Imaging of the Upper Airway

Imaging of the upper airway of the horse is a valuable adjunct to upper respiratory endoscopy.  The major imaging modalities used in evaluating the upper airway are radiography,  ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI).  Conditions of the nasal passage, paranasal sinuses, pharynx and larynx, and guttural pouches can  be assessed using various combinations of these techniques.  

Radiography 

Preparation and Image Acquisition 

Radiographs of the equine head, in particular those of the paranasal sinus region, are notoriously  challenging to interpret. However, they are inexpensive to produce, easy to obtain, and are  widely available both in referral settings as well as in ambulatory practice.  

Four projections (lateral-lateral, left and right obliques, and dorsoventral) are generally sufficient  to examine the nasal cavity or paranasal sinuses and a lateral-lateral view is adequate to examine  the laryngeal/pharyngeal region. Depending upon the size of the horse and x-ray detector, a  rostral and a caudal image may need to be acquired to completely examine the entire nasal cavity and sinus region.  

Interpretation 

Abnormalities involving the paranasal sinuses often manifest as abnormal soft tissue or fluid  opacities involving either part or all of a sinus region. A fluid line may be present. Lesions as  diverse as primary or secondary sinusitis, paranasal sinus cyst, ethmoid hematoma will appear  similar on radiographic images. The tooth roots can be evaluated for signs of tooth root abscess.  In cases of trauma, fractures as well as paranasal sinus fluid accumulation may be identified, but  interpretation can be challenging.^1-3  

In the laryngeal-pharyngeal region, the anatomic positions of the palate, the palatopharyngeal  arch, hyoid apparatus, and larynx can be assessed.⁴ Abnormal accumulation of fluid within the  guttural pouches, air in the esophagus, and foreign bodies may be appreciated.  

Ultrasonography 

Preparation and Image Acquisition 

Although ultrasonography yields little information about the interior of the paranasal sinuses, it  is a very useful technique to image the abaxial aspects of the laryngeal region. It is inexpensive  and does not require general anesthesia. While sedation is not always necessary, it can facilitate  patient cooperation and relaxation to allow extension of the head permitting access to the laryngeal region. The hair can be clipped or soaked with alcohol. A 7-10 megahertz linear or  microconvex transducer will provide good resolution and adequate penetration.  

Lateral, oblique, and ventral windows can be used to image the larynx.⁵ The arytenoid cartilages,  thyroid cartilage, cricoid cartilage, and cricoarytenoideus lateralis muscle are evaluated in  longitudinal and transverse planes from a lateral window. In most horses, the lateral portion of  the cricoarytenoideus dorsalis muscle may be imaged from an oblique window, as can the retropharyngeal area. From the ventral window, the basihyoid bone and lingual process, vocal  folds, thyroid cartilage, and cricoid cartilage are imaged.  

Interpretation 

Ultrasonographic evaluation of the larynx is used to augment the findings of upper airway  endoscopy in various laryngeal disorders.^6-9 Increases in echogenicity of the cricoarytenoideus  lateralis and cricoarytenoideus dorsalis muscles suggestive of denervation due to recurrent  laryngeal neuropathy can be appreciated (Fig. 1). In cases of poor performance or abnormal upper airway noise with equivocal results of resting upper airway examination, ultrasonography  of the cricoarytenoideus lateralis and cricoarytenoideus dorsalis muscles can provide additional  information in recommending appropriate treatment. This is especially useful in cases where a high speed treadmill examination is not performed.  

The extent of arytenoid chondritis can be more accurately determined and the disease process  can be followed with serial ultrasound examinations. Ultrasonography allows evaluation of arytenoid thickness, identification of perilaryngeal abscessation, and impingement of the  inflamed and enlarged arytenoid on surrounding structures.  

In addition to assisting with the diagnosis of more common upper airway conditions, unusual  congenital malformations can be more fully evaluated by imaging the abaxial aspect of the larynx. Retropharyngeal masses (abscesses, enlarged lymph nodes) can be readily identified in  the cranial neck region. Ultrasound guidance can be valuable if masses require aspiration or  surgical drainage.  

Magnetic Resonance Imaging 

Patient Preparation and Image Acquisition 

When considering MRI of the upper airway, a bore large enough to accommodate the region of  interest is required. Current standing magnet designs cannot accommodate the equine head, so general anesthesia is required. Magnetic resonance imaging examinations of the sinonasal or  laryngeal regions can generally be completed in 60-90 minutes, depending upon the size of the  area examined. Images may be acquired in any geometric plane. We have used intravenous  gadolinium-based contrast media to more accurately characterize tooth root infection, vascular  malformations, cyst-like structures, and abscessation.  

Interpretation 

Magnetic resonance imaging provides excellent information about soft tissue and bony structures in the head.^10-12 The locations of fractures in cases of trauma and any associated soft tissue damage can be determined with a much higher degree of certainty than with radiography. The  extent of anatomic malformations or mass lesions can be appreciated. Abnormalities such as tooth root abscess (Fig. 2), oronasal fistula, ethmoid hematoma, and paranasal sinus cyst may  appear similar on radiographic images, but can be differentiated from one another using MRI.

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