Further diagnostics
Mit der Kombination homogene Lichtquelle und Kopflupe lassen sich die vorderen Augenanteile Lider, Bindehaut, Hornhaut, vordere Augenkammer, Iris, Linsenvorderfläche (mit fokalem Licht oder Spaltlicht bis hin zur Linsenrückfläche) gut untersuchen.
A hand-held slit lamp is the tool of choice for a precise examination of the anterior structures of the eye. This allows even the smallest changes to be detected. The examination distance is small (80/100 mm) and the magnification is high.
The intraocular pressure can be measured using a rebound tonometer or aplanation tonometer. It is important to pay attention to the possibility of errors with each measurement. Even the slightest pressure from outside on the globe can distort the measured intraocular pressure. There is a risk of false positive results, particularly in the case of painful eye diseases, if the eyelids have to be opened manually. Conductive anesthesia of the eye and eyelid retractors are helpful here. Sedation leads to a reduction in the intraocular pressure.
After several doses of local anesthetic eye drops, the index finger is used to palpate the conjunctiva of the eyelid and, if necessary, the conjunctiva bulbi. This examination is used to determine hardening or palpable changes in the eyelid and their extent.
This examination method allows the dorsal vitreous area to be viewed in detail. Horses often suffer from internal eye inflammation. In the worst case, this can lead to complete blindness. Examining the vitreous body as completely as possible can provide early indications of uveitis.
Indirect ophthalmoscopy is used to view the fundus of the eye through cloudy media. It is fascinating to be able to see the brightly colored fundus of the large horse's eyes. Changes can be noticed with this overview examination and then focused and examined precisely with the direct ophthalmoscope.
This test checks the patency of the nasolacrimal duct. The exit is ventrally in the nostril, often at the transition from pigmented to unpigmented skin (upper lateral in donkeys). A teat cannula or another blunt cannula is inserted here and the nasolacrimal duct is rinsed with 0.9% saline solution or BSS (balanced salt solution). The administration of local anesthetic eye drops, block anesthesia of the infraorbital nerve and/or sedation can be helpful.
The intraocular pressure can be measured using a rebound tonometer or aplanation tonometer. It is important to pay attention to the possibility of errors with each measurement. Even the slightest pressure from outside on the globe can distort the measured intraocular pressure. There is a risk of false positive results, particularly in the case of painful eye diseases, if the eyelids have to be opened manually. Conductive anesthesia of the eye and eyelid retractors are helpful here. Sedation leads to a reduction in the intraocular pressure.
The Seidel test is used to detect small leaks in the cornea. If, for example, there is a small leak after a corneal suture through which aqueous humor leaks out, a visible clear trickle will appear immediately after applying fluorescein as evidence of the leak.
Ultrasound examination is always helpful when the inner eye cannot be seen. It is possible to get a good overview of the condition of the inner eye with almost any ultrasound probe from 7.5 MHz. The lacrimal gland can be examined with narrow ultrasound probes.
Mit Sonden über 10 MHz werden die Bilder immer genauer und es können z.B. auch Tumore und Einlagerungen besser dargestellt werden. Es sind auch Hornhautveränderungen, wie z.B. ein Hornhautabszess, Veränderungen in der vorderen Augenkammer und Linsentrübungen darstellbar. Insbesondere bei der Beurteilung des Glaskörpers müssen Fehlermöglichkeiten in Form von Überinterpretation wegen Artefakten beachtet werden.
Here, the time between the last blink and the first appearance of a dry spot on the cornea is measured. This serves as a measure of the stability of the tear film. After the first blink, the eyelids are held open and the fluorescein on the cornea is observed with the cobalt blue filter. As soon as dark islands appear, the time is stopped. In horses, the normal value is around 7-9.5 seconds. Local anesthetic eye drops shorten this time.
The STT is performed without prior anesthesia. A measuring strip is inserted into the ventral conjunctival sac, more in the temporal third. If the value is less than 10 mm after 1 minute, a pathological "dry eye" can be assumed. Keratoconjunctivitis sicca (KCS, disease of the tear-producing organs) is very rare in horses. (Normal value: 15-20 mm/30 sec or 11 mm to ≥ 30 mm/min)
A vision test in which horses are led over harmless obstacles to see if they can see them. This can be done while covering one eye.
