1. Thyroxine (inactive form) also called T4 (90% of the secretion). 2. Triiodothyronine (active form) also called T3 (10% of the secretion).

Transcription

1 A Introduction The nomenclature of the thyroid gland comes from its close relation to the thyroid cartilage (the thyroid cartilage was named like this because thyroid means shield and it is shielding the vocal cords). ; Location: At the lower part of the anterior neck (at the root of the neck), extending at the level of C5-T1 vertebrae. ; Function: The vocal cords are thyroarytenoid ligaments which are attached to the thyroid cartilage on its posterior surface. Secretion of hormones which are: 1. Thyroxine (inactive form) also called T4 (90% of the secretion). 2. Triiodothyronine (active form) also called T3 (10% of the secretion). It s responsible for the metabolism in your body. 3. Calcitonin Secreted by the Para-follicular cells (these cells come from the neural crest). Its function is mainly reducing the serum calcium. ; Structure : It has a butterfly shape with two lobes which look like cones or pears and we call them: The left lobe (lobus sinister) The right lobe (lobus dexter) These lobes are attached in the middle by a narrow passage or tissue and we call it isthmus. 1

2 The isthmus of the thyroid It is passes anterior to the 2 nd -4 th tracheal rings (mostly to 2 nd -3 rd but sometimes to the 4 th ) Sometimes above the isthmus there is an extra-thyroid tissue extended superiorly (cranially), this extra-thyroid tissue is usually located to the left of the midline, this tissue is not always present, it is only present in half of the population and we call it the pyramidal lobe (because it is pyramidal in shape). If the pyramidal lobe is present, almost 70-80% of it is attached to the hyoid bone by a fibromuscular band, and you ll rarely find it alone without attachment. If the fibromuscular band is present, the muscular part of it is called the elevator of the thyroid gland (AKA Levator Glandulae Thyroideae muscle). The pyramidal lobe and the fibromuscular band are an embryological remnant of the thyroglossal duct. The thyroglossal duct goes from the dorsum of the tongue in the pharynx to the root of the neck where the cells migrate there and proliferate to form the thyroid gland, when the duct starts to obliterate, the superior remnant is called foramen cecum but inferiorly it will form the fibromuscular band, and as it goes closer to thyroid isthmus, it still has some follicular thyroid cells which will be the pyramidal lobe. As a conclusion, the fibromuscular band and the pyramidal lobe are the inferior remnant of the thyroglossal duct while the superior remnant is the foramen cecum. 2

3 ; Fascial Covering of Thyroid Gland If we look to the thyroid gland, there are two fascial coverings: Capsule of thyroid Firstly, we have an external layer covering the thyroid, the trachea, the larynx and behind pharynx and esophagus, this fascial covering is referred to as the pretracheal layer of the deep cervical fascia. At the level of C6 vertebra, the larynx ends and the trachea starts behind it, the pharynx ends and the esophagus starts. In any part of the body there is skin, superficial fascia (fat) and the deep fascia, but the deep fascia of the neck is unique because it is divided into 4 parts: The investing layer of the deep cervical fascia (because it invests all the neck) and this is the first layer. Prevertebral layer which is covering the posterior muscles around the cervical vertebrae. Pretracheal layer which is located Anteriorly and covers the anterior muscles and the viscera there (like the thyroid, larynx, trachea and esophagus), this layer is divided into two layers: Muscular layer: covering the strap muscles (sternohyoid, sternothyroid and thyrohyoid). Visceral layer: covering the thyroid, larynx, pharynx, trachea and esophagus. The carotid sheath containing the internal carotid artery, internal jugular vein and the vagus nerve. 3

