Anatomy of the rabbit nasolacrimal system
The lagomorph nasolacrimal system is unique.8,18 There is a single, relatively large (2-4 mm) lacrimal punctum that is located deep within in the ventromedial fornix of the lower eyelid (Fig 1).1,8,14,18 In large, laboratory rabbits, the location of the punctum has been described as 3 mm from the medial canthus and 3 mm from the inner lid margin.14,18 This opening may be pigmented.
The punctum leads into a short canaliculus, which then opens into the lacrimal sac, a dilation of the nasolacrimal duct.11 The nasolacrimal duct exits the lacrimal sac through a small lacrimal foramen in the lacrimal bone, close to the apices of the first two maxillary cheek teeth. The duct then enters the maxilla where it is encased within a bony canal, and continues in a rostroventral direction before deviating mediodorsally around the apex of the first maxillary incisor tooth.1,3,4,11,14,18 The duct exits at a small nasal ostium several millimeters posterior to the ventromedial aspect of the alar fold, just a few millimeters into the mucocutaneous junction of the nose.8,11,14,15,18
The rabbit nasolacrimal duct is relatively long and narrow and its course is quite tortuous in rabbits (Fig 2).4 Obstruction of the nasolacrimal system most frequently occurs at two narrow constrictions.1,4,8,14,16,18 The proximal narrowing is found where the duct passes through the lacrimal foramen as it enters the maxillary bone. The distal narrowing is located at the apex of the first maxillary incisor tooth.3,8,11,12
Dacryocystitis, or inflammation of the lacrimal sac, is a common problem in rabbits.1,8,16 Clinical signs consistent with dacryocystitis and nasolacrimal duct obstruction include unilateral epiphora and/or purulent ocular discharge (Fig 3).4,8,11 Chemosis and hyperemia consistent with conjunctivitis is also frequently observed.1,8,11,16 however dacryocystitis can sometimes be differentiated from bacterial conjunctivitis by placing pressure on the skin beneath the medial canthus and expressing purulent material from the nasolacrimal punctum.1,8,16 In some severe cases of dacryocystitis, the lacrimal sac may be visibly and palpably distended with discharge.1,8 The presence of chronic purulent discharge can result in corneal edema, keratitis, or even corneal ulceration.1,8,11,18 Additional secondary findings include nasal discharge, panophthalmitis; damp, matted fur on the nose and periocular region, as well as hair loss and red, crusted skin around the medial canthal region.8,11 Attempts to irrigate the nasolacrimal duct are usually unsuccessful, with fluid or debris flushing back out into the eye or opalescent, gritty tears flushed out.3,7,12
Dacryocystitis is often precipitated by obstruction of the nasolacrimal outflow duct.16 The circuitous route of the nasolacrimal duct brings it into close association with the roots of maxillary incisor and premolar teeth.1,8,11,16,18 Therefore, periapical disease of maxillary incisor and cheek teeth roots, specifically elongation, soft tissue inflammation, abscessation, and/or associated osteomyelitis and bone remodeling, can result in stenosis and eventual blockage of nasolacrimal outflow.1,8,16,18 In a retrospective study of 28 rabbits with dacryocystitis, oral examination revealed underlying dental disease in 53% of cases.8 Nasolacrimal duct stenosis usually presents as epiphora initially, but reduced tear flow can also promote bacterial overgrowth leading to inflammation and erosion of duct submucosa.1,16 This inflammation alters the tear film, which becomes viscous and gritty, further blocking the duct at its narrowest points.1
Less frequently, a primary bacterial infection can be established within the nasolacrimal system.8,17 Historically, this has been described as a component of upper respiratory infection (e.g. Pasteurellosis), but other bacteria have also been reported (see Culture below).1
Objectives of nasolacrimal flushing
Nasolacrimal flushing serves both diagnostic and therapeutic functions. Nasolacrimal flushing can be used to evaluate the patency of the duct and collect samples for culture or cytology. When managing the clinical patient, nasolacrimal flushing is used to irrigate the duct thereby improving tear flow and removing debris. Nasolacrimal flushing may be successful in curing primary bacterial infections and this technique can also significantly improve secondary dacryocystitis caused by underlying dental disease.
Nasolacrimal flushing is a two to three-person procedure. One person is needed to restrain the rabbit, another is needed to pass the cannula and flush the nasolacrimal duct. An optional third person can manipulate the light source as needed and collect fluid or debris that passes from the nares.
