Is it megaesophagus or a normal variation?

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.

Allison Zwingenberger

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

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 as well.

Image 1: The cricopharyngeus muscle is outlined by air in the cervical esophagus.

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 same series.

Image 2: A normal, transient collection of air cranial to the heart.

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 3: Fluid in the caudal esophagus on a left lateral projection.

Megaesophagus

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.

Image 4: Generalized megaesophagus and aspiration pneumonia.

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.

Esophagrams

Sometimes megaesophagus is not visible at all on plain radiographs. If the animal has signs of a swallowing disorder, you'll need an esophagram to make the diagnosis. When the megaesophagus is more severe or static, administering some liquid barium and taking a radiograph will be enough to show you the outline of the dilated esophagus. Fluoroscopy is valuable in more subtle cases.

One of the most common causes of focal megaesophagus that I see is esophagitis. It usually occurs in the caudal thoracic esophagus, and can be a primary disease or secondary to another disorder such as hiatal hernia. This focal dysmotility might be visible only on fluoroscopy. Liquid barium usually is enough to make the diagnosis of esophagitis using fluoroscopy.

Image 5: An esophageal stricture is the cause of aspiration pneumonia seen on survey radiographs.

Esophageal strictures often are not visible on survey radiographs. You may see only persistent, small collections of air in the esophagus, and/or aspiration pneumonia (Image 5). Barium and fluoroscopy often are needed to diagnose esophageal strictures, and to describe their degree of stenosis, location and extent (Image 6). Barium-soaked kibble can be helpful in cases where there is a mild esophageal stricture. Solid-food particles may slow the bolus in a stenotic area, which is much less obvious when viewing a liquid bolus.

Image 6: An esophageal stricture is the cause of aspiration pneumonia seen on survey radiographs.

Even if you have made the diagnosis of megaesophagus on plain radiographs, keep in mind that additional radiographs or fluoroscopy could be indicated. Imaging is useful in monitoring response to treatment for megaesophagus, or for following secondary disease such as aspiration pneumonia. Fluoroscopy can help diagnose additional esophageal problems such as dyssynchrony of the pharyngeal swallowing mechanism or hiatal hernia.

Diagnosing megaesophagus

• Recognize variations of normal (transient).

• Persistent abnormal dilation of the esophagus with gas or fluid.

• If static, plain radiographs may be all you need.

• Use fluoroscopy in cases of less severe or dynamic disease.

• Follow up imaging for response to treatment and monitoring of complications such as aspiration pneumonia.

Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.