Transcranial magnetic stimulation confirms hyperexcitability of occipital cortex in migraine

We hypothesized that the hyperexcitability of occipital cortex neurons may predispose migraine subjects to develop spreading depression, the putative basis of migraine with aura (MwA). To date there is no direct physiologic correlate confirming this in patients. Accordingly, we evaluated the differences in the threshold of occipital cortex excitation between MwA patients and normal controls (C) using transcranial magnetic stimulation (TMS).

TMS was performed using the Cadwell MES 10 stimulator. A circular coil 9.5 cm in diameter was applied to the occipital scalp (7 cm above the inion). Stimulator intensity was increased in 10% increments until subjects reported visual phenomena or 100% intensity was reached. Stimulation intensity was then fine-tuned to determine the threshold at which phosphenes were just visualized.

Eleven MwA patients, mean age 37 +/- 7 years, were compared with 11 C, mean age 37.7 +/- 7 years. The difference in the proportion of subjects with phosphene generation between MwA patients and C was significant (MwA patients 100% versus C 27.3%, p = 0.001). The mean threshold level for MwA patients was 44.2 +/- 8.6 versus 68.7 +/- 3.1 for C (p = 0.0001). All threshold levels for MwA patients were lower than the lowest threshold for C; the MwA patient with the lowest threshold had an aura after stimulation.

The threshold for excitability of occipital cortex is lower in MwA patients compared with C. This is a direct neurophysiologic correlate for clinical observations that have indicated hyperexcitability of the occipital cortex in migraineurs.