Speaker - Dr. Lan

Yue Lan
Director of Rehabilitation Medicine Department of Guangzhou First People's Hospital

Live Broadcast Excerpts

Good evening, fellow experts and comrades. I’m very glad to receive the invitation, and tonight I will share with you some of the applications of non-invasive neuromodulation technology, mainly transcranial magnetic stimulation, in swallowing function research and rehabilitation. I will introduce you from the following four aspects. The first one is to introduce the neural regulation mechanism of dysphagia after stroke; then I will share with you the two research projects of the National Natural Science Foundation of China in 2015 and 2017. Some achievements in this area; Finally, I would like to share with you some of the more successful rehabilitation cases we have done.

01 Swallowing hemispheric dominance

Swallowing is composed of two-level centers, the first-level center is the cortical and subcortical structures, and the second-level center is the brainstem center. The current mainstream view is that the cortical and subcortical structures can mainly control voluntary swallowing, that is, the cognitive, preparation and oral stages in our swallowing stages, which can be started and stopped at will; the brainstem center is our secondary center, which is a relatively low center. It mainly controls our reflex swallowing, mainly our pharyngeal and esophageal stages. Once the swallowing reflex phase is initiated, it is irreversible and can only proceed downwards.

The latest research results show that this reflexive swallowing can also be partially regulated in the cortex and cortical center, which is a theoretical basis for our rehabilitation treatment for swallowing patients.

The current mainstream view is that swallowing activity is dominated by both cerebral hemispheres, but it does not have an absolute dominant hemisphere like the language center or is completely dominated by the opposite side like the motor center. 

Although swallowing is dominated by both cerebral hemispheres, there are also cases of unilateral dominance, that is, one hemisphere plays a dominant role in swallowing, and the other side plays a supporting role. It may be that one hemisphere dominates 70 percent and the other hemisphere dominates 30 percent.

For example, when we use TMS assessment, the motor evoked potential MEP evoked by one cerebral hemisphere is greater than that of the other side; when using fMRI detection, we can also find that the cerebral cortex activation degree of one hemisphere is greater than that of the other side. These are all manifestations of swallowing hemispheric dominance.

First, in a 2005 study of 20 healthy subjects, when TMS was used to measure suprhyoid muscles, it was found that 55 percent of the subjects had the left dominant hemisphere, 40 percent had the right dominant hemisphere, and 5 percent had no significant dominant hemisphere; When this pharyngeal muscle group was used as the target, it was found that the left dominant hemisphere accounted for about 85 percent, the right dominant hemisphere accounted for 15 percent, and no significant dominant hemisphere accounted for 0 percent.

In addition, in an article published in 2004 using fMRI to study swallowing activity, if different brain regions are used as target regions for research, it will be found that the proportions of the three are not the same. This study shows that there is a dominant hemisphere in swallowing activities, but individual differences are relatively large. Some people have the left dominant hemisphere, some people have the right hemisphere, and some people have no obvious dominant hemisphere. It shows that swallowing activity is relatively complicated compared with other physiological activities.

02 Theory of TMS in the treatment of dysphagia

So what's the theoretical basis for how the hemispheres of the human brain work? The activity of the brain hemispheres is very complex, and no one theory can explain all the brain activities. One of the more important theories is called competitive inhibition, which means that the left and right hemispheres of the brain act in a dynamic balance of inhibition and excitation, which is the basis of our brain physiological activity. When a person performs a specific action such as swallowing, the corresponding functional area of the cerebral cortex will generate a dominant excitation center, while other functional areas of the cortex are in a state of inhibition, which is in line with the laws of our human physiological activities and is very energy-saving and efficient. In a pathological state, when one hemisphere is damaged, the balance of mutual inhibition is broken. As shown in the figure, when the left hemisphere of the brain is damaged, the left side of the brain decreases the inhibition of the right side, and the right side of the brain is activated abnormally. This abnormal activation in turn doubles the inhibition of the left side, further reducing its activity. It is not conducive to the recovery of the brain function of the affected side, and it is manifested as a further decline in the motor function of the hemiplegic side.

Specifically, when it comes to the brain regulation mechanism used in the rehabilitation of swallowing function, there are two main theories: one is that the swallowing function is regulated by the activity of the cerebral cortex on both sides, and the swallowing center of one hemisphere is damaged, which is compensated by the healthy side of the brain. It can improve the patient's swallowing function; another point of view is based on what I just said to correct the "imbalance" of competitive inhibition between the two hemispheres, the activity of the damaged lesion and its surrounding brain areas may be enhanced, and even "new" brain areas may appear. Activation, thereby restoring control of the lower central nervous system and improving swallowing function. In these two theories, the intervention methods of TMS will also be different. At present, the advantages and disadvantages of the two methods are still under constant debate.

In recent years, there have been many studies using TMS to treat dysphagia after stroke. Since these two theories have not yet formed a conclusion, most of the research conclusions are not very consistent. Some studies use unilateral stimulation, such as using low-frequency rTMS to suppress the healthy hemisphere, or using high-frequency rTMS to stimulate the affected hemisphere to restore the balance between the hemispheres. A large number of literature reports have proved that it has a clear effect on dysphagia after stroke. Some experts also performed bilateral stimulation. For example, in this article, high-frequency rTMS stimulation was performed on the motor cortex of the pharynx on both sides of 22 stroke patients. It was found that the swallowing function of patients in the real stimulation group improved. The effect can last for more than two months, indicating that bilateral treatment is effective for this swallowing function.

Another study conducted in 2017 compared bilateral and unilateral stimulation. In 35 patients with unilateral hemisphere stroke, high frequency rTMS of 10hz were used to stimulate the motor cortex of the suprhyoid muscle, and it was found that swallowing function was significantly improved in both patients with bilateral and unilateral stimulation, and the improvement was more obvious with bilateral stimulation than with unilateral stimulation.

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