Interictal Abnormalities of Gamma Band Activity in Visual Evoked Responses in Migraine: An Indication of Thalamocortical Dysrhythmia?

Lack of cold pressor test-induced effect on visual-evoked potentials in migraine

In patients with migraine, the various sensory stimulation modalities, including visual stimuli, invariably fail to elicit the normal response habituation. Whether this lack of habituation depends on abnormal activity in the sub-cortical structures responsible for processing incoming information as well as nociception and antinociception or on abnormal cortical excitability per se remains debateable. To find out whether inducing tonic pain in the hand by cold pressure test (CPT) alters the lack of visual-evoked potential (VEP) habituation in migraineurs without aura studied between attacks we recorded VEPs in 19 healthy subjects and in 12 migraine patients during four experimental conditions: baseline; no-pain (hand held in warm water, 25°C); pain (hand held in cold water, 2–4°C); and after-effects. We measured P100 amplitudes from six blocks of 100 sweeps, and assessed habituation from amplitude changes between the six sequential blocks. In healthy subjects, the CPT decreased block 1 VEP amplitude and abolished the normal VEP habituation (amplitude decrease to repeated stimulation) in patients with migraine studied between attacks; it left block 1 VEP amplitude and abnormal VEP habituation unchanged. These findings suggest that the interictal cortical dysfunction induced by migraine prevents the cortical changes induced by tonic painful stimulation both during pain and after pain ends. Because such cortical changes presumably reflect plasticity mechanisms in the stimulated cortex, our study suggests altered plasticity of sensory cortices in migraine. Whether this abnormality reflects abnormal functional activity in the subcortical structures subserving tonic pain activation remains conjectural.

A Note on the Digiti Quinti Sign in Hemiplegic Migraine

The digiti quinti sign (DQS) was described as an indication of mild hemiparesis. It consists of a wider space between the fourth and fifth fingers at the affected side when the patient extends both arms horizontally to the front with the palms down. The three successively examined sporadic hemiplegic migraine patients presented herein disclosed no other neurological abnormality except the interictally present DQS on the same side of the motor deficits. This sign is perhaps clinically useful for the diagnosis of hemiplegic migraine.

Cortical inhibition and habituation to evoked potentials: relevance for pathophysiology of migraine

Dysfunction of neuronal cortical excitability has been supposed to play an important role in etiopathogenesis of migraine. Neurophysiological techniques like evoked potentials (EP) and in the last years non-invasive brain stimulation techniques like transcranial magnetic stimulation (TMS) and transcranial direct current stimulation gave important contribution to understanding of such issue highlighting possible mechanisms of cortical dysfunctions in migraine. EP studies showed impaired habituation to repeated sensorial stimulation and this abnormality was confirmed across all sensorial modalities, making defective habituation a neurophysiological hallmark of the disease. TMS was employed to test more directly cortical excitability in visual cortex and then also in motor cortex. Contradictory results have been reported pointing towards hyperexcitability or on the contrary to reduced preactivation of sensory cortex in migraine. Other experimental evidence speaks in favour of impairment of inhibitory circuits and analogies have been proposed between migraine and conditions of sensory deafferentation in which down-regulation of GABA circuits is considered the more relevant pathophysiological mechanism. Whatever the mechanism involved, it has been found that repeated sessions of high-frequency rTMS trains that have been shown to up-regulate inhibitory circuits could persistently normalize habituation in migraine. This could give interesting insight into pathophysiology establishing a link between cortical inhibition and habituation and opening also new treatment strategies in migraine.

Serotonin and migraine: a reconsideration of the central theory

The 5-hydroxytryptamine (5-HT) has been implicated in migraine pathophysiology for the past 50 years. A low central 5-HT disposition associated with an increase in 5-HT release during attack is the most convincing change of 5-HT metabolism implicated in migraine. Peripheral studies on plasma/platelet have not generally shown low 5-HT levels. Studies on 5-HT reactivity showed hypersensitivity, also expressed as reduced tachyphylaxis (habituation), which successively was evidenced as the most characteristic marker of an altered sensory neurotransmission. Even the gender and seasonal variations of 5-HT parameters seem to agree with a low 5-HT turnover with receptoral hypersensitivity. The interpretation of the effects of some serotonergic drugs and recent neuroimaging studies give major evidence for this cascade of events. Although the exact mechanism that links abnormal 5-HT neurotransmission to the manifestation of head pain has yet to be fully understood, a deficit on 5-HT descending pain inhibitory system is still probably today the most implicated in migraine pathophysiology. This short review focuses and discusses the alteration of peripheral and central 5-HT parameters in migraine patients.

Is the cerebral cortex hyperexcitable or hyperresponsive in migraine?

Although migraineurs appear in general to be hypersensitive to external stimuli, they maybe also have increased daytime sleepiness and complain of fatigue. Neurophysiological studies between attacks have shown that for a number of different sensory modalities the migrainous brain is characterised by a lack of habituation of evoked responses. Whether this is due to increased cortical hyperexcitability, possibly due to decreased inhibition, or to an abnormal responsivity of the cortex due a decreased preactivation level remains disputed. Studies using transcranial magnetic stimulation in particular have yielded contradictory results. We will review here the available data on cortical excitability obtained with different methodological approaches in patients over the migraine cycle. We will show that these data congruently indicate that the sensory cortices of migraineurs react excessively to repetitive, but not to single, stimuli and that the controversy above hyper- versus hypo-excitability is merely a semantic misunderstanding. Describing the migrainous brain as 'hyperresponsive' would fit most of the available data. Deciphering the precise cellular and molecular underpinnings of this hyperresponsivity remains a challenge for future research. We propose, as a working hypothesis, that a thalamo-cortical dysrhythmia might be the culprit.