Anatomy clinical correlates: Mediastinum

We all know how important the heart is, but everyone forgets about the mediastinum, which is the area between the two lungs where the heart actually sits!The mediastinum doesn't just house the heart, but many important structures in the thoracic cavity from the superior thoracic aperture down to the diaphragm.So let's take a closer look at the clinical conditions that can affect the mediastinum and the structures within it.First up, there is widening of the mediastinum, which can be seen on a chest x-ray.Since the mediastinum contains so many structures, each of them can contribute to pathological widening.It can be observed after a trauma that causes laceration or dissection of the great-vessels, typically the aorta, which can cause hemorrhaging into the mediastinum.Other times, malignant tumors such as lymphomas can produce massive enlargement of mediastinal lymph nodes and widening of the mediastinum.Another cause of mediastinal widening is heart hypertrophy due to congestive heart failure.Now, next up, there’s the esophagus.The esophagus may have up to four normal anatomic constrictions as it descends, however there are three sites of constriction that can occur specifically in the posterior mediastinum caused by three structures it meets on its way down: two thoracic constrictions with the first being where the esophagus is crossed by the arch of the aorta, and second where it is crossed by the left main bronchus, and one diaphragmatic constriction where it passes through the esophageal hiatus of the diaphragm.The fourth site of constriction occurs in the cervical region before the esophagus enters the mediastinum, where constriction may be caused by the cricopharyngeus muscle.At these locations, there’s a slower passage of substances and is where foreign objects that are swallowed are most likely to lodge.These narrowings can be seen in chest radiographs of a person who underwent a barium swallow study.These areas are also at risk of stricture after ingesting caustic liquids such as cleaning products.Don’t try this at home!!Now, even though we try to prevent foreign objects going into our esophagus, one thing we are okay with is the use of transesophageal echocardiography, or TEE for short.TEE is a device which uses ultrasound within the esophagus to show images of the cardiac structures, particularly the left atrium which makes up the majority of the posterior heart and directly anterior to the esophagus.In addition to the left atrium, TEE can visualize the atrial septum and mitral valve.TEE allows for assessment of conditions such as atrial enlargement due to mitral stenosis or regurgitation, which is important as severe enough left atrial enlargement can cause external compression on the esophagus leading to dysphagia..Furthermore, the descending aorta lies posterior to the esophagus, so TEE can visualize aortic abnormalities such as dissection or aneurysm.Ok, now, a little bit higher up, there are the recurrent laryngeal nerves, which supply all intrinsic muscles of the larynx, except the cricothyroid which is supplied by the external laryngeal nerve, a branch of the superior laryngeal nerve.You might be thinking, what does this have to do with the thorax?Well, procedures in certain thoracic regions, like a mediastinotomy, or disease in the superior mediastinum such as esophageal cancers and mediastinal lymph node enlargement can injure these nerves and affect the voice, leading to hoarseness or even loss of voice called aphonia.Furthermore, as the left recurrent laryngeal nerve wraps around the arch of the aorta, any dilation of the arch of the aorta can stretch and damage this nerve.Damage to both recurrent laryngeal nerves at the same time would lead to bilateral paralysis of the vocal cords, and would require intubation.Speaking of the aorta, let’s have a look at some variations of the aortic arch.Sometimes there may be a right arch of the aorta which courses to the right of the trachea instead of its usual course to the left of the trachea.In rare cases, a double arch of the aorta can occur which forms a ring around the esophagus and trachea, which can compress these structures and potentially result in difficulty with breathing and swallowing.The aorta can also be subject to something called coarctation of the aorta, which is when the aortic arch or thoracic aorta has an abnormal narrowing or stenosis of the aortic lumen.This causes an obstruction of blood flow distal to the stenosis and to the inferior part of the body.The most common site for a coarctation is near the ligamentum arteriosum.If the stenosis is proximal to the ductus arteriosus, also called preductal coarctation, this can cause the ductus arteriosus to remain patent at birth instead of closing off to become the ligamentum arteriosum, ultimately allowing blood from the pulmonary trunk to bypass the stenosis.When the coarctation is inferior to the ligamentum arteriosum, called postductal coarctation, good collateral circulation develops between the proximal and distal parts of the aorta through the intercostal and internal thoracic arteries.As the subclavian arteries are proximal to this stenosis, they are not affected so their internal thoracic artery branch can provide blood to the intercostal arteries, which in turn provide blood to the descending aorta distal to the stenosis.The collateral vessels may become so large that they cause a notable pulsation in the intercostal spaces and erode the adjacent surfaces of the ribs called rib notching, which is visible on a chest x-ray.Other clinical signs may include hypertension in the upper extremities, and a weak and delayed pulse in the lower extremities.The aorta is also susceptible to dissection.Aortic dissection happens when there’s a tear in a layer of the aorta called the tunica intima, and this results in a false lumen within the aorta.Aortic dissection is associated with things such as hypertension, bicuspid aortic valve, and connective tissue disorders, where individuals may present with sudden onset chest pain radiating to the back and unequal blood pressures between the arms.There are two types of dissection: Stanford type A and Stanford type B.Type A is a dissection that involves any part of the ascending aorta, so think of A for Ascending Aorta.These typically start at the sinotubular junction and extend to the aortic arch, but can also encompass the descending aorta.Type B refers to all other dissections involving only the descending aorta and originate close to the left subclavian, so think Below the subclavians.The aorta can also suffer traumatic lesions which lead to aortic ruptures.These are most often a result of the rapid deceleration that is frequently associated with car crashes, but can also happen because of a fall from height.The most common site of injury is the aortic isthmus, just distal to the left subclavian artery origin, as this is tethered by the ligamentum arteriosum and is thought to be the transition zone between the mobile ascending aorta and the fixed descending aorta.Unfortunately over 80% of people die from this before reaching the hospital.And just before we wrap up, let’s quickly look at something called subclavian steal phenomenon.With subclavian steal phenomenon, the vertebral artery on the same side as an occluded, or blocked subclavian artery “steals” blood from the contralateral subclavian artery circulation.This happens when the occlusion is proximal to the vertebral artery, so blood from the contralateral vertebral artery flows to the basilar artery and then continues as retrograde flow through the ipsilateral vertebral artery.This allows blood flow to the area supplied by the occluded subclavian vessel, and is often asymptomatic.When subclavian steal becomes symptomatic, it is considered subclavian steal syndrome.Symptoms include upper extremity ischemia and neurological symptoms such as dizziness or vertigo.Alright, as a quick recap.A wide mediastinum can be caused by trauma, various malignant tumors or mediastinal enlargements, aortic dissection or rupture, and heart hypertrophy.Foreign bodies or strictures in the esophagus usually occur at the site of the esophagus’ impressions, such as where the cricopharyngeus muscle, the aorta and left main bronchus and the diaphragm leave a mark.

Injuries to the recurrent laryngeal nerve lead to hoarseness and aphonia.A transesophageal echocardiography is an ultrasound technique that allows visualisation of cardiac structures and the aorta from inside the esophagus.Coarctation of the aorta happens when the aortic arch or thoracic aorta has an abnormal narrowing that diminishes the caliber of the aortic lumen.Aortic dissection occurs when the aortic intimal lining tears creating a false lumen within the aorta, and it is either type A mainly involving the ascending aorta or type B only involving the descending aorta.