The term “auricle” is a biological descriptor used to identify two distinct structures in human anatomy. It refers to the externally visible part of the ear, often called the pinna. Separately, the term also describes a small, ear-shaped pouch extending from the upper chambers of the heart, known as the atria. This dual usage applies to both the auditory and circulatory systems, requiring clear differentiation to understand the role of each structure.
The Auricle of the Ear: Structure and Role
The external ear auricle, or pinna, is the funnel-shaped, visible structure attached to the side of the head. Its complex folds and curves are formed primarily by a plate of elastic cartilage covered by a thin layer of skin. The only part lacking cartilaginous support is the earlobe (lobule), which consists of fibro-fatty tissue.
This structure’s primary function is to collect and direct sound waves. The ridges and depressions of the auricle gather airborne vibrations and funnel them efficiently into the external auditory canal. The unique shape also assists in localizing sound, allowing the brain to determine the direction of origin.
The auricle also serves a minor protective role for the opening of the ear canal. By directing sound, it ensures that acoustic signals are concentrated before traveling toward the middle ear and the tympanic membrane. The auricle is supplied by multiple sensory nerves, including branches of the trigeminal and great auricular nerves.
The Auricle of the Heart: Location and Purpose
The cardiac auricles, more accurately called the atrial appendages, are small, muscular pouches extending from the right and left atria. The left atrial appendage (LAA) is notable, lying anteriorly in the atrioventricular groove. Each appendage is lined with rough, muscular ridges called pectinate muscles, which distinguish them from the smooth walls of the main atrial chambers.
The primary function of the atrial appendages is to serve as a small reservoir, allowing for a temporary increase in atrial volume during high blood flow. The LAA also plays a role in regulating blood volume and pressure through the release of natriuretic peptides. These hormones manage the body’s fluid balance.
In normal cardiac rhythm, the auricles contract with the rest of the atria, ensuring smooth blood movement into the ventricles. Their minor physiological roles in regulating blood dynamics remain important. They are capable of decompression, temporarily mitigating high atrial blood pressure, especially in the left atrium.
Clinical Significance of the External Ear Auricle
The exposed and cartilaginous nature of the external ear makes it susceptible to clinical issues stemming from trauma or infection. An auricular hematoma is a common injury, typically resulting from blunt force trauma seen in contact sports. This trauma causes blood to collect between the perichondrium (the connective tissue layer) and the underlying cartilage.
If the hematoma is not promptly drained, the separation disrupts the blood supply to the cartilage, which relies on the perichondrium for nutrients. Untreated hematomas can lead to cartilage death and subsequent fibrous tissue formation, resulting in the permanent disfigurement known as “cauliflower ear.” Another concern is perichondritis, an infection of the perichondrium causing painful swelling and redness, often sparing the earlobe since it lacks cartilage.
Perichondritis is frequently caused by trauma, such as burns, insect bites, or high ear piercings that penetrate the cartilage. The most common causative bacterium is Pseudomonas aeruginosa. Timely treatment with antipseudomonal antibiotics is necessary to prevent tissue destruction and subsequent deformity, as the infection can rapidly deprive the cartilage of blood.
Clinical Relevance of the Cardiac Auricle
The left atrial appendage (LAA) is medically relevant because it is the most common site for blood clot (thrombus) formation in patients with a specific heart rhythm disorder, Atrial Fibrillation (AFib). AFib is the most common sustained arrhythmia. During AFib, the atria do not contract effectively; instead, they quiver rapidly and irregularly.
This uncoordinated electrical activity leads to blood stasis, especially within the complex, trabeculated structure of the LAA. Blood pooling in this pouch creates an environment conducive to clot formation. The LAA is the source of up to 90% of cardiac-related clots in individuals with non-valvular AFib.
If a thrombus breaks free from the LAA, it can travel through the bloodstream, leading to an embolic stroke, which is a major complication of AFib. Patients with AFib face a five-fold higher risk compared to those with a normal heart rhythm. To mitigate this danger, patients are often prescribed anticoagulant medications (blood thinners) to prevent clot formation.
For patients who cannot tolerate long-term blood thinners due to high bleeding risk, Left Atrial Appendage Occlusion (LAAO) may be performed. Procedures utilizing devices like the Watchman device physically seal off the LAA. Permanently closing this pouch prevents potential clots from escaping and entering the systemic circulation, thereby reducing the patient’s stroke risk.