Neurological

Functional inhibition of the occipital nerve likely occurs within the trigeminocervical complex

Blocking the greater occipital nerve (GON) with lidocaine can help treat various headache and facial pain syndromes, and according to study results published in Migraine, nociceptive activity is likely to occur primarily at the C2 level of the spine.

Previous studies have confirmed the benefits of pharmacological blockade of the GON in patients with cluster headache and trigeminal neuralgia, and this treatment strategy may also be effective for migraines or drug overuse headaches.

The aim of the current study was to determine the exact location of the interaction between the occipital and trigeminal nerves and the mechanisms of this interaction.

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The double-blind, placebo-controlled, randomized study enrolled 25 healthy volunteers who were invited to quantitative sensory tests using trigeminal thermal stimuli and stimulus-induced changes in neuronal activity in the brain stem measured by functional MRI techniques.

After completing the Beck Depression Inventory and the German version of the Patient Health Questionnaire for the determination of depression, anxiety and other psychiatric illnesses, mechanical detection thresholds as well as heat and cold detection and pain thresholds were evaluated as a starting value.

At the second visit, sensory tests were performed to assess the detection and pain thresholds on each trigeminal branch on either side. Participants were randomized to receive either lidocaine or placebo on the left GON with a second sensory test after 10 minutes and an MRI scan 30 minutes after the intervention.

At the start of the study, there were no differences between the groups in the mechanical detection thresholds. There were no differences in the intensity and discomfort scores of nasal stimulus between GON block with lidocaine and placebo treatment.

The pharmacological GON block with lidocaine had no influence on mechanical detection, thermal detection or pain threshold in healthy volunteers. In the subjective assessment of numbness and anesthesia in the ipsilateral occipital area, the general anesthesia rates were significantly higher in the lidocaine group compared to the placebo group (92% vs. 0% in each case).

Data from functional MRI showed that left-sided injection of lidocaine was associated with reduced nociceptive neuronal activity in the contralateral trigeminocervical complex, while placebo had no significant effect on nociceptive trigeminal activation.

Based on these results, the researchers speculated that the point of interaction between the trigeminal and occipital entrances within the trigeminocervical complex occurs at the level of C2, where the nociceptive signals converge on the same nuclei, cross to the contralateral side, and from there connect to higher brain centers .

One of the limitations of the study was the inability to assess activation at the C2 level due to technical limitations.

“Application of the lidocaine GON block will likely reduce neural input from the occipital nerve and affect the summary of neural activity that is transmitted to the higher processing centers without affecting the peripheral trigeminal nerve as such,” the researchers concluded.

reference

Hoffmann J, Mehnert J, Koo EM, May A. The blockade of the greater occipital nerve modulates nociceptive signals within the trigeminocervical complex. J Neurol Neurosurg Psychiatry. Published online July 26, 2021. doi: 10.1136 / jnnp-2021-326433.

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