The Jones test tests the patency of the nasolacrimal duct. If fluorescein is injected into the eye as part of the fluorescein test, it can be checked whether and when fluorescein comes out of the exit of the nasolacrimal duct. This should happen within 5 minutes, but can take up to 20 minutes.
Initial diagnostics
The eye examination begins with the initial assessment of the ability to orientate. Attention is paid to symmetries/asymmetries, skin changes, injuries, eyelash position, eyelid pinching, eyelid swelling, eyelid changes and tearing. The reaction to threatening gestures and the eyelid reflex are also checked.
In particular, the pupil size, direct and consensual pupillary reflex, reaction to bright light or light reflex, fundus reflex (color in the pupil or of the light reflected from the fundus) and eye color (iris) are examined here.
Theoretically, it is possible to use only one light source without any other aids to examine the horse's eye when the pupil is dilated (short-acting: tropicamide, effective over days or weeks: atropine). This is a very limited method with the risk of missing findings. A magnifying glass greatly improves the view of the front eye structures up to and including the front surface of the lens. More detailed or special examinations are required for a more precise diagnosis.
Spezielle Diagnostik
partly in cooperation with referral clinics (*)
In this examination, altered corneal tissue and deposits are spread on a slide, dried, stained and examined under a light microscope. A major role is played by the exclusion of a fungal infection, which can be very dangerous. In addition, eosinophil granulocytes (inflammatory cells that often play a role in non-healing corneal wounds) can be detected.
The histological examination of tissue samples is carried out in special institutes for animal pathology. Tissue sections are prepared and stained. The cross-sections can then be examined at the cellular level in a microscopic image. This examination is used in particular to identify tumor cells and to assess their degree of differentiation.
Lissamine green has the same staining properties as rose bengal, except that it does not stain epithelial cells even when the tear film is not intact and it is less toxic and irritating.
In corneal infections, bacteria can trigger dangerous corneal ulcers. The bacteriological examination determines the germ and the resistance test determines which antibiotic is effective. Reserve antibiotics may only be used after a resistance test to avoid antibiotic resistance. In emergency situations, it is important to obtain results from the bacteriological examination with resistance test as quickly as possible.
If the nasolacrimal duct is blocked and flushing and catheterization do not improve the patency, it may be necessary to introduce iodine-based contrast media or a catheter with a metal stylet into the nasolacrimal duct and to take X-rays. This can determine the location of a narrow spot. For example, it can provide evidence of dental disease if the narrow spot is near a tooth root.
Rose Bengal stains dead and degenerated cells, as well as mucus. In addition, if the tear film is not intact, it can stain normal cells depending on the dose and thus indicate a disruption of the tear film.
Magnetic resonance imaging uses magnetic fields to visualize tissues, rather than X-rays. This can be used to visualize organ changes and tumors in particular. This examination is performed under general anesthesia.
Computer tomography can be used to create cross-sectional images of body parts using X-ray technology. As with other imaging techniques, great progress is constantly being made in the ability to depict tissue structures and the use of equipment. For example, it is now routinely possible to take head images while standing in order to show, for example, damage to the bony orbit caused by retrobulbar tumors.
The Seidel test is used to detect small leaks in the cornea. If, for example, there is a small leak after a corneal suture through which aqueous humor leaks out, a visible clear trickle will appear immediately after applying fluorescein as evidence of the leak.
This technique uses infrared light in the wavelength range of approximately 800 to 1400 nm to obtain 2- and 3-dimensional images of cornea and retinal structures at micrometer resolution.
This technique can be used to test for ametropia (e.g. nearsightedness and farsightedness). The significance of nearsightedness or farsightedness in horses is not yet known. Horses can compensate for visual impairments very well, so that horses with a missing eye or a severely damaged eye often have little or no noticeable impairment. This procedure is currently in the testing phase.
The retinal function can be tested using ERG (electroretinogram). This procedure can determine either a complete failure or an undisturbed function. Everything in between is difficult to assess. Night blindness, such as CSNB (Congenital Stationary Night Blindness) in tiger-spotted horses, can be demonstrated. It is important to use contact electrodes that are suitable for the horse.
FVEP (Flash Visual Evoked Potentials) represent the functionality of the visual pathway between the retina and the brain. As with the ERG, further research and some effort are required to enable routine use with clear statements as the result of the examination.