Spike latency and response properties of an excitable micropillar laser

We present experimental measurements concerning the response of an excitable micropillar laser with saturable absorber to incoherent as well as coherent perturbations. The excitable response is similar to the behavior of spiking neurons but with much faster time scales. It is accompanied by a subnanosecond nonlinear delay that is measured for different bias pump values. This mechanism provides a natural scheme for encoding the strength of an ultrafast stimulus in the response delay of excitable spikes (temporal coding). Moreover, we demonstrate coherent and incoherent perturbations techniques applied to the micropillar with perturbation thresholds in the range of a few femtojoules. Responses to coherent perturbations assess the cascadability of the system. We discuss the physical origin of the responses to single and double perturbations with the help of numerical simulations of the Yamada model and, in particular, unveil possibilities to control the relative refractory period that we recently evidenced in this system. Experimental measurements are compared to both numerical simulations of the Yamada model and analytic expressions obtained in the framework of singular perturbation techniques. This system is thus a good candidate to perform photonic spike processing tasks in the framework of novel neuroinspired computing systems.

Spatiotemporal Chaos Induces Extreme Events in an Extended Microcavity Laser

Extreme events such as rogue waves in optics and fluids are often associated with the merging dynamics of coherent structures. We present experimental and numerical results on the physics of extreme event appearance in a spatially extended semiconductor microcavity laser with an intracavity saturable absorber. This system can display deterministic irregular dynamics only, thanks to spatial coupling through diffraction of light. We have identified parameter regions where extreme events are encountered and established the origin of this dynamics in the emergence of deterministic spatiotemporal chaos, through the correspondence between the proportion of extreme events and the dimension of the strange attractor.

Temporal summation in a neuromimetic micropillar laser

Neuromimetic systems are systems mimicking the functionalities or architecture of biological neurons and may present an alternative path for efficient computing and information processing. We demonstrate here experimentally temporal summation in a neuromimetic micropillar laser with an integrated saturable absorber. Temporal summation is the property of neurons to integrate delayed input stimuli and to respond by an all-or-none kind of response if the inputs arrive in a sufficiently small time window. Our system alone may act as a fast optical coincidence detector and paves the way to fast photonic spike-processing networks.

Relative refractory period in an excitable semiconductor laser

We report on experimental evidence of neuronlike excitable behavior in a micropillar laser with saturable absorber. We show that under a single pulsed perturbation the system exhibits subnanosecond response pulses and analyze the role of the laser bias pumping. Under a double pulsed excitation we study the absolute and relative refractory periods, similarly to what can be found in neural excitability, and interpret the results in terms of a dynamical inhibition mediated by the carrier dynamics. These measurements shed light on the analogy between optical and biological neurons and pave the way to fast spike-time coding based optical systems with a speed several orders of magnitude faster than their biological or electronic counterparts.

Increased sural nerve epineurial blood flow in human subjects with painful diabetic neuropathy

AIMS/HYPOTHESIS

The pathogenesis of painful diabetic neuropathy remains unknown. As a consequence we still do not have any effective, rational treatments and a greater understanding of the mechanisms is urgently required. Previous studies have shown no consistent morphological differences in the nerves of patients with and without painful neuropathy. The aim of this study was to compare epineurial haemodynamics in patients with chronic painful and painless neuropathy.

METHODS

The techniques of microlightguide spectrophotometry and fluorescein angiography were used to measure epineurial intravascular oxygen saturation and blood flow respectively. Eleven patients with painful and eight with painless neuropathy were studied, with the groups matched carefully in terms of severity of neuropathy and diabetes control.

RESULTS

Intravascular oxygen saturation was higher in the painful neuropathy group compared to those without pain (median 73.8% vs 67.7%, respectively; p=0.021). Fluorescein rise time was also faster in those with painful symptoms (median 18.3 s vs 53.6 s; p=0.046) indicating higher epineurial blood flow in these subjects.

CONCLUSION/INTERPRETATION

These results indicate that there are distinct differences in haemodynamics within the epineurium of the sural nerve in subjects with painful and painless neuropathy. Haemodynamic factors could therefore have an important role in the pathogenesis of neuropathic pain and might offer further insight into potential treatments for this distressing condition.

Management of degenerative changes and stenosis of the lumbar spinal canal secondary to cervical spinal cord injury

We describe the case of a 47-year-old female who sustained a C5/6 fracture with C6 complete spinal cord injury 26 years ago. She presented with increased spasticity of the lower extremities, the abdominal wall and episodes of autonomic dysreflexia. Imaging of the spine revealed post-traumatic kyphosis at the level of the injury and degenerative changes of the lumbar spine with marked facet joint hypertrophy at the level of L4/5 causing severe spinal canal stenosis. Discussants of this case comment on the possible pathophysiological mechanisms causing autonomic dysreflexia, especially the development of degenerative changes, Charcot arthropathy and the role of tethering mechanisms. The diagnostic options and management approaches are also discussed.

