Raptor Ophthalmology: The Ophthalmic Exam

Introduction

A bird is a wing guided by an eye… Rochon-Duvigneaud: Lex Yeux et La Vision Des Vertebres

In many birds, the eye is the most important sensory organ, and even partial impairment of vision has far-reaching consequences. Unfortunately, ocular lesions are a common finding during ophthalmic examination in birds of prey.

Assessment of visual function

Visual deficits are often detected earlier in falconry birds, because they may be observed flying, hunting, perching, and interacting with conspecifics.

Thoroughly question the caretaker or directly observe the bird from a distance.

• How is the bird’s food intake? A bird that has recently become blind often fails to find food in its cage but will eat readily when handfed.
• Is the bird reluctant to fly? Birds with severe vision loss are disinclined to fly, while birds with visual impairment may consistently miss the perch or even fly into a wall or large object.
• Is head posture normal? Unilateral vision loss is sometimes associated with abnormal head carriage ranging from subtle head tilt to opisthotonos. A bird with unilateral blindness will keep the sighted eye positioned toward activity in its environment. Some birds will fixate on a brightly colored object and follow its motion.
• Birds with visual deficits may also become apprehensive when activity is near and demonstrate an accentuated startle reaction.

Evaluation of restrained birds

Knowledge of normal raptor behavior is helpful when performing an ophthalmic examination. Buteos such as the red-tailed hawk (Buteo jamaicensis) are fairly calm when restrained, while accipiters like the sharp-shinned hawk (Accipiter striatus) are high-strung. Accipiters are very susceptible to the stresses imposed by excessive handling. Falcons are intermediate in disposition, while many owls are relatively sedate. If manual restraint induces considerable stress (often first evidenced by open mouth breathing), consider inhalant anesthesia to facilitate the exam. If prolonged handling of an accipiter is required, anesthesia should be used.

• When beginning an ophthalmic exam, first assess the general appearance of the head and periocular region.
• Look in the ears, particularly in owls, since recent trauma may cause bruising in the aural canal (Figures 1 and 2).

• Use a focused light source such as a Finoff transilluminator (Fig 3) to evaluate ocular position, symmetry of the globes, as well as pupil size and symmetry (Fig 4 and Fig 5).
  

  Figure 3. Use of a transilluminator to evaluate the eye of a barred owl (Strix varia). Photograph provided by Dr. Christal Pollock. Click on image to enlarge.

  

  Figure 4. Ptosis in a great horned owl (Bubo virginianus). Photograph donated by Dr David Williams MA VetMB PhD CertVOphthal FRCVS.

  

  Figure 5. Although a slight degree of anisocoria may be a normal finding, a profound difference in pupil size is always clinically significant. Image provided by Dr. Ed Ramsay. Click on image to enlarge.

• Pupillary light reflex (PLR) should be very brisk with contraction occurring faster than dilation. A slow reflex is usually indicative of profound cranial trauma. Although complete decussation of the optic nerve occurs at the chiasm, a slight consensual pupillary response has been verified in chickens experimentally. Clinically, it is rare to appreciate a true consensual PLR. An aberrant consensual PLR can occur from light passing from one globe, across the exceedingly thin interorbital septum directly into the other globe.
• When evaluating menace, the initial response in accipiters, buteos and falcons may be similar to that seen in mammals such as blinking or head movement. However many birds (especially owls) will demonstrate only change in pupil size or movement of the nictitans. With prolonged handling, an accipiter may lose its menace response.
• Confirm eyelid mobility and function by gently tapping the lateral or medial canthus with a finger or cotton-tipped applicator. Eyelids may only close partially, but the nictitans will move completely and briskly. If indicated, the leading edge of the eyelids or nictitans may be grasped and extended with an atraumatic instrument such as Graefe forceps.
• The cornea should be clear and moist, and the iris should have a smooth, uniform texture. Focus a bright light source on the cornea and use magnification to observe any scatter of light within the anterior chamber to assess chamber clarity.
• The posterior segment can be examined by direct ophthalmoscopy (PanOptic^™, Welch Allyn), or ideally using indirect ophthalmoscopy with a focal light source. Use a 20-30 diopter hand held lens for birds with relatively large pupils like raptors (Fig 6).

 

Although bird pupils cannot be reliably dilated with autonomic drugs, pupils can usually be sufficiently dilated by simply darkening the room. Inhalant anesthesia, including air sac perfusion anesthesia, may also be used for complete ophthalmic examination.

Choroidal vasculature is clearly visible in most young owls, which have a paucity of pigment in their retinal epithelium (Fig 7).

Choroidal vessels are harder to appreciate in most diurnal raptors and in many cases are totally obscured by the overlying pigmented retinal epithelium. In these birds, the fundus is typically gray or brown to reddish-brown due to the pigment in the underlying retina and choroid.

The dominant feature during fundic exam is the pecten. The pecten is typically located temporoventral within the eye. At the base of the pecten, a thin white line surrounding its perimeter represents the periphery of the optic disk (Fig 8).

