Migraine and the Brainstem: The Role of the Brainstem in Attack Generation

The brainstem is one of the oldest and most fundamental structures in the human nervous system. Located at the base of the brain, connecting the cerebral hemispheres to the spinal cord, it regulates vital functions including breathing, heart rate, blood pressure, sleep, and wakefulness. It also contains the principal relay stations for pain processing and is the site of origin of several cranial nerves including the trigeminal nerve whose activation generates migraine pain. The brainstem's role in migraine goes beyond simply being the location of the trigeminal nucleus. It is increasingly understood to be an active participant in the generation and modulation of migraine attacks.

The Brainstem as a Migraine Generator

The concept of a migraine generator, a brain region whose activity initiates attacks, has been associated with the brainstem since early neuroimaging studies in the 1990s. A landmark positron emission tomography study by Weiller and colleagues published in 1995 identified increased activity in the dorsal rostral pons, a region of the upper brainstem, during spontaneous migraine attacks. Crucially, this brainstem activation persisted after the headache was successfully treated with sumatriptan, suggesting that it was not merely a response to pain but an independent feature of the attack state.

The specific brainstem region identified in these studies, the dorsal raphe nucleus and locus coeruleus, contains the primary cell bodies of the serotonergic and noradrenergic systems that project widely throughout the brain and modulate pain sensitivity, mood, sleep, and arousal. Dysregulation of these systems during a migraine attack could lower the threshold for trigeminal activation, increase sensitivity to sensory stimuli, and alter the emotional and cognitive state that characterizes the migraine experience.

The Trigeminal Nucleus Caudalis

The trigeminal nucleus caudalis is a structure in the lower brainstem that serves as the primary relay station for pain signals from the trigeminal nerve. Pain impulses traveling along trigeminal nerve fibers from the meninges and cerebral blood vessels synapse in the trigeminal nucleus caudalis before being relayed to the thalamus and cortex where they are consciously perceived as headache pain.

The trigeminal nucleus caudalis also receives descending input from higher brain regions including the hypothalamus, periaqueductal gray, and rostral ventromedial medulla. These descending pathways can either inhibit or facilitate pain transmission at the level of the trigeminal nucleus caudalis, providing a mechanism by which brain states, stress, and mood influence pain sensitivity and migraine vulnerability.

Sensitization of the trigeminal nucleus caudalis during migraine attacks, a process called central sensitization, amplifies pain signals and lowers the threshold for subsequent trigeminal activation. This central sensitization is mediated in part by neuroinflammatory processes occurring in the brainstem itself.

Brainstem Modulation of Pain

The periaqueductal gray matter, a region of the midbrain that is anatomically part of the brainstem, is a critical node in the descending pain modulation system. It integrates information from higher cortical and limbic regions and projects to the rostral ventromedial medulla, which in turn modulates trigeminal pain transmission in the nucleus caudalis.

Dysfunction in the descending pain modulation system has been proposed as a contributing factor in migraine. If the inhibitory descending pathways are impaired, trigeminal pain signals may be amplified rather than suppressed, increasing the likelihood and severity of migraine attacks. Studies of periaqueductal gray function in migraine patients have identified structural and functional changes consistent with this hypothesis.

Brainstem Symptoms During Migraine

Certain migraine subtypes produce symptoms that directly reflect brainstem involvement. Migraine with brainstem aura, formerly called basilar-type migraine, is characterized by aura symptoms that originate from the brainstem including dysarthria, vertigo, tinnitus, hypacusis, diplopia, ataxia, and decreased level of consciousness. These symptoms reflect transient disruption of brainstem function during the aura phase and are distinct from the cortical aura symptoms that characterize common migraine with aura.

Vertigo and dizziness are common interictal symptoms in people with migraine and may reflect baseline abnormalities in brainstem vestibular processing that are exacerbated during attacks. Vestibular migraine, a condition characterized by episodes of vertigo with migraine features, is now recognized as a distinct migraine subtype whose pathophysiology involves brainstem vestibular nuclei.

Implications for Treatment

The brainstem's role in migraine has several treatment implications. The serotonergic systems originating in the brainstem dorsal raphe nucleus are targeted by triptans, which are serotonin receptor agonists. The effectiveness of triptans in migraine reflects in part their modulation of brainstem serotonergic activity in addition to their peripheral effects on trigeminal nerve terminals and meningeal blood vessels.

Preventive medications including tricyclic antidepressants and serotonin-norepinephrine reuptake inhibitors influence brainstem monoaminergic systems and may reduce migraine frequency through their effects on descending pain modulation and trigeminal sensitization.