V Busch, W Jakob, T Juergens, W Schulte-Mattler, H Kaube & A May (2006). “Functional connectivity between trigeminal and occipital nerves revealed by occipital nerve blockade and nociceptive blink reflexes” Cephalalgia 26:50-55 (click author to see paper)
This is the second in a series of four papers exploring the question—how can occipital nerve block possibly help cluster headache, since the occipital nerve is at the back of the head, and cluster attacks are at the front of the head?
AUTHORS’ ABSTRACT: Headache syndromes often suggest occipital and neck involvement, although it is still unknown to what extent branches of segment C1-C3 contribute actively to primary headache. Pain within the occipital area may be referred to the trigeminal territory. However, a modulation of trigeminal transmission by affecting cervical input in humans has not been elucidated so far. A convergence of cervical and trigeminal input at the level of the caudal part of the trigeminal nucleus in the brainstem has been suggested due to anatomical and neurophysiological studies in animals. We examined the R2 components of the nociceptive blink reflex responses in 15 healthy subjects before and after unilateral nerve blockade of the greater occipital nerve with 5ml prilocain (1%). R2 response areas (AUC) decreased and the R2 latencies increased significantly after the nerve blockade only on the side of injection. AUC and latencies on the non-injection side remained stable. Thresholds for sensory or pain perception did not differ significantly between the repeated measurements on both sides. Our findings extend previous results related to anatomical and functional convergence of trigeminal and cervical afferent pathways in animals and suggest that the modulation of this pathway is of potential benefit in primary headache disorders.
Dr. Sewell’s comment:
Many people have noticed a connection between cluster attack pain, which is generally around the eye and in the temple, and neck pain. The same connection holds true of migraine, so it is odd that neck pain isn’t part of the diagnostic criteria of any headache syndrome. One Norwegian study in 2002 noted that 75% of tractor drivers get a headache, but only when they spend all day driving with their heads turned or looking over their shoulders. And yet, it is well-known to doctors that the eye-bone simply isn’t connected to the neck-bone. So what’s the connection?
In our last installment, we discussed a rat paper in which the investigators showed that there were some neurons in the neck that received input from both the trigeminal nerve (which carries pain from the front of the head) and the occipital nerve (which carries pain from the back of the head and the neck), and also that stimulating the occipital nerve made those neurons more sensitive to trigeminal pain. In other words, stimulating the back of the rat’s head made the front of the rat’s head more painful. In today’s paper, a different set of investigators in Hamburg decide to kick it up a notch by seeing if the same holds true of humans as well.
But how do they do this? Unfortunately, rat people have all the advantages over us human investigators. We can’t simply saw the side off someone’s head, poke around and see what’s going on, nor can we drive electric probes into the spinal cord in people’s necks. No, we have to be clever instead.
This particular bit of German cleverness involves the “blink reflex”. This reflex is what’s occurring when you blink both eyes when something touches either cornea, or you see something rapidly approaching your eye. It can also happen with a bright flash of light, or a loud sound. Like any reflex, only two neurons are involved—the first is the trigeminal nerve, which carries sensation from the face down to the medulla in the brainstem, where it synapses with both facial nerves, which carry the command back up to the eyelids to blink. Elapsed time: One tenth of a second.
By measuring brainstem responses using electrodes, researchers have been able to break the blink reflex response into three “wiggles”, somewhat creatively named “R1”, “R2”, and “R3”. The first synapse, where the trigeminal nerve hits the pons in the brainstem, appears to generate R1 (at the ten-millisecond point). The other two, R2 and R3, occur on both sides, at 35 milliseconds and 80 milliseconds, and seem to come a little later in the medulla, once the electrical signal travels down there. Now, what’s interesting about R2 is that it seems to be generated by pain only, not touch. For R1 and R3, touch is sufficient. By measuring R2, we can tell if a stimulus is painful or not, without having to ask!
So here’s what Dr. Busch and his team did: They took 15 healthy volunteers, and attached electrodes just above both of everyone’s eyebrows. They gave electrical shocks of steadily increasing intensity, making everyone blink, and measured the R1, R2, and R3 components of the blink reflex. Then they numbed everyone’s occipital nerve on one side. You can find it yourself—put one finger on the bony bit sticking out at the back of your head, and another on the bony bit sticking out just behind and below your ear. Feel along the ridge; halfway between the two points, you should find your occipital nerve. It will feel like a “pressure point”, because it is. They then repeat the shocks, and measured R1, R2, and R3 again.
What they found was interesting. Everyone felt pain, of course; everyone blinked. But the R2 was now delayed by five milliseconds—it occurred at 40 milliseconds when the occipital nerve was numbed, as opposed to 35 milliseconds beforehand. The “R2-response area” was also decreased, from over 6000 µVms to below 3500 µVms on both sides. I don’t really know what that means, and the authors don’t explain, but by gum, there was a change!
So what does it mean? It means that it looks as if the eye-bone is, in fact, connected to the neck bone. Somehow, numbing the neck caused a measurable difference in the blink reflex. Just as stimulating the neck in rats made the neurons responding to the face more sensitive, it seems that numbing the neck and back of the head in humans is somehow making the neurons responding to the face less sensitive. The authors conclude, “the occipital nerve probably has an excitatory influence on trigeminal circuits, that can be reduced by selective inhibition of that nerve due to an anaesthetic blockade.” If they’re right, it could explain why occipital nerve blocks help cluster headache, and also why neck injuries can cause them.