2.8 Autonomic Effects of Trigeminal Nerve Injury
By Jeffrey A. Brown, MD, FACS, FAANS (USA)
Editor’s Note: When you have trigeminal nerve pain (TNP), do you sometimes have a red, runny, swollen, teary, or droopy eye on the same side as your pain? How about a stuffy or runny nose, or fullness in your ear? These are some autonomic corollaries or consequences of TNP. Although not everyone experiences these autonomic effects, it is important to know what they are and why they occur. In this chapter, Dr. Brown talks about the autonomic corollaries of TNP and why they occur.
In 1908, a physiologist first described something called the “oculocardiac reflex.” Push on your eyeball and your heart rate drops. As it turns out, this is the first understanding of the role of the trigeminal system in the control of those elements of the body that we cannot consciously control. The eyeball is innervated by the trigeminal nerve. Irritate it, and things happen in your body, not just in your face. Irritate it enough, and your heart can even (briefly) stop beating.
This is an autonomic function, where autonomic refers to the involuntary functions of the nervous system, the functions that one cannot consciously control. It relates to the working elements of the internal organs of the body, the muscles of the heart, blood vessels, stomach, and intestines and the functions of the lungs and the sweat and salivary glands. The autonomic nervous system also aides in digestion, relaxation, and even “instinctual” emergency responses to injury or potential injury.
Any form of injury, even minor irritation, to the trigeminal peripheral branches in the face will have some autonomic consequence because the wiring of the trigeminal nerve is intimately intertwined with the autonomic nervous system within the brain stem. It is the brain stem that runs the unconscious activity of the body. Think of it as the body’s operating system, analogous to that of your iPhone or computer. If it gets reprogrammed, your phone can work better, but if it is damaged, things can literally go haywire. (Haywire is the thin, too-flexible wire used to hold bales [or bundles] of hay in the fields. Left alone, the wires tend to get intolerably tangled.)
One principle: The more intense the injury to the trigeminal system, the more widespread the effects, even down to the control of your digestive system. Why? The trigeminal nerve nucleus in the brain stem has connections to other nuclei that connect to the vagus nerve. The vagus nerve is appropriately named because it has “vague” or diffuse (autonomic) effects throughout the body. These interconnections go through another nucleus called the nucleus solitarius. A nucleus is the energy center of each nerve and the cable that emerges and enters into it. The output of the vagus nerve can be parasympathetic or sympathetic. One excellent way of understanding the difference is to use the rhyming phrases “rest and digest” and “feed and breed” to describe the parasympathetic plethora of functions. Some examples of parasympathetic functions include the following: The heart rate is slowed, and airway muscles in the lungs are tightened, reducing the amount of work the lungs must do. Energy is diverted to the bowels to help with digestion. The parasympathetic system puts the body to rest.
In contrast, the sympathetic system puts your body on alert. When activated, it enlarges the pupils in your eyes so you can see better. It diverts energy away from the digestive system to provide needed muscle energy to run away, along with increasing the heart rate to provide more oxygen to those working muscles, and even activating energy stores in the liver.
Together, the systems keep the body in balance. It is a complex computer system, indeed.
The problem is that, with trigeminal injury, the response can be a mixture of the two types of responses. Depending on the severity of the induced injury, it can also be inhibitory, instead of stimulating, to the parasympathetic system.
The point remains, however, that injury to the trigeminal nerve can be either at the peripheral level (in the face) or at the ganglion level (inside the skull and closer to the brain stem), at the nerve root level (between the ganglion and the brain stem) or at the brain-stem level. The closer one gets to the brain stem, the greater the consequences of injury.
Think of the brain and the brain stem as a complex electrical system. Electrons in the wiring can be moved quickly (high frequency, high voltage, high current) or more slowly (low frequency, low voltage, low current). Voltage refers to the power or strength of the pushing force delivered to the electrons. Frequency refers to how often those pushes are delivered. Current refers to the consequence of those two forces working together, leading to the quantity (volume) of electrons running through a wire in a given time period. By varying any one of these forces, if electrical energy is delivered to the brain stem, there can be different effects on the body.
I first began to research this topic when studying the effects of balloon compression on the trigeminal nerve while trying to understand how the operation to treat TN really worked. One important effect I observed was that, when the nerve root is compressed during the operation by the balloon, the heart rate will briefly slow. Blood pressure will drop, and then there will be a brief rebound rapid heart rate and rise in blood pressure. Why? Research in an animal model shows that low-frequency stimulation (at less than 50 cycles/second) of the lower portions of the trigeminal complex in the brain stem and spinal cord leads to a decrease in heart rate, a drop in blood pressure, a reduced breathing rate, and an increase in the motility of the gastric digestive system. This appears to happen because the sympathetic system is inhibited rather than the parasympathetic system being activated. The sympathetic nerves are activated by epinephrine. The parasympathetic nerves are activated by a different chemical, acetylcholine.
These responses differ from those seen with other reflexes such as the oculocardiac reflex; the diving reflex, which is a consequence of submersion of the face in cold water; or the nasopharyngeal reflex that results from noxious irritation of the mucosa of the nose.
All these responses are complex mixtures of chemical outpourings from things that happen to the face, that is, the trigeminal nervous system. What happens if you squeeze the trigeminal nerve at its root, as happens with balloon compression rhizotomy? The nerve gives one big “ouch,” and out pours a surge of electrical discharge that yields a mixture of cholinergic and anticholinergic mediated effects. What happens in the body then depends on the unpredictable mix of stimulating and inhibiting nerve channels that are turned on.
Disease can do this as well. One way it does so is by causing inflammation, or the release of irritating chemicals. The inflammatory response is the body’s effort to bring more blood flow to injured tissue to jump-start the body’s reparative mechanisms. In the short term, it is helpful. If it persists, it is not.
What can we take away from this discussion?
There is, for example, a disease entity called SUNCT, which is an acronym for short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing. All of these descriptive parts of the name are signs of trigeminal nerve irritation. Is this really a different entity, or is it another in the confusing plethora of manifestations of trigeminal nerve injury? From the preceding discussion, one could understand it to be the latter. TN is that, too.
TN is better understood, in my opinion, as a single form of TNP, meaning trigeminal nerve injury. It can manifest itself with brief bursts of short circuits, stabbing pain; or it can be felt by constant pain, often burning; or it can be a mixture of the two. Usually intermixed with the neuropathic (electrical, nerve injury) pain, there can be a residual aching pain. Aching pain is not nerve pain. It is what is called nociceptive pain. This is the type of pain transmitted to the brain by nerve endings in response to injury of the body. The pain felt in the face from neuralgia attacks, through another pathway in the that nucleus solitarius, can evoke this different form of pain as a secondary response.
In summary, the trigeminal nerve is intimately intertwined with the autonomic system of the body, and as such, any disease or injury to the nerve will cause an autonomic response that can be something so seemingly innocuous as a red face or possibly something as devastating as a full cardiac arrest. The difference is in the severity.
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