4 The most important layer is the visceral part of the pretracheal layer which covers the thyroid gland, parathyroid glands larynx, trachea, pharynx and esophagus, and this is the external thyroid covering of the gland. When there is hemorrhage or bleeding post operatively in the thyroid, the blood will be confined within the pretracheal layer, this will lead to accumulation of the blood and pressing over the trachea leading to suffocation (this is a very dangerous situation in thyroid surgeries). Secondly, there is a deeply covering over the gland which is the fibrous capsule of the thyroid (AKA Capsula fibrosa glandulae thyroideae). This fibrous capsule is actually the one which sends septa, and it is the one that sends dens connective tissue posteriorly to the larynx and trachea to fix the gland in its position. The thyroid gland has a profuse blood supply, so there are blood capillaries (arteries and veins) and lymphatic blood capillaries surrounding the gland, these plexuses or networks are found between the pretracheal and the fibrous capsule. ; Relations of the Thyroid Gland Anterio-laterally: Mainly it is covered by muscles. Upper part, there is omohyoid muscle (the muscle between the hyoid and the scapula). Lower part, there is sternocleidomastoid (SCM) (it is the mark line for thyroid surgeries, because when you want to do a cut, the cut should extend from the anterior border of SCM to the other anterior border of SCM, because this muscle is crossing anteriolaterally to the lower border of the thyroid gland). Middle part, there is sternohyoid and sternothyroid, these two muscles provide a firm attachment for the gland anterio-laterally (so when there is any swelling in the gland as in inflammation or goiter, it can t go anteriorly and the sternothyroid muscle is attached to the oblique lines of the thyroid cartilage, so there is no upward expansion of the gland, so the only way to the gland to grow is inferiorly and posteriorly). 4

5 Anterio-medially: Upper part: there is the thyroid and the cricoid cartilages. Lower part: there is the upper 5 th -6 th tracheal rings. Sometimes when the physicians want to do tracheostomy, they prefer the lower rings (5 th or 6 th ) to make the incision there to avoid the isthmus. Posterior-laterally: There, is the main blood vasculature which is in the carotid sheath. The carotid sheath contains the common carotid artery that will continue as the internal carotid artery at the level of C3-C4, internal jugular vein and the vagus nerve. In the carotid sheath, the most medial (deep) structure is the artery while the most laterally (superficial) is the vein and the most posterior is the nerve. Posterior-medially: There, is the recurrent laryngeal nerve (a branch from the vagus nerve) which passes through the groove between the trachea and the esophagus. Posteriorly: There is the inferior thyroid artery and the parathyroid glands. The posterior relations of the thyroid gland are in close relation, so any damage to any of these structures during thyroid surgery results in a very severe condition as the following: If you cut the recurrent laryngeal nerve, this will affect the vocal cords, then this will lead to difficulty in breathing and speech. If you cut the inferior thyroid artery, this will lead to bleeding, this bleeding will confined in the pretracheal layer, and then suffocation will occur. If you remove the parathyroid glands, this will lead to loss of the parathyroid hormone and eventually decrease in serum calcium (this will lead to tetany). 5

6 ; Vasculature of Thyroid Gland Arterial Blood Supply Superior thyroid artery: It comes from the external carotid artery, and reaches the gland from above. As soon as the superior thyroid artery penetrate the pretracheal fascia, it will divide into a large anterior branch and small posterior branch (so it supplies the anterio-superior aspect but mainly to the anterior aspect of the gland). Inferior thyroid artery: It comes from the thyrocervical trunk which is a branch from the subclavian artery. When it arises from the thyrocervical trunk, it goes up and medially to reach the gland from behind (posterio-inferiorly), so it supplies the posterior-inferior aspect, but mainly the posterior aspect of the gland as well as the parathyroid glands which are located in the posterior aspect of the gland. The subclavian artery has its branches in a clockwise order as the following; superiorly vertebral, anteriorly thyrocervical then internal thoracic and posteriorly costo-cervical. Thyroid Ima Artery: Thyrocervical means branch to the thyroid and to the neck (the cervical branch gives transverse cervical and suprascapular arteries). Only in 10% of the population (not always present). Ima means the lowest, because it arises from the lowest position which is the brachiocephalic artery, but sometimes the origin of the Ima artery varies, but most commonly it will arise from the brachiocephalic. It ascends in the midline anterior to the tracheal rings, so it sends small branches to the trachea, and ends up in the isthmus. 6

