Surgical Anatomy of Neck

Well, before we get into the surgery of the neck, it’s mandatory to understand that the anatomy in the neck is, although very reliable and fixed, but it is a challenge because there are too many white structures tightly packed together, and one has to revisit this anatomy with surgical dissection in mind. Now, the neck is conventionally also divided into various levels; like level 1a, which is the submental triangle; level 1b, which is the submandibular triangle; and then, of course, level 2, 3, and 4, which are, respectively, in relation to the upper one third, the middle one third, and the lower one third of the sternocleidomastoid muscle. Level 2 can further be divided into 2a and 2b based on whether the nodes are– and the zone is medial or lateral to the spinal accessory nerve.

And, similarly, level 4 can be divided into 4a and 4b depending upon whether it is medial or lateral to the internal jugular vein. Now, generally, these are the conventional levels, level 1a, 1b 2, 3, 4a, 4b, and level 5, which, again, can be divided into 5a and 5b depending upon whether the part of the triangle is superior to the spinal accessory nerve or inferior to it. So these are the usual divisions which are relevant when one is performing the neck dissections which are an essential part of management of head and neck cancers. As we all know, the survival drops by 50% if there are nodes, and especially if there is extra-nodal extension.

Level 6 is a very special level, which is not conventionally counted in the neck section for standard head and neck cancers. And it’s actually for those thyroid or central malignancies, like larynx, et cetera. It includes the pretracheal and paratracheal nodes. Of course, we will discuss that at the end of this presentation.

Now, the pattern of spread of nodes of the lymphatics and the malignancies of head and neck is pretty predictable. And it is depicted in this cartoon where you can see that mostly from the face and scalp, eyelids, chin, external ear, middle ear, floor of mouth, the nasal cavity, it’s 1a, 1b. And in parotid, it could be 1a, 1b, level 2. And one can actually find that post auricular, it can go to level 5.

So it’s fairly predictable. So, therefore, today and in the likely future, the neck dissections are going to become more and more selective. And, in fact, super selective one can only dissect out the levels which are likely to be draining the concern malignancy. So they would vary as you can see in the colors that are appearing. And one can make out that since it is predictable, it’s possible to proceed, to do very, very ultra-selective dissections.

Now, what about surgical anatomy which is relevant to a surgeon operating in the head and neck, and especially when you’re doing neck dissections. Of course, platysma is the bread and butter for the skin and all flaps of the neck. Blood is supplied by facial artery and facial nerve. And it is actually also called a fat sandwich. This is missing in the midline and also in the posterior triangle.

So platysma is something mandatory for the survival of a flap. And, unlike in the cavities of the body, like abdomen or thorax, we cannot retract beyond a point in the neck. We need to, therefore, raise flaps in order to access some corners and difficult areas which we’ll see as we go along. Now importantly, as I mentioned, the posterior flap should be kept smaller because the platysma is deficient, and one should not be searching for platysma in the midline. Most flaps are subplatysmal.

Commonly heard, Erb’s point is a point where the spinal accessory nerve is just about two centimeters superior and coming out. Now, how do you identify the Erb’s point? It serves as a very important landmark for reaching the spinal accessory nerve. This is the point where greater auricular nerve winds around the sternocleidomastoid muscles and other cervical plexus branches also come out, which we’ll see as we go along.

The other important structure is marginal mandible nerve, which is also called as a ramus mandibularis. Now this nerve is very important as it’s a branch of facial nerve, of course, and from the cervical division. It is important because it supplies the angle of the mouth and its injury can lead to drooling of saliva and dropping of the angle of the mouth, a very important nerve. And this actually is responsible for the pattern of incisions that you make when you’re doing neck dissections, especially the horizontal incision. It is usually kept about a centimeter and a half or two centimeters below the mandible to preserve this nerve.

Important nerve to see, dissect, and preserve. And that is what you will see in the video. We’ll discuss the Hayes-Martin technique and the other techniques to preserve it.

The fourth lighthouse is spinal accessory nerve and Erb’s point relationship. Some people take the crossing of the external jugular vein, or the sternomastoid also, as a reference point for the Erb’s point, but it’s important that, not only would it help you find the accessory nerve, which is a very important structure to be preserved during neck dissections, we’ll talk about what is preserved and what is sacrificed. It is a very, very important structure to be preserved, therefore we need to identify it in the posterior triangle. And it is roughly about two centimeters superior to the Erb’s point that the spinal accessory nerves gets into the posterior triangle.

Of course, another very important structure is hypoglossal nerve. And it has a relationship with the carotid bifurcation. Common carotid bifurcates roughly at the level of thyroid cartilage, or some people would say at the level C6 transverse process of C6 vertebra.

However, it’s important to identify in the bifurcation for various reasons. One, if you’re handling the tissue close to the bifurcation, you’re likely to stimulate the vagal or the baroreceptors, and that can lead to vasovagal. And this is the point where we usually alarm the anesthetist about the likely collapse and therefore the bradycardia that can happen.

Exactly 2 centimeters superior to the carotid bifurcation is where we find the hypoglossal nerve. So there’s is a rule of 2 centimeters for the Erb’s point and spinal accessory nerve and carotid bifurcation and the hypoglossal nerve, respectively. As I mentioned at the beginning, I’m repeating that anatomy in the head and neck is by in large fixed. It amounts to cutting on a dotted line, but one needs to know the anatomy very clearly.

Phrenic nerve is another very important structure that must be dissected and saved. And it’s in the posterior triangle lower down. It’s the only structure that goes lateral to medial in the neck, all other structures go medial to lateral or from superior to inferior position.

On the left side, another vital structure is thoracic duct which, if found, should be ligated. Importantly, if there is doubt, just ligate it because the chylous fistula can be a real nightmare. It can be a challenge as a post-operative complication.

And very often in a patient who’s fasting overnight, one may not be able to see it very clearly because it doesn’t show up. It only is detected, in most cases, once it is damaged. So therefore the moment it is damaged or there is a suspicion, should be ligated.

Now, I’ll conclude by mentioning about safety layer, which I call as a holy layer in the neck. It’s a prevertebral fascia which is forming this layer. And it covers both the brachial plexus, the phrenic nerve, and if you stick superficial to this, it’s unlikely that one would damage either the phrenic nerve or the brachial plexus. So it’s very, very important to stick to that.