Equipment required includes:
- Topical anesthetic drops (0.5% proparacaine hydrochloride or 0.5% bupivacaine)
Clinical Tip: Most rabbits tolerate cannulation of the punctum with topical anesthesia and generally do not require sedation unless additional diagnostics are performed (see below). A conscious rabbit can also sneeze out any expelled material that accumulates within the nasal cavity and nasopharynx, thereby reducing the risk of aspiration of any purulent material.1
- Sedation or general anesthesia (optional, see topical anesthetic drops above) can serve to reduce patient stress and improve compliance.
- Warm, sterile, physiologic saline or eye flush solution (~2-3 ml per eye).8 Acetylcysteine can be added to the irrigation fluid to break down obstructive debris.8,17 Always irrigate with saline first.
- 20- to 27-gauge cannula: Irrigate the nasolacrimal duct using a soft Teflon intravenous catheter with the stylet removed, a soft plastic irrigating cannula, or a tomcat catheter in a large rabbit.15 Select a new catheter for each eye.1,4
Clinical Tip: Select the largest cannula that passes comfortably. If the gauge is too narrow, a good seal cannot form and sufficient pressure cannot be achieved to facilitate flushing.
- 3-6 ml luer lock syringe
- Adjustable, bright light source
- Fluorescein dye
Clinical Tip: Use fluorescein to stain the cornea and identify concurrent corneal defects. A common site of corneal ulceration associated with dacryocystitis is the nasoventral quadrant.17 Fluorescein dye can also be used to test subtle nasolacrimal duct patency (Jones test).
- Sterile petri dish or collection cup (optional)
- Microtip culturette swabs (optional)
Complications are possible, but fortunately rare, during nasolacrimal flushing.
- In patients with dacryocystitis, the nasolacrimal duct is often inflamed and potentially fragile.3 The risk of iatrogenic damage to the lacrimal sac and nasolacrimal duct is greater in these patients when metal cannulas are used.1,15
- If the lacrimal sac is ruptured, particularly with aggressive or high pressure flushing, fluid can be forced into periorbital tissues. This can result in discomfort from swelling at the medial canthus or even exophthalmos.1 Although this fluid is usually resorbed within a few hours and the position of the eye will return to normal 3,17 , analgesia and possibly antibiotics may be warranted since retrobulbar abscessation has been reported.3,17,20
- Occasionally, rupture of the nasolacrimal duct can create a new drainage exit into the nasal cavity. This rupture may lead to resolution of clinical signs, even though the nasolacrimal system remains obstructed.20
- It is also possible to inadvertently scratch the cornea during nasolacrimal flushing, particularly if the rabbit suddenly jumps.1,3
Nasolacrimal flushing is a relatively easy, straightforward technique.7
- First, warm the sterile fluid to be infused in syringe(s).
- Instill topical anesthetic into the lower conjunctival sac approximately 1-2 minutes before beginning the procedure.
- If the rabbit is anesthetized, position the head so that the nose is lower than the base of the skull to reduce the risk of aspiration.3
- Visualize the lacrimal punctum by gently retracting and everting the lower eyelid. The punctum will initially appear as a slit, but the opening will widen with further eversion
- Attach the syringe to the cannula, then gently insert the cannula into the punctum at a 45-90°angle. Advance the cannula 3-10 mm in a ventromedial direction.9,12 The cannula should slip easily into the duct. Never force the cannula, and take care to avoid contact with the cornea.1,3
- Gently flush the duct using saline solution, ophthalmic solution, or in select cases, aceytylcysteine.17 Always begin with saline.
- Apply short, gentle bursts of pressure if the cannula does not flush easily. Gently advancing and withdrawing the cannula 1-2 mm may relieve an area of occlusion. Flushing can then be attempted again.20 Covering the punctum opening with a moistened cotton-tipped applicator while flushing may also help.
- Carefully irrigate the nasolacrimal duct until clear fluid exits the nares.
- Select a new cannula when flushing the contralateral duct to prevent the spread of infectious organisms.1
Management of dacryocystitis involves cannulation and flushing of the duct followed by frequent administration of appropriate topical ophthalmic antibiotics. Systemic therapy may also be indicated in severe cases.
Frequent irrigation is often required to relieve an obstruction and/or manage a serious infection.1,7 This can range from every 3 to 4 days, to multiple flushes per week, or even daily for serious infections.1,17,18 Irrigation is an important part of treatment however frequent flushing can be very stressful for the rabbit so weigh the risks and benefits of your treatment regimen carefully.17
Nasolacrimal flushing is performed with sterile saline, ophthalmic antibiotic solution, acetylcysteine, or some combination.1,8,10 If two or three nasolacrimal flush attempts are unsuccessful, consider contrast radiography of the nasolacrimal system (see Additional diagnostics below).
