Megaesophagus is a condition in which the esophagus has reduced peristalsis, and has poor tone at rest. The esophagus can
have a mild, focal motility problem, or the entire organ may be dilated and functioning poorly.
These varations in severity mean that megaesophagus can have a variety of radiographic appearances. Both focal and generalized
megaesophagus can be congenital, or acquired secondary to inflammation, foreign bodies, neuromuscular disease or idiopathic
causes. Since a radiograph is a snapshot in time of the dynamic process of swallowing, it can be hard to decide what is a
variation of normal, and what qualifies as an esophageal motility problem.
Variations of normal
Image 1: The cricopharyngeus muscle is outlined by air in the cervical esophagus.
There are variations in the normal appearance of the esophagus one should recognize. The first is a small amount of air in
the cervical esophagus, just caudal to the cricopharyngeus muscle (Image 1). It often outlines the cricopharyngeus muscle,
or upper esophageal sphincter, that lies dorsal to the laryngeal cartilages (circled, Image 1). The muscle appears oval, and
the air usually is triangular in shape. The cricopharyngeus sometimes gets mistaken for a foreign body because of its size.
This transient accumulation of air is more common in animals under general anesthesia, but is seen in conscious radiographs
Image 2: A normal, transient collection of air cranial to the heart.
The second variation of normal is a triangular pocket of air in the thoracic esophagus, just cranial to the heart base (Image
2). Small amounts of air like these should clear with the next swallow, and are usually not seen on other radiographs of the
Image 3: Fluid in the caudal esophagus on a left lateral projection.
Finally, if you are taking three-view thoracic series, you'll often see some fluid in the caudal esophagus on the left lateral
projection (Image 3). This is because the esophagus and cardia of the stomach are on the left, and the increased pressure
from abdominal organs causes some reflux of gastric contents. The key to recognizing these variations is that they are transient.
If you take another radiograph, they should be cleared.
Image 4: Generalized megaesophagus and aspiration pneumonia.
Focal or generalized megaesophagus can cause persistent accumulations of air, or larger amounts of air, to accumulate in these
sites or other portions of the esophagus. The most common appearance of generalized megaesophagus is to see two diverging
or parallel soft-tissue lines dorsal to the trachea and caudal vena cava, and ventral to the aorta (Image 4). The dilated
portion of the esophagus might fill with fluid or food material, especially in the case of an obstructive process such as
a stricture or vascular ring anomaly.
Aspiration pneumonia is a common complication of many swallowing disorders because food boluses re-enter the pharynx. Three
projections of the thorax are valuable in detecting alveolar disease. The most common location for aspiration pneunomia is
the right middle lung lobe (asterisk in Image 4), which you often see clearly only on a left lateral projection. The lobe
is located in a ventral position, and the pneumonia often is in the most dependent portion. Subtle disease often is hidden
by the mediastinum on a d/v or v/d projection, and not visible on the right lateral projection.