Discussion

Introduction:
Ultrasound is an extremely useful modality to assess both soft tissue structures and bony margins. While its use in regions such as the limbs, abdomen and thorax is routine for equine practitioners, its utility extends to other areas as well, including the head and cranial neck. One of the major benefits of ultrasonography is its portability and availability, allowing clinicians to examine patients either in a clinic or farm setting.
While ultrasound examinations of the head and cranial neck can be carried out with a rectal transducer, a higher-frequency linear tendon probe (8 - 12 MHz) can provide enhanced detail and improved operator comfort. A microconvex trandsducer, with its small footprint, is another useful tool. Examinations of this region can often be performed without clipping of the haircoat, but improved resolution will be obtained if hair is removed. The region to be examined should be soaked with warm water or isopropyl alcohol prior to scanning.

Eye:
Examination of the globe and orbit can be enhanced with the use of ultrasonography as an ancillary diagnostic tool. In some situations, it may be difficult to perform a complete ophthalmic examination due to periocular swelling of the soft tissues or opacity of the cornea. Transpalpebral ultrasonography can overcome some of these challenges. Retinal detachment, orbital or eyelid abscessation, and lens luxation all have characteristic ultrasonographic appearances, and prompt diagnosis can lead to more rapid treatment (Whitcomb 2002; Scotty et al. 2004; Hallowell and Bowen 2007). A detached retina appears as a 'seagull wing' or 'V' shape in the posterior aspect of the globe with the point of the 'V' at the retinal attachment of the optic disc. Mass lesions, such as neoplasia or abscessation, may be identified in the eyelids or in the retrobulbar space. Identification of the lens in an abnormal position is characteristic of lens luxation.

Trauma and infection:
While radiography has long been a mainstay in cases of head trauma, ultrasound can often provide additional detail about fracture fragments of the facial bones or orbital rim. It is also an excellent modality with which to assess unusual swellings in any area, including the head (Vaughan 2011). Signs of an infectious process, including abscessation and cellulitis, or the presence of a foreign body may have important implications for case management. In this highly vascular and innervated region, ultrasound can help determine the most appropriate approach for foreign body retrieval or other surgical intervention, especially if soft tissue swelling has distorted the anatomy.

Temporomandibular joint:
Temporomandibular joint disorders may be a cause of abnormal behaviour, poor performance, or pain. Ultrasonography allows assessment of the synovial environment, fibrocartilagenous disc, and bony margins of the joint (Rodriguez et al. 2007). As with any joint, it should be assessed for the presence and character of synovial effusion, synovial membrane proliferation, and osteophyte formation.

Larynx:
Ultrasonography of the larynx has proven to be useful for diagnosis of a variety of equine upper airway conditions, including recurrent laryngeal neuropathy, arytenoid chondritis, and fourth branchial arch disorders (Chalmers et al. 2006; Garrett et al. 2009, 2011; Garrett
2010). Horses with recurrent laryngeal neuropathy (characterised by the inability to fully abduct the arytenoid cartilage during dynamic endoscopic examination) have hyperechogenic cricoarytenoideus dorsalis, cricoarytenoideus lateralis, and vocalis muscles on the affected side as compared to the normal side. Ultrasonography is especially useful in horses with questionable arytenoid cartilage movement during resting upper airway endoscopy (e.g. Havermeyer grades II and III) as an ancillary diagnostic aid, especially if dynamic endoscopy cannot be performed.
Arytenoid chondritis can be confirmed using laryngeal ultrasonography as well, as ultrasonography allows assessment of the majority of the arytenoid cartilage and its abaxial border. This is especially useful if arytenoid cartilage enlargement is equivocal during resting upper airway endoscopic examination or if arytenoid granulomas or chondromas are present and involvement of the body of the arytenoid cartilage is unable to be assessed. Chondritic arytenoid cartilages are enlarged with irregular margins and abnormal echogenicity within the cartilage. Peri-laryngeal or intra- cartilage abscessation may be apparent as well. Repeat examination is useful to monitor response to treatment.
Additional anatomic malformations, including fourth branchial arch defects can be definitively diagnosed using laryngeal ultrasonography. These anatomic malformations exhibit a variety of abnormalities during resting and dynamic endoscopic examination. On the affected side, horses with fourth branchial arch defects lack a cricothyroid articulation and the thyroid lamina extends dorsal to the muscular process of the arytenoid cartilage. Laryngeal ultrasonography is invaluable to elucidate the cause of the problem, as they may be misdiagnosed as recurrent laryngeal neuropathy, dorsal displacement of the soft palate, axial deviation of the aryepiglottic folds, or dynamic laryngeal collapse.