Clinical Tip: If ophthalmic antibiotic drops or ointment are used, keep the risk of intestinal dysbiosis in mind since rabbits frequently groom and ingest some topical medications.
Suitable first line ophthalmic antibiotics for use in rabbits with dacryocystitis include chloramphenicol, aminoglycosides, fluoroquinolones, such as ciprofloxacin or ofloxacin, and triple antibiotic combinations (aminoglycoside/polymyxin B/bacitracin).7,8 Depending on the severity of the clinical signs, acetylcysteine, vitamin A ointment, or nonsteroidal agents can also be applied topically.8
Rabbits with advanced disease will also require systemic antibiotic therapy alone or in combination with nonsteroidal anti-inflammatory drugs.1,8,18 Williams (2012) recommends a short course of oral azithromycin.18
Management of obstruction
If cannulation from the proximal punctum is not possible, retrograde nasolacrimal duct cannulation via the distal nasal ostium may be attempted while the patient is under general anesthesia.15,18,19 Cannulation with a monofilament nylon suture (0 to 2-0) has also been described to restore duct patency 7, however, as the majority of intractable obstructions are extraluminal in origin, this approach is often unsuccessful.20
It is imperative that underlying dental disease is appropriately managed 1, however, significant bone remodeling is often present.16,17 Therefore efforts to manage dacryocystitis in these patients must be considered palliative because the changes to the nasolacrimal system are generally irreversible 16,17, and many cases of dacrocystitis will not resolve regardless of how aggressive the therapy instituted.1,20
Culture and cytology
- Sample purulent discharge from the punctum or the ventral conjunctival fornix with a microtip culturette that has been moistened with sterile distilled water.6,8 Avoid contact with the lashes and eyelid skin to minimize contamination with normal skin flora, and keep in mind that use of topical anesthetic can affect culture results.6 Material or fluid exiting the nasal meatus can also be collected into a sterile petri dish.7,11,12 If clinical signs are bilateral, collect a separate sample for each eye.6
- Cytological evaluation includes direct and cytocentrifuged slides stained with a modified Wright-Giemsa stain.11 Healthy rabbits exhibit a low cell count (median 500 cells/ μL) with a predominance of heterophils and an absence of other polymorphonuclear cells.11
- Interpretation of culture results is challenging since many bacteria isolated can be part of the normal bacterial flora of the ocular surface and culture results must be interpreted in conjunction with clinical signs.2 Bacteria frequently recovered from conjunctiva of apparently health rabbits include Corynebacterium spp., Staphylococcus spp., Pasteurella spp., Moraxella spp., Streptococcus viridans, Micrococcus spp., and Neisseria sp.2,6,12 Dacryocystitis has mainly been associated with Gram-positive bacteria, such as Staphylococcus spp. and Streptococcus spp.17 Other bacteria that have been reported include Gram-negative organisms, such as Pasteurella, Pseudomonas, and Moraxella sp.13
In one survey of rabbits with dacryocystitis, dental malocclusion and tooth root elongation were observed in over 50% of cases.8 Therefore it is imperative that underlying dental disease be recognized and treated.1 Perform a complete oral examination to look for evidence of dental disease.1,8,18
Clinical workup of dacryocystitis should also include skull or dental radiographs, if indicated 1, to assess the position, shape, and structure of tooth roots as well as their relationship to the eye or surrounding bone.3
Plain radiographs can be difficult to interpret. Dacryocystography, or contrast radiography of the nasolacrimal system, can be used to outline the path of the nasolacrimal duct and identify the site of an obstruction, when present.1,3 Under heavy sedation or general anesthesia, the contrast agent is slowly infused into the nasolacrimal duct.13 The recommendations for volume and type of contrast agent have varied in the literature from 0.2-0.5 ml of non-ionic contrast agent to 1-2 ml of a dilute preparation of a water-soluble organic iodide contrast, such as iothalamate sodium/meglumine.5,9,15,18
Contrast-enhanced computed tomography can also be useful in determining the exact location of bony abnormalities and the extent of nasolacrimal system damage. Lacrimal endoscopy has also been described in rabbits in an experimental setting.10
For images of CT dacryocystography in the rabbit, view Figure 9 in the open access article Hou K, Ai T, Liu R, et al. Modeling chronic dacryocystitis in rabbits by nasolacrimal duct obstruction with self-curing resin. J Ophthalmol. 2017;2017:3438041. doi: 10.1155/2017/3438041.9