7 The isthmus receives its blood supply from blood capillary plexuses, but sometimes when the isthmus is large, it receives its blood supply from the thyroid Ima artery. When the surgeons want to do tracheostomy, in the 5 th or 6 th tracheal rings, they have to pay attention to the fact that only 10% of the people have a thyroid Ima artery, so they have to ligate this artery before doing any further surgical procedures in tracheostomy. A The relation between tracheostomy and thyroid gland We have 3 main regions to go through in tracheostomy: The 2 nd tracheal ring above the thyroid gland (this is the best position), at this position you need to retract the isthmus only. The 5 th or 6 th tracheal rings: at this position you have to pay attention to the presence of Ima artery (the second option). The 3 rd and 4 th tracheal rings: in some patients who have large neck or short trachea at this position you have to cut the isthmus (the last option). A Notes about the tracheostomy The link of the tracheostomy video: video that was shown by dr.allouh: ( The first thing in tracheostomy or any surgical procedure in the neck is doing a horizontal incision from the anterior border of SCM to the other anterior border of SCM (you do a horizontal incision to follow the dermatomes so the scar healing will be minimal). We use the Kocher s forcipes to ligate the Ima artery. To perform the tracheostomy; First, you should use a scalpel to cut between the tracheal rings in the soft tissue, after that you take a curved scissor to cut the tracheal ring from the right and the left, therefore widening the opening. The bottom aspect of the tracheal ring is sutured to the skin to prevent it from going back and close the opening. Finally, we do suction in the trachea to remove any blood or any fluids. 7

8 Venous Drainage We have 3 main pairs of veins: Superior thyroid veins: - Drain in the internal jugular vein. Middle thyroid veins: - The largest. - Drain in internal jugular. Inferior thyroid vein: - Drain in the left brachiocephalic vein Sometimes the inferior thyroid veins join together forming one large vein we call it thyroideus plexus impar (which means the single inferior thyroid vein), it is also drains in the left brachiocephalic vein. Lymphatic Drainage of Thyroid Gland It is important when there is a cancer in the thyroid, when you remove the thyroid gland, you should remove the lymph nodes around it, so you should know the lymphatic drainage to know which lymph nodes you should remove along with the thyroid. Lymphatic plexus on the surface of the gland drains through lymphatic vessels into: Superior drainage goes to the prelaryngeal lymph nodes (also known as Delphian LN), if they are present, they will give you a prophecy that there is malignancy. Inferior drainage goes to the pretracheal lymph nodes. Medial drainage goes to the paratracheal lymph nodes. Lateral drainage goes to the inferior deep cervical lymph nodes (AKA jugulo-omohyoid LN). 8

9 ; Innervation of Thyroid Gland Autonomic nervous control Sympathetic: From the superior (mainly), middle and inferior cervical sympathetic ganglia. It reaches through the superior and inferior Thyroid periarterial plexuses. Parasympathetic: From the vagus nerve. It reaches via superior & recurrent laryngeal nerve. The autonomic innervation isn t responsible for the secretions of the thyroid, it is just a vasomotor (means it controls the blood vessels by constriction or dilation). The pituitary gland is responsible for the activity and hormonal secretion of the thyroid gland by TSH (so the regulation is by the endocrine system not the nervous system). A Thyroidectomy It is the surgical removal of the thyroid gland. Indications: 1. Cancer (adenocarcinoma) 2. Goiter: It is an abnormal enlargement of the thyroid gland due to: Iodine deficiency: - This will make the gland inactive, so the pituitary starts to increase the secretion of TSH to stimulate the gland, and this will stimulate the growth of the gland not the production of hormone leading to hypothyroidism. - This problem was resolved by iodized salt to prevent iodine deficiency. Hashimoto's thyroiditis/disease: - It is the most common cause of goiter in the developed countries as in North America. - It is an autoimmune disease, the body produces antibodies to attack the thyroid follicular cells by destruction of TSH receptors located on them, and so the cells will not respond to TSH leading to hypothyroidism. It was the first disease to be recognized as an autoimmune disease in

