to be responsible for this paradox. Arsenic levels fell rapidly when acetarsol was withdrawn and were indistinguishable from pretreatment values within 4 weeks. Short-term acetarsol therapy offers a useful additional measure when local steroids have failed to control ulcerative proctitis; it appears to be safe and formal controlled comparisons with other therapeutic options are therefore legitimate.

Adult↗
PubMed1989

The significance of plasma phytanic acid levels in adults.

The presence of phytanic acid in tissues and plasma has been considered diagnostic of heredopathia atactica polyneuritiformis (Refsum's disease), but recently slightly raised plasma phytanic acid levels have been reported in other conditions. Forty two normal people were found to have a phytanic acid level of 0-33 mumol/l. Fourteen patients with heredopathia atactica polyneuritiformis had a plasma phytanic acid level before treatment of 992-6400 mumol/l. Five patients with retinitis pigmentosa but not heredopathia atactica polyneuritiformis had plasma levels of 38-192 mumol/l. It was concluded that some patients with retinitis pigmentosa without heredopathia atactica polyneuritiformis but a raised plasma phytanic acid may represent a group of patients with a disease or diseases as yet uncharacterised apart from the retinal condition.

Adolescent↗
PubMedDate not supplied

Technical approaches to hemisphere-selective transcranial magnetic brain stimulation.

Clinical application of transcranial magnetic brain stimulation is mainly used to determine central motor conduction times. With the stimulation coil (Magstim 200, Novametrix) centered conventionally over the midline of the skull convexity and using high stimulus intensities, which are often required in pathological states, the motor cortices of both hemispheres are usually activated simultaneously. Under this condition it is not possible to determine from which hemisphere the descending excitatory volleys to a particular motoneurone pool originate and how the input to the lower motoneurons is organized (uni-/bilateral, ipsi-/contralateral). This limitation can be overcome by two different techniques for selective stimulation of the motor cortex of one hemisphere without coactivation of the other even when using maximal stimulus intensities: 1. A large 12 cm (outer diameter) stimulation coil could be used for selective stimulation when a) the magnetic field radiated over the non-stimulated hemisphere is modified by using a prototype coil shield covering the half of the coil over the nonstimulated hemisphere in combination with b) placing the coil away from the midline towards the preferentially excited hemisphere. The coil shield consists of a sheet of a nickel iron alloy which alters the time course of the induced currents by reducing the initial rate of current intensity change (dI/dt). 2. The use of a smaller 6.5 cm (outer diameter) coil also provided a useful tool for selective stimulation of one hemisphere but was restricted to subjects with low excitation thresholds. In subjects with high excitation thresholds the described use of the large stimulation coil is advisable.

Adult↗