Charcot joint of the spine, a cause of autonomic dysreflexia in spinal cord injured patients

STUDY DESIGN

Case report of two subjects.

OBJECTIVE

Charcot joints of the spine as a cause of Autonomic Dysreflexia in spinal cord lesions.

SETTING

Stoke Mandeville Hospital, UK.

METHOD

Two patients with long standing spinal cord lesions developed symptoms of headaches and sweating associated with sitting up and transfers. In both cases no other cause was found to account for Autonomic Dysreflexia.

RESULT

Charcot Joints of the spine below the level of injury were demonstrated in both cases and symptoms resolved with prolonged bed rest.

CONCLUSION

As care of spinally injured patients continues to improve, they live longer and lead a more active lifestyle, it is expected that the incidence and prevalence of Charcot's joints will increase. Therefore the knowledge and heightened awareness of this entity, early diagnosis and detection with plain X-rays for urinary surveillance, may reduce the morbidity in spinal cord injured patients.

A new minimally invasive technique to show nerve ischaemia in diabetic neuropathy

AIMS/HYPOTHESIS

Experimental studies have shown that abnormalities of nerve microcirculation are important factors in the pathogenesis of diabetic neuropathy but there have been few clinical studies. We have applied microlightguide spectrophotometry to measure intravascular oxygen saturation (HbO2%) and blood flow in human sural nerve.

METHODS

We studied ten patients with mild-moderate sensory motor diabetic neuropathy, nine patients without neuropathy and nine control subjects. We took 300 measurements of oxygen saturation under direct visual control through a 1.9 mm rigid endoscope over three regions of the nerve. Spectrophotometric measurements of nerve fluorescence were taken after an intravenous injection of sodium fluorescein and the rate of increase in nerve fluorescence (rise time) was used as an indicator of nerve blood flow.

RESULTS

Nerve oxygen saturation was reduced in patients with neuropathy compared with control subjects (67.1 +/- 2.2% vs 76.7 +/- 2.1%, p = 0.006). Fluorescein rise time was prolonged in patients with neuropathy compared with the control group (48.5 +/- 7.0 s vs 14.0 +/- 3.1 s, p = 0.001) suggesting impaired nerve blood flow. There was a correlation between rise time, nerve oxygen saturation, glycaemic control and sural nerve sensory conduction velocity (p < 0.01).

CONCLUSION/INTERPRETATION

The combination of microlight-guide spectrophotometry and micro-endoscopy provides a valuable minimally invasive technique for clinical investigation of nerve microcirculation. We have shown reduced nerve oxygenation and impaired blood flow in diabetic neuropathy and these findings strongly support a central role of microvascular disease in the pathogenesis of diabetic neuropathy.

Type I Chiari deformity presenting with profound sinus bradycardia: case report and literature review

Type I Chiari deformity presents with diverse symptoms and signs which can be attributed to compression of structures at the foramen magnum. Bradycardia as a result of medullary compression has not been reported previously. A patient is described with type I Chiari deformity who presented with episodic profound sinus bradycardia for which a pacemaker was inserted before the diagnosis of cerebellar ectopia was finally made. Surgical decompression proved curative.

Spinal extradural arteriovenous malformation presenting with recurrent hemorrhage and intermittent paraplegia: case report and review of the literature

Symptomatic spinal extradural arteriovenous malformations (AVMs) are uncommon. They usually present with slowly progressive myelopathy and/or radiculopathy and only rarely with spinal extradural hemorrhage (SEH). Histopathologic features of a true spinal extradural AVM causing an overt SEH have only previously been described in four cases. A further case of a histologically confirmed spinal extradural AVM causing SEH in a 50-year-old otherwise medically normal Caucasian woman is presented. Recurrent hemorrhage resulted in intermittent paraplegia and acute chest/shoulder pain--diagnosed initially as hysteria. Interesting sequential myelographic, computed tomographic, and magnetic resonance imaging studies are presented, and the literature on spinal extradural AVMs presenting with SEH is reviewed.

Idiopathic spinal extradural lipomatosis in a non-obese otherwise healthy man

Spinal extradural lipomatosis is rare and is usually associated with long-term steroid administration or obesity. It is most commonly thoracic in situation. We present a unique case of spinal extradural lipomatosis in a 20-year-old non-obese, clinically normal man. The role of MRI in investigation and the management options are discussed.