The recurrent laryngeal nerve that “takes a detour”, is this irrefutable evidence from fish to humans?

Many friends have heard that thyroid surgery can easily cause hoarseness in patients due to damage to the recurrent laryngeal nerve. With the high incidence of thyroid diseases, the anatomical term recurrent laryngeal nerve has gradually become widely known.

When otolaryngologists and head and neck surgeons are protecting the recurrent laryngeal nerve during surgery, they often wonder: Why does the recurrent laryngeal nerve take such a "detour" when the human structure is so precise and simple?

1. Relics of evolution

First, we need to understand how the recurrent laryngeal nerve takes a "detour". The recurrent laryngeal nerve branches off from the vagus nerve from the brainstem, goes down into the chest cavity, bypasses the aortic arch on the left, bypasses the right subclavian artery on the right, goes up along the neck into the throat, and innervates the laryngeal muscles. If the recurrent laryngeal nerve is compared to a high-speed rail line, it is like building a railway from Beijing to Guangzhou in order to get from Beijing to Shanghai, and then going north from Guangzhou to Shanghai. This "detour" without any reason has made many anatomists jokingly say that the recurrent laryngeal nerve is a patch that the Creator hurriedly made after discovering a design flaw.

Of course, this "patch theory" is purely a joke. The reason why the recurrent laryngeal nerve takes a "detour" can be traced back to our ancient ancestors - fish.

The evolution of quadrupeds (copyright image from the library, reprinting may cause copyright disputes)

In the body structure of fish, blood vessels such as the aortic arch are parallel to the recurrent laryngeal nerve, and are basically located in the head or gills of fish. However, with the evolution of organisms, our ancestors walked from water to land, and the structure of their bodies also changed. As fish evolved into amphibians, reptiles, and finally mammals, not only four legs appeared, but also the structure of the neck. During this evolutionary process, the recurrent laryngeal nerve was gradually pulled toward the chest cavity by the aortic arch, becoming longer and longer, and eventually forming the current "detour" structure.

This shadow of evolution is also evident in human embryonic development. During human embryonic development, the formation of the recurrent laryngeal nerve also follows an ancient pattern. It is first formed in the neck, and then gradually changes with the changes in the position of the large blood vessels, extending downward to the vicinity of the heart and then returning to the larynx. This path seems circuitous and unnecessary in adults, but in the early stages of embryonic development, its relative position to the blood vessels is reasonable and somewhat similar to that of our fish ancestors. It can be seen that the process of embryo formation is like the process of evolution, billions of years in a glance.

Humans are not the only ones who have the problem of the recurrent laryngeal nerve taking a long detour. Among the existing animals, the one that takes the longest detour is the giraffe. The recurrent laryngeal nerve of a giraffe is about 4 to 5 meters long. It can be seen that the longer the neck, the farther it can go. According to scientists' estimates, if the species that have ever existed on Earth are counted, the champion of the recurrent laryngeal nerve taking a long detour is the various dinosaurs with long necks. The treasure of the Shanghai Natural History Museum, the Hechuan Mamenchisaurus fossil, is 22 meters long and has a neck of nearly 10 meters. I still remember when I went to the old Natural History Museum as a child, the huge Mamenchisaurus fossil gave people a terrifying sense of oppression in the dark and empty hall. With such a long neck, the recurrent laryngeal nerve may be nearly 20 meters long in one round trip. It is hard to imagine how Mamenchisaurus made a sound. When it wanted to call, its mouth opened first, but the vibration of the throat might be half a beat slower. Such a scene inevitably makes people feel that Mamenchisaurus is a bit silly and cute.

The holotype specimen of Hechuan Mamenchisaurus stands in the Chengdu Natural History Museum (Chengdu University of Technology Museum). (Source: Xinhua News Agency)

2. Clinical Challenges

In clinical work, the "detour" recurrent laryngeal nerve also poses a great challenge to ENT doctors. Whenever a patient suffers from hoarseness and is found to have paralyzed vocal cords on one side during examination, our work is like the communicators checking telephone lines on the battlefield during World War II, looking for the "broken line" along the way. The recurrent laryngeal nerve starts from the brainstem and ends at the larynx to control the vocal cords. Compression or invasion of the vagus nerve segment, chest lungs, aortic arch, thyroid gland, etc. along the way will cause nerve damage and eventually cause vocal cord paralysis.

The recurrent laryngeal nerve is also a branch of the vagus nerve (copyright image from the library, reprinting and use may cause copyright disputes)

Compared with various nerve injuries that have occurred, our ENT doctors encounter the recurrent laryngeal nerve more often during thyroid surgery. When the recurrent laryngeal nerve passes through the thyroid gland, it is located in the tracheoesophageal groove behind the thyroid gland, and is locally close to the thyroid gland. When the thyroid gland is surgically treated, it is very easy to injure the recurrent laryngeal nerve. Over the years, surgeons have developed various surgical techniques and methods to avoid or protect the recurrent laryngeal nerve. In recent years, the application of intraoperative nerve monitoring technology has provided important assistance for nerve protection. This monitoring allows doctors to detect and identify the location of the recurrent laryngeal nerve during surgery, so that they know themselves and the enemy.

Sometimes, we occasionally encounter a variant recurrent laryngeal nerve that "does not take a detour", that is, the "non-recurrent laryngeal nerve". The incidence of the "non-recurrent laryngeal nerve" on the right side is greater than that on the left side. Different reported data vary, but it is basically around one percent. This is similar to our experience in clinical work. Basically, for every hundred or so thyroid surgeries, we may encounter a case of non-recurrent laryngeal nerve. Unlike the detour of the recurrent laryngeal nerve to the chest cavity, the non-recurrent laryngeal nerve goes directly to its destination, often catching doctors off guard during surgery. Nerves do not appear where they should appear, but nerves appear where they should not appear. Although this anatomical variation itself is not bad, it can easily lead to doctors making mistakes due to insufficient anticipation during thyroid surgery.

The story of the recurrent laryngeal nerve seems to have been told from ancient times to the present day.