Evoked potentials and transcranial magnetic stimulation in migraine: published data and viewpoint on their pathophysiologic significance

Evaluating the effects of spatial frequency on migraines by using pattern-reversal visual evoked potentials

OBJECTIVE

To clarify the effects of contrast and spatial frequency in patients with migraine by means of pattern-reversal visual evoked potentials (PVEPs).

METHODS

PVEPs were obtained from 14 patients who had migraine without aura (MO), 11 patients who had migraine with aura (MA), and 25 age-matched, healthy controls (CO). PVEPs were binocularly recorded with a reversal rate of 1Hz (2 reversal/s) at 3 spatial frequencies (0.5, 1.0 and 4.0 cpd) at high (98%), medium (83%) and low (29%) contrast. N75, P100 and N135 latency and the amplitudes of P50-N75, N75-P100 and P100-N135 were analyzed.

RESULTS

Increased amplitude of PVEPs in patients with migraines were revealed at 3 different spatial frequencies in all components. The MO and the MA showed increased amplitudes mostly in high contrasts (98%). These findings were detected more at a high spatial frequency (4.0 cpd) than at a low spatial frequency (0.5 cpd). Increased amplitude with prolonged latency of N135 were found both in MO and MA at 4.0 cpd.

CONCLUSIONS

We conclude that pattern stimuli of high contrasts may be particularly effective in uncovering abnormal cortical reactivity which may be modified in the primary and secondary visual cortex in the interictal state of migraine.

SIGNIFICANCE

These findings indicate that there is abnormal visual cortex processing in patients with migraine.

Nociceptive Blink Reflex and Visual Evoked Potential Habituations Are Correlated in Migraine

BACKGROUND

Lack of habituation, as reported in migraine patients between attacks for evoked cortical responses, was also recently found for the nociceptive blink reflex (nBR) mediated by brainstem neurons. It is not known if both brain stem and cortical habituation deficits are correlated in the same patient, which would favor a common underlying mechanism.

OBJECTIVE

To search for intraindividual correlations between habituation of pattern reversal-visual evoked potentials and that of the nociception-specific blink reflex in migraineurs and in healthy volunteers (HV).

METHODS

We recorded 15 HV and 15 migraine without aura patients between attacks. Habituation for visual evoked potentials was measured by comparing the N1-P1 amplitude change (%) between the first and sixth block of 100 sequential averaged responses. Habituation for the nBR was defined as the percentage change of the R2 response area between the 1st and 10th block of five averaged EMG responses, elicited by stimulating the right side every 2 minutes for 32 minutes. We also calculated the slope of N1-P1 amplitude and R2 response area changes from the first to the last response and the correlation with attack frequency.

RESULTS

A significant habituation deficit in both cortical and brain stem evoked activity characterized on average the group of migraineurs compared to controls. In migraine patients, but not in HV, we found a significant positive correlation between habituation of pattern reversal-visual evoked potentials and that of the nociception-specific blink reflex both for the degree of habituation between first and last blocks of averagings (r = 0.703; P = .003) and for the habituation slope (r = 0.751; P = .001). Moreover, nBR habituation was positively correlated with attack frequency (r = 0.548; P = .034).

CONCLUSION

The positive correlation between visual evoked potential and nBR habituations is consistent with the idea that in migraine the same neurobiological dysfunction might be responsible for the habituation deficit both in cortex and brain stem. As nBR habituation increases with attack frequency, its interictal deficit is unlikely to be due to trigeminal sensitization.

