LENS LUXATION IN A CROSSBREED TERRIER

History

A four year old, female neutered Yorkshire Terrier X Jack Russell Terrier presented to the clinic for routine annual vaccination. The owner reported an "odd looking" left eye for the past four weeks with some corneal cloudiness.

Examination

A full clinical examination was undertaken along with an ophthalmic examination as per Appendix 1. All relevant findings are recorded below.

OS: - Anterior luxated lens evident (fig. 37)

- Central patchy corneal oedema

- Immature cataract in lens

- Darkened iris compared with OD

- Dilated pupil c.f. OD (fig. 38)

- Dazzle reflex absent

- Menace response absent

- PLR, direct and indirect - absent

OD: - Subtle iridodenesis

- No cataract evident

- Small aphakic crescent dorsally

STT L = 22mm/minute, R = 18mm/minute

IOP L = 52mmHg, R = 20mmHg

Figure 37: On presentation, anteriorly luxated lens with immature cataract.

Figure 38: On presentation, anisocoria (dilated left pupil).

Diagnosis

OS - complete lens luxation with secondary glaucoma, blind globe

OD - lens subluxation

Treatment

Enucleation OS, via a transpalpebral approach

Continued monitoring OD

Follow Up

Twelve months later the iridodenesis in the left eye had become more obvious together with phacodenesis. Vitreal wisps were evident at the lens equator. Surgical lens removal is being considered but cost is an issue.

Discussion

Lens luxation can occur either as a primary or a secondary event. Primary lens luxation is due to a zonular defect, believed to be inherited, causing "bizarre reticulate formations" of zonular fibres (1). The terrier breeds especially are over represented in the groups of dogs showing primary lens luxation. The Miniature Bull Terrier, Tibetan Terrier, Parson Jack Russell Terrier, Lancashire Heeler and Border Collie are encompassed within the BVA/ KC Eye Scheme for this condition.

The dog in this case was a small terrier from a Jack Russell mother and a Yorkshire Terrier father so although it was a mixed breed, it had a genetic inheritance predisposing to primary lens luxation. The owner reported that the eye seemed sore about one month previously then had become progressively more opaque. This suggested that the lens had become cataractous post luxation. This fact, along with the bilateral nature of the disease, made the presumptive diagnosis that of primary lens luxation.

Lens luxation can occur as a secondary event after other intraocular disease such as cataract formation or glaucoma. This may be due to mechanical stretching of the zonules or inflammatory degeneration. There was no evidence of other intraocular disease in the contralateral eye despite evidence of lens instability. The dog was also within the most common age range for luxation of the lens in primary disease.

An aphakic crescent is seen dorsally due to rupture of the dorsal zonules first. This is believed to be due to those zonules supporting the weight of the lens breaking first, although all zonules are equally affected by the degeneration. Anterior dislocations are reported more frequently than posterior dislocations, however luxated lenses can move between these two positions (2).

Glaucoma was originally believed to be involved in the pathogenesis of this disease but serial tonometry on affected dogs showed that this is not the case (1). However glaucoma is a serious secondary complication associated with lens luxation. This may either be from inflammatory debris or vitreous clogging the aqueous outflow, or the mass of the anteriorly displaced lens obstructing outflow. Secondary glaucoma is much more frequently reported with anterior rather than posterior lens luxation. In this case the glaucoma had already damaged the left eye to such an extent that surgery to remove the displaced lens would be pointless so enucleation was performed.

The next question to be addressed is the remaining eye which is also affected by degeneration of the zonules and is therefore also at risk of lens luxation. Medical management of these cases involves continued monitoring of the lens, alongside the use of miotics to try to trap the lens posterior to the iris where the risk of secondary glaucoma is much less. However there is a risk that the lens will still displace forward of the iris despite miosis. Also the dog is left with a small pupil and a functionally aphakic eye, significantly affecting vision.

The treatment of choice is to surgically remove the lens (3). This has been traditionally done to the lens through an "open sky" technique, removing the lens complete within its capsule. This can be done immediately post luxation or as a prophylactic procedure prior to luxation once the problem has been identified. Some surgeons recommend performing the procedure on the subluxated lens at the same time as removing the luxated lens (4). The prognosis is improved by removing the lens by phacoemulsification prior to luxation. Using the "open sky" technique negates the benefit of performing the procedure prophylactically for long term prognosis (5). Even with prophylactic surgery prior to luxation there is a significant risk of retinal detachment post surgery due to loss of support from the vitreal face.

Glaucoma is a risk even with surgery, probably due to the vitreal wisps that occur prior to luxation around the lens equator through the disrupted zonules, along with inflammatory debris.

It is usual in the UK to leave these dogs aphakic post surgery, which does give significant visual compromise. Surgery has been described to place a lens by suturing the haptics of the replacement lens into the ciliary sulcus (5). Although the results were not as good as "in the bag" lenses, vision is still better than with no lens in place at all.

In this case the remaining lens, although becoming progressively more unstable over time, has yet to luxate. The owners are presently considering the option of prophylactic lens extraction.

References

1. Curtis R. (1990) Lens luxation in the dog and cat. Veterinary Clinics of North America: Small Animal Practice. Vol. 20 (3) 755 - 773.

2. Curtis R, Barnett KC, Lewis SJ (1983) Clinical and pathological observations concerning the aetiology of primary lens luxation in the dog. The Veterinary Record 112, 238 - 246.

3. Glover TL, Davidson MG, Nasisse MP, Olivero DK (1995) The intracapsular extraction of displaced lenses in dogs: a retrospective study of 57 cases (1984 - 1990). Journal of the American Animal Hospital Association 31, 77 - 81.

4. Nasisse MP, Glover TL. (1997) Surgery for lens instability. Veterinary Clinics of North America : Small Animal Practice. Vol 27 (5) 1175 - 1193.

5. Manning S, Renwick P, Heinrich CL, Cripps P. (2006) The surgical management of lens dislocation in the dog: a retrospective study of 102 cases (155 eyes) (1994 - 2004). BSAVA Congress Clinical Research Abstracts.