10 3. Benign nodules or cysts formation: - If the benign nodules are uni-nodular, there will be hyperthyroidism, whereas if there is multi-nodular or multi-cyst this will lead to euthyroidism (it means no increase in the activity nor decrease). 4. Graves disease: - AKA as basedow s disease or exophthalmic goiter. - It is an autoimmune disease that affects the thyroid gland causing it to enlarge & become hyperactive leading to hyperthyroidism. - The autoantibodies bind and activate TSH receptors ( we call them thyroid stimulating immunoglobulins, because they have similar structure to the TSH). - It has 3 main signs: 1. Goiter..(جحوظ العين).2 Exophthalmos 3. Thickness of the skin over the tibia (known as pretibial myxedema). A Risks of Surgery in Thyroid Gland 1. Recurrent laryngeal nerve injury: - If the injury was unilateral, this will lead to hoarseness of the voice and mild dysphonia, but if the injury was bilateral, this will lead to impairment of breathing because of the paralysis of the vocal cords, and also it will lead to loss of speech. 2. Postoperative Bleeding: - When the surgeon removes the thyroid gland sometimes he didn t notice any bleeding in the area, so this will lead to hemorrhage that is confined within the space surrounded by pretracheal fascia, this will compress the trachea causing difficulty in breathing and finally suffocation. - We should give at least 2 minutes to look at the region before closure to make sure that everything is ligated. 3. Parathyroid glands removal: 10

11 - It is a very severe problem because the parathyroid glands have variable locations especially the inferior ones, and sometimes they are embedded within the thyroid tissue. - We need at least 2 parathyroid glands to maintain the normal level of the hormone. - Inadvertent removal of parathyroid glands will decrease the levels of serum Ca2+, this will increase the permeability of Na+ in the nerve endings initiating an action potential, and this will cause Tetany (it means involuntary muscle spasm). - The tetany starts in the hands and feet leading to what we call trousseau s sign. After the surgeon does thyroidectomy, he will call another doctor or nurse to stay awake beside the patient s bed to keep an eye on him, because if the trousseau s sign starts to appear the doctor must give the patient IV Ca+2 to prevent further decrease in Ca+2 which will lead to suffocation and death. Tetanus it is an infection caused by C.tetani, this bacteria produces a neurotoxin which will cause damage in some neurotransmitters of the CNS, so this will lead to generalized spasm, and it mainly starts in the muscles of the jaw (masticatory muscles). A A Short Clinical Case (Homework) As an ER resident, a patient who was exposed to a car accident was admitted to your department with a broken mandible and obstructed upper airway. Your first aim is to maintain the patient breathing by applying a tracheostomy procedure. Based on this fact, please answer the following questions: 1. What are the soft tissues that you need to cut through in order to reach the trachea? 2. What is the preferred tracheal site for tracheostomy? 3. What procedures will you adopt to protect the thyroid isthmus and thyroid Ima artery if present? 11

12 A Notes about thyroidectomy video: Firstly, we make a horizontal incision from the anterior border of SCM to the other anterior border of SCM. Then, we make another horizontal incision in the platysma muscle, then after this muscle you cut in a vertical line to prevent the cutting of the strap muscle, then we put a retractor on each side. The link of thyroidectomy video: ( =zlaaiytsxnk&oref=https%3a%2f% 2Fwww.youtube.com%2Fwatch%3F v%3dzlaaiytsxnk&has_verified=1) It is not the exact video but it is very similar to it. When you gain access to the thyroid gland, find the pyramidal lobe and then find the Delphian LNs to roll out malignancy. Then after that, you should cut the isthmus to gain access to the posterior aspect of the gland. In the ligation process the superior thyroid artery is in close relation with the superior laryngeal nerve away from the gland, but they are separated from each other when they reach the gland, so the ligation of the superior thyroid artery should be close to the gland to avoid damage to the superior laryngeal nerve while the inferior thyroid artery it reaches the gland in an oblique way but the recurrent laryngeal nerve reaches the gland in a vertical way so they are away from each other when they are away from the gland but they become very close when they are close to the gland so the ligation of the inferior thyroid artery should be away from the gland to avoid damage to the recurrent nerve. Sometimes, if the goiter is benign you can leave some part of the posterior aspect of the thyroid to avoid the loss of parathyroid glands, so that makes you sure that the parathyroid are still there, while if the goiter is malignant, you have to look for the parathyroid and isolate them. If you remove the parathyroid gland inadvertently, you can transplant the parathyroid glands under the skin of the arm (in the superficial fascia of the arm). Transformers Team A 12