Ancillary testing

The ophthalmic techniques used in other animals can be performed on birds, the only limiting factor is generally size.

Assessment of tear production

Standard Schirmer tear test (STT) values have been reported in birds using 2-5 mm filter strips. Mean reported values for a 1-minute STT using 5 mm strips were 4.1 to 14.4 mm/min in Falconiforms, and 10.7 to 11.5 mm/min in Accipitriforms. Owls show consistently lower values (< 3mm/min).

Culture

Obtain culture samples after applying a single drop of topical anesthetic. Use a small calcium alginate or rayon swab (Mini-tip, Beckton Dickenson Microbiology Systems or Calgiswab, Spectrum Laboratories) to avoid contact with the skin or feathers. To obtain a conjunctival sample, use the less mobile upper eyelid. Sample the center and edge of corneal lesions. If a descemetocele is suspected do not obtain a sample until the bird is under general anesthesia.

Cytology

Indications for cytology include conjunctivitis, corneal ulcers with inflammatory infiltrate, and periocular mass lesions. Administer a drop of topical anesthetic 15-20 seconds before sampling. Gently exfoliate corneal or conjunctival cells from lesions with a small, sterile brush or spatula (Storz, St. Louis, MO) and needle aspirate masses. Prior to needle aspirate, direct application of a cotton-tipped applicator soaked with topical anesthetic to the needle entry site will decrease stress and facilitate obtaining the specimen.

Intraocular pressure

Apply topical anesthetic to the corneal surface 10-15 seconds before measuring intraocular pressure (IOP). IOP in raptors typically ranges from 9-12 mmHg. Although Schiotz tonometry may be used in large birds, applanation tonometry (Tonopen, Mentor O&O) provides reproducible readings on eyes with a minimal corneal diameter of 9 mm. Use of rebound tonometers (TonoPen XL, Reichert) has been described in chickens and Eurasian eagle owls (Bubo bubo), although Jeong et al found that values obtained with the rebound tonometer were significantly higher than values obtained with the applanation tonometer.

Imaging

Use radiographs, CT or MRI to evaluate the orbital contents and scleral ossicles. Ocular ultrasound using a 12 MHz transducer or higher may help to examine an eye with advanced corneal or lens opacification, closure of the pupillary opening, or retrobulbar lesions. Ocular ultrasound also helps to identify low-lying retinal detachment.

Electrodiagnostics

The electroretinogram (ERG) assesses retinal photoreceptor function. Primary indications for ERG are to rule out retinal photoreceptor degeneration as a cause for visual dysfunction and to assess retinal function before cataract surgery.

Assessment before release

When determining if a bird is suitable for release, assessment of visual function must certainly be taken into account. Although the amount of debate on this topic is substantial, there is little data to provides guidance on how ocular lesions impact survival potential after release back to the wild. Use of release-training enclosures that allow an accurate determination of the raptor’s ability to capture live prey is strongly recommended, although the ability to catch prey in a controlled environment does not necessarily translate to successful survival skills in the wild.

The following are prerelease parameters that I take into account:

• Chronicity: A lesion that has been present for months should not be weighed as heavily as a recent injury. Scars in raptors are associated with chorioretinal hyperpigmentation and depigmentation. Scars appear flat and well demarcated (Fig 9), while active lesions appear raised and irregular with blurry margins.
  

  Figure 9. Chorioretinal scarring in a golden eagle (Aquila chrysaetos). Photograph provided by Dr. Christopher Murphy.

• Location: Axial lesions of the optical media and foveal lesions of the retina are the most severe. Peripheral lesions may have little effect on vision. Foveal lesions are to be assessed more harshly than peripheral fundus lesions.
• Extent: Small focal lesions are less significant than extensive lesions or lesions involving multiple ocular structures.
• Symmetry: Bilateral lesions should be weighted more heavily than unilateral lesions, especially if they involve axial optical media and/or foveae.
• Other body systems: The impact of ocular lesions may exacerbate the debilitating effects of other body system lesions. While a bird may be able to compete successfully with mild, unilateral impairment of wing function, it might starve to death if it also has significant loss of vision.
• Age: It is estimated that 75% of free-ranging raptors do not survive their first year of life. For this reason, I weight lesions more heavily in young, inexperienced birds, than in mature birds that have honed their hunting skills.
• Species: Ocular lesions are weighted more heavily in diurnal birds of prey because owls possess highly developed hearing. For instance, it has been shown that barn owls (Tyto alba) are capable of capturing mice in absolute darkness using only auditory cues.

Should a one eyed bird be released? All of the above factors need to be taken into consideration in answering this question. It is worth noting that at bird banding stations where healthy migrating raptors are captured, evaluated and banded, a number of raptors are identified in robust good health with unilateral vision. If a bird is healthy, has unilateral vision loss and there is no possibility of placing it for educational purposes and the only options are to release it or euthanize it, strongly consider release.

References