Early angiographic changes of intra-aneurysmal flow after flow-diverter stent treatment are not predictive of therapeutic success.

BACKGROUND AND PURPOSE: Flow-diverter stents (FDS) are new devices for the endovascular treatment of intracranial aneurysms (IAs) promoting progressive aneurysmal thrombosis. To date, the delay of aneurysmal exclusion remains unclear. We evaluated the correlation between angiographic changes in the first 24 hours and 12-month occlusion in aneurysms treated with FDS. METHODS: We retrospectively analyzed the intra-aneurysmal flow by evaluating the in-flow and out-flow delays on preoperative, immediate postoperative, 24-hour and 12-month follow-up angiography. Dichotomy of in-flow and out-flow within the aneurysm was considered as the time of contrast filling and time of contrast washing relatively to the parent artery. The delay times were compared and correlated with the therapeutic success of FDS at 12 months of follow-up. RESULTS: Out of 14 treated IAs, in 13 consecutive patients, n = 10 (71%) aneurysms showed complete occlusion at 12 months. Between immediate postoperative and 24-hour control, 10 aneurysms (71%) demonstrated in-flow modification, with eight increasing, two decreasing and four having no change. There were no statistical differences in therapeutic success in relation to the different flow-related profiles of intra-aneurysmal flux.Out-flow modifications were found in 11 aneurysms (79%) between immediate postoperative and 24-hour control, with five increasing, six decreasing and three having no change. Similar to the in-flow changes, there were no statistical differences in therapeutic success relative to the flow-related profiles. CONCLUSIONS: Early angiographic changes after FDS placement are very frequent, but are not correlated with the 12-month technical success of flow-diversion techniques.

A Re-Examination of Hebbian-Covariance Rules and Spike Timing-Dependent Plasticity in Cat Visual Cortex in vivo.

Spike timing-dependent plasticity (STDP) is considered as an ubiquitous rule for associative plasticity in cortical networks in vitro. However, limited supporting evidence for its functional role has been provided in vivo. In particular, there are very few studies demonstrating the co-occurrence of synaptic efficiency changes and alteration of sensory responses in adult cortex during Hebbian or STDP protocols. We addressed this issue by reviewing and comparing the functional effects of two types of cellular conditioning in cat visual cortex. The first one, referred to as the "covariance" protocol, obeys a generalized Hebbian framework, by imposing, for different stimuli, supervised positive and negative changes in covariance between postsynaptic and presynaptic activity rates. The second protocol, based on intracellular recordings, replicated in vivo variants of the theta-burst paradigm (TBS), proven successful in inducing long-term potentiation in vitro. Since it was shown to impose a precise correlation delay between the electrically activated thalamic input and the TBS-induced postsynaptic spike, this protocol can be seen as a probe of causal ("pre-before-post") STDP. By choosing a thalamic region where the visual field representation was in retinotopic overlap with the intracellularly recorded cortical receptive field as the afferent site for supervised electrical stimulation, this protocol allowed to look for possible correlates between STDP and functional reorganization of the conditioned cortical receptive field. The rate-based "covariance protocol" induced significant and large amplitude changes in receptive field properties, in both kitten and adult V1 cortex. The TBS STDP-like protocol produced in the adult significant changes in the synaptic gain of the electrically activated thalamic pathway, but the statistical significance of the functional correlates was detectable mostly at the population level. Comparison of our observations with the literature leads us to re-examine the experimental status of spike timing-dependent potentiation in adult cortex. We propose the existence of a correlation-based threshold in vivo, limiting the expression of STDP-induced changes outside the critical period, and which accounts for the stability of synaptic weights during sensory cortical processing in the absence of attention or reward-gated supervision.