Needle-like structures discovered on positively charged lightning branches.

Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends-called positive and negative leaders1. Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30-300-megahertz frequency band2-8 that can be remotely sensed and imaged with high spatial and temporal resolution9-11. Positive leaders propagate more continuously and thus emit very little high-frequency radiation12. Radio emission from positive leaders has nevertheless been mapped13-15, and exhibits a pattern that is different from that of negative leaders11-13,16,17. Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders9,12,16,18-20, which may lead to current pulses that both reconnect positive leaders to negative leaders11,16,17,20-22 and cause multiple cloud-to-ground lightning events1. The disconnection process is thought to be due to negative differential resistance18, but this does not explain why the disconnections form primarily on positive leaders22, or why the current in cloud-to-ground lightning never goes to zero23. Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures-which we call 'needles'-that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times.

[Necrotic toes in a female patient using interferon]. / Een interferongebruikster met necrose aan de tenen.

BACKGROUND: Interferon has an important role in treatment of viral hepatitis, multiple sclerosis and solid and non-solid tumours. Ischaemia and necrosis of the extremities are relatively little-known adverse effects of treatment with interferon. CASE DESCRIPTION: A 44-year-old woman was treated with interferon-beta for relapsing-remitting multiple sclerosis. She developed ischaemia and necrosis of the right lower extremity. Extensive laboratory and imaging investigations offered no clear diagnosis, leading to suspicion of a connexion with interferon-beta treatment. Discontinuation of interferon lead to rapid healing, clearly suggesting a relationship between interferon and the vascular complications. CONCLUSION: Vascular complications during treatment with interferon seem to arise fairly frequently, and can even occur many years after commencing therapy. When vascular symptoms similar to Raynaud's syndrome develop, careful monitoring of the disease course and treatment with vasodilators if required is important. If this is insufficiently effective, interferon should be discontinued. Prompt recognition can prevent significant morbidity in these patients.

Accuracy and inter-observer variation in the classification of dysarthria from speech recordings.

BACKGROUND: Dysarthria may be classified as flaccid, spastic, ataxic, hypokinetic, choreatic, dystonic, or mixed. We hypothesized that in routine neurological practice the reliability and accuracy of perceptual analysis alone in the classification of dysarthria is low and that this classification is mainly based on the clinical context rather than on the perception of speech. We therefore studied the accuracy and the inter- observer agreement in the classification of dysarthrias on the basis of perceptual analysis alone. METHODS: Seventy two neurologists and neurological trainees classified recorded speech samples of 100 patients as flaccid, spastic, ataxic, extrapyramidal, or mixed dysarthria, or as not dysarthric. All observers were blinded to the patients' final diagnosis, which was based on all clinical features and investigations. In the analysis the observers were arranged in eight groups of nine observers, or four paired groups with similar levels of clinical experience. Together, the observers in a given group rated all 100 recordings. RESULTS: The accuracy of the classification was poor (35 % were classified correctly) and the inter-observer agreement between paired groups low (kappa 0.16 to 0.32). The level of experience in neurology did not have a significant influence. CONCLUSION: Neurological trainees as well as experienced neurologists have great difficulty in identifying specific types of dysarthria on the basis of perceptual analysis alone. In clinical practice this probably means that most neurologists will classify dysarthria in the context of other features from neurological examination or ancillary investigations.