Visual disturbances and migraine

The visual system as target of non-invasive brain stimulation for migraine treatment: Current insights and future challenges

The visual network is crucially implicated in the pathophysiology of migraine. Several lines of evidence indicate that migraine is characterized by an altered visual cortex excitability both during and between attacks. Visual symptoms, the most common clinical manifestation of migraine aura, are likely the result of cortical spreading depression originating from the extrastriate area V3A. Photophobia, a clinical hallmark of migraine, is linked to an abnormal sensory processing of the thalamus which is converged with the non-image forming visual pathway. Finally, visual snow is an increasingly recognized persistent visual phenomenon in migraine, possibly caused by increased perception of subthreshold visual stimuli. Emerging research in non-invasive brain stimulation (NIBS) has vastly developed into a diversity of areas with promising potential. One of its clinical applications is the single-pulse transcranial magnetic stimulation (sTMS) applied over the occipital cortex which has been approved for treating migraine with aura, albeit limited evidence. Studies have also investigated other NIBS techniques, such as repetitive TMS (rTMS) and transcranial direct current stimulation (tDCS), for migraine prophylaxis but with conflicting results. As a dynamic brain disorder with widespread pathophysiology, targeting migraine with NIBS is challenging. Furthermore, unlike the motor cortex, evidence suggests that the visual cortex may be less plastic. Controversy exists as to whether the same fundamental principles of NIBS, based mainly on findings in the motor cortex, can be applied to the visual cortex. This review aims to explore existing literature surrounding NIBS studies on the visual system of migraine. We will first provide an overview highlighting the direct implication of the visual network in migraine. Next, we will focus on the rationale behind using NIBS for migraine treatment, including its effects on the visual cortex, and the shortcomings of currently available evidence. Finally, we propose a broader perspective of how novel approaches, the concept of brain networks and the integration of multimodal imaging with computational modeling, can help refine current NIBS methods, with the ultimate goal of optimizing a more individualized treatment for migraine.

Arguments against the role of cortical spreading depression in migraine

Cortical spreading depression (CSD) is a wave of increased electrocortical activity and vasodilation, followed by sustained decreased activity and prolonged vasoconstriction. Although the discovery of CSD has been ascribed to Leão, rather than vasoconstriction, he only observed a depression of neural activity combined with vasodilation, with much weaker stimulation than used by his followers. There is a longstanding belief that CSD underlies migraine aura, with its positive symptoms such as mosaic patterns and its negative symptoms such as scotoma, and a similar propagation speed and vasoreaction pattern. However, there are many arguments against this theory. CSD is difficult to evoke in man, and electroencephalography (EEG) readings are not flattened during migraine (as opposed to EEG during CSD). Moreover, in contrast to CSD, migraine can occur bilaterally, and is not accompanied by a disrupted blood-brain barrier, increased cerebral metabolism, or cerebral cell swelling. Calcitonin gene-related peptide, which is thought to be characteristic of migraine pain, is increased in the blood from the external jugular vein during migraine in humans, but not during CSD in cats or rats. Moreover, CSD does not explain the appearance of premonitory symptoms or allodynia, long before the actual onset of aura. In addition, there is a variation in the pain mechanisms of migraine and CSD, and in their reaction to transcranial magnetic stimulation and several pharmacologic interventions. Finally, the origin of putative CSD in migraine is currently unknown.

Clinical impact of migraine for the management of glaucoma patients

Migraine is a common and debilitating primary headache disorder that affects 10-15% of the general population, particularly people of working age. Migraine is relevant to providers of clinical eye-care because migraine attacks are associated with a range of visual sensory symptoms, and because of growing evidence that the results of standard tests of visual function necessary for the diagnosis and monitoring of glaucoma (visual fields, electrophysiology, ocular imaging) can be abnormal due to migraine. These abnormalities are measureable in-between migraine events (the interictal period), despite patients being asymptomatic and otherwise healthy. This picture is further complicated by epidemiological data that suggests an increased prevalence of migraine in patients with glaucoma, particularly in patients with normal tension glaucoma. We discuss how migraine, as a co-morbidity, can confound the results and interpretation of clinical tests that form part of contemporary glaucoma evaluation, and provide practical evidence-based recommendations for the clinical testing and management of patients with migraine who attend eye-care settings.

Migrainous scintillating scotoma and headache is ocular in origin: A new hypothesis

Brain neuronal dysfunction has been implicated in pathogenesis of migraine but direct evidence is lacking. Scintillating scotoma of migraine is generally believed to originate at the visual cortex. While cortical spreading depression is a relatively late physiological alteration in migraine, its protective role in neuronal ischaemia is increasingly being recognized. Atenolol, nadolol, or verapamil prevent migraine but do not readily cross the blood-brain barrier or critically influence any brain or peripheral neuronal function. Typical migraine headache, aura, or scintillating scotoma has not been reported following enucleation or evisceration of the eye. In humans, pain and temperature fibres from only the ophthalmic division of the trigeminal nerve reach the upper cervical spinal segments. Pain in migraine attacks including occipital and nuchal discomfort reflects selective involvement of the ophthalmic nerve. Photophobia is largely a retinal reflex involving the ophthalmic division of the trigeminal nerve. Key clinical features of the migrainous scintillating scotoma are consistent with retinal origin. Spreading depression in the retina is well-established. A subtle regional ocular sympathetic deficit prevails in migraine patients and possibly impairs regulation of intraocular choroidal blood volume and intraocular pressure. Several first-line migraine prophylactic agents lower the intraocular pressure. The neuro-ophthalmological basis for a monocular origin of migrainous scintillating scotomata due to mechanical deformation of the posterior segment of the corneo-scleral envelope consequent to choroidal venous congestion and rise in intraocular pressure is presented. Study of distribution and displaceability of the migrainous scintillating scotoma can settle its site of origin. Headache of migraine possibly arises from a similar mechanical deformation of the anterior eye segment followed by antidromic discharge in the trigeminovascular system. Lateralizing negative deficits such as homonymous hemianopia probably reflect vasospastic complications of migraine. A rational explanation for the most characteristic clinical features of migraine and a new template to elucidate the pharmacological basis of anti-migraine drugs is offered.