Multilevel somatosensory system disinhibition in children with migraine

Although migraine is characterised by an abnormal cortical excitability level, whether the central nervous system is hyper- or hypo-excitable in migraine still remains an unsolved problem. The aim of our study was to compare the somatosensory evoked potential (SEP) recovery cycle, a marker of the somatosensory system's excitability, in a group of 15 children suffering from migraine without aura (MO) (mean age 11.7+/-1.6 years, five males, 10 females) and 10 control age-matched subjects (CS) (mean age 10.9+/-2.1 years, six males, four females). We calculated the SEP's latency and amplitude modifications after paired electrical stimuli at 5, 20 and 40 ms interstimulus intervals (ISIs), comparing it with a single stimulus condition assumed as the baseline. In MO patients, the amplitudes of the cervical N13 and of the cortical N20, P24 and N30 responses at 20 and 40 ms ISIs showed a higher recovery than in CS (two-way ANOVA, P<0.05). Since, the SEP recovery cycle depends on the inhibitory interneuron function, our findings suggest that a somatosensory system disinhibition takes place in migraine. This is a generalized phenomenon, not limited to the cerebral cortex, but concerning also the cervical grey matter. The SEP recovery cycle reflects the intracellular concentration of Na(+), therefore, the shortened recovery cycle in our MO patients suggests a high level of intracellular Na(+) and a consequent depolarized resting membrane potential, possibly due to an impaired Na(+) -K(+) ATPase function in migraine.

Processing of global form and motion in migraineurs

Previous studies have identified anomalies of cortical visual processing in migraineurs that appear to extend beyond V1. Migraineurs respond differently than controls to transcranial magnetic stimulation of V5, and can demonstrate impairments of global motion processing. This study was designed to assess the integrity of intermediate stages of both motion and form processing in people with migraine. We measured the ability to integrate local orientation information into a global form percept, and to integrate local motion information into a global motion percept. Control subjects performed significantly better than migraineurs on both tasks, suggesting a diffuse visual cortical processing anomaly in migraine.

Visually evoked phase synchronization changes of alpha rhythm in migraine: Correlations with clinical features

OBJECTIVE

This study aimed to compute phase synchronization of the alpha band from a multichannel electroencephalogram (EEG) recorded under repetitive flash stimulation from migraine patients without aura. This allowed examination of ongoing EEG activity during visual stimulation in the pain-free phase of migraine.

METHODS

Flash stimuli at frequencies of 3, 6, 9, 12, 15, 18, 21, 24, and 27 Hz were delivered to 15 migraine patients without aura and 15 controls, with the EEG recorded from 18 scalp electrodes, referred to the linked earlobes. The EEG signals were filtered in the alpha (7.5-13 Hz) band. For all stimulus frequencies that we evaluated, the phase synchronization index was based on the Hilbert transformation.

RESULTS

Phase synchronization separated the patients and controls for the 9, 24 and 27 Hz stimulus frequencies; hyper phase synchronization was observed in patients, whereas healthy subjects were characterized by a reduced phase synchronization. These differences were found in all regions of the scalp.

CONCLUSIONS

During migraine, the brain synchronizes to the idling rhythm of the visual areas under certain photic stimulations; in normal subjects however, brain regions involved in the processing of sensory information demonstrate desynchronized activity. Hypersynchronization of the alpha rhythm may suggest a state of cortical hypoexcitability during the interictal phase of migraine.

SIGNIFICANCE

The employment of non-linear EEG analysis may identify subtle functional changes in the migraine brain.

Coupling between visual evoked cerebral blood flow velocity responses and visual evoked potentials in migraneurs

Neurovascular coupling may be altered in migraneurs. Therefore, visual evoked potentials (VEP) and visually evoked cerebral blood flow velocity responses (VEFR) were simultaneously recorded in 30 healthy controls and 30 migraneurs interictally using a checkerboard stimulus with visual contrasts of 1%, 10% and 100%. The VEFR were measured in the posterior cerebral artery using transcranial Doppler and VEP were recorded from occipital leads. We found an increase in VEFR and VEP in both the healthy and migraneur groups (P < 0.01). VEFR were significantly higher in migraneurs (P < 0.01), while VEP did not significantly differ between the groups (P > 0.05). Regression showed a significant association between VEP and VEFR in both healthy controls (r = 0.66, P < 0.01) and migraneurs (r = 0.63, P < 0.01). The regression coefficient of migraneurs (b = 0.88, SE = 0.08) was significantly higher than that of healthy controls (b = 0.55, SE = 0.07) (P = 0.04). We conclude that neurovascular coupling is increased in migraneurs interictally.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.