The term lens luxation means complete dislocation of the crystalline lens from its natural site in the hyaloid fossa, while partial displacement is called subluxation (Fig. 1).
In order for the lens to move out of its natural site, there must be partial or complete rupture of the zonular fibres (suspensory ligaments) that tether the ciliary processes to the equator of the lens. Following rupture of these fibres the lens may move forwards into the anterior chamber, backwards in the vitreous chamber or remain blocked in the papillary hole. In particular stages or in chronic states the lens can be completely luxated posteriorly and become located in the vitreous cavity on the floor of the eye. During these movements, the lens may assume a medial, lateral, ventral or, more rarely, a dorsal position. If the lens moves in a retrograde direction, the anterior chamber becomes deeper and, in some cases, asymmetrical. These movements can only occur when there is concomitant degeneration of the vitreous humour or vitreous syneresis, characterized by a type of liquefaction, with consequent loss of the consistency of the vitreous humour and rupture of the anterior part of the hyaloid membrane. In contrast, anterograde dislocation of the lens decreases the depth of the anterior chamber and shifts the iris forwards. Sometimes the lens moves freely from a posterior position in the vitreous chamber towards the anterior chamber and vice versa.
Lens luxation enables observation of the equatorial zone of the lens, which cannot usually be evaluated unless a biomicroscope is used with the pupil widely dilated. The luxation creates a characteristic and pathognomonic sign called the aphakic crescent. This is an area of the pupil not covered by the lens which allows a reflection from the fundus to be seen; this reflection is usually crescent-shaped and can be of variable size, depending on the degree of displacement of the lens. The aphakic crescent can be located dorsally, medially, centrally or laterally depending on whether the lens has shifted ventrally, laterally, dorsally or medially, respectively (Fig. 2). During luxation or subluxation of the lens it is often possible to see parts of the prolapsed vitreous humour in the anterior chamber. The clinical signs of phakodonesis and iridodonesis may follow partial or complete displacement of the lens. The term phakodonesis indicates a slight movement of the lens within the eyeball following movement of the globe itself, while iridodonesis means trembling of the iris following its loss of contact with the lens structure behind it. Chronic dislocation of the lens can cause complete opacification of the lens or cataract.
AETIOLOGY AND PATHOGENIC MECHANISMS
Luxation of the lens can be classified as congenital, primary or secondary.
The congenital form is often associated with other ocular abnormalities and is present at birth or develops in the early period of life.
Primary luxation is said to be present in animals in which it is not possible to identify a triggering cause. It occurs at a higher incidence in some breeds, such as:
- Australian Blue Heeler
- Border Collie
- Wire Fox Terrier
- Jack Russell Terrier
- Miniature Bull Terrier
- German Shepherd dog
- Sealyham Terrier
- SharPei
- Tibetan Terrier
- Italian Volpino
Primary luxation is more often bilateral, although the two eyes are not necessarily affected contemporaneously. The age at onset varies depending on the breed (mean age from 3 to 6 years) and although the underlying cause has not been defined in many dogs, it is thought that a genetic mechanism may be one of the factors triggering this pathology.
Secondary luxationof the lens is usually a consequence of chronic inflammation – of various causes – which weakens and then ruptures the zonular suspensory ligaments or, less frequently, may be the result of acute trauma. Intraocular tumours may also cause mechanical displacement and consequent subluxation or luxation of the lens as a result of the growth of the neoformation. In the case of glaucoma with secondary buphthalmos the zonular fibres become detached due to the increased volume of the eyeball. In the case of a swollen cataract the fibres rupture because of increased volume of the lens and concomitant inflammation (lens-induced phacolytic uveitis).
DIAGNOSIS
Subluxation and luxation of the lens can be diagnosed clinically through the use of appropriate instruments that enable sufficient magnification for correct observation of the anatomical structure of the lens. In most cases of subluxation, a mydriatic drug, possibly short-acting, must be given in order to be able to identify the edge of the lens and, thereby, diagnose its displacement. In cases in which the cornea is opaque, for any reason, and the anatomical structures behind it cannot be seen, an ultrasound examination, using a probe with a suitable frequency, may be helpful.
COMPLICATIONS
Lens luxation may be followed by secondary complications of the eye such as the development of glaucoma, secondary lesions of the cornea due to contact between the lens in the anterior chamber and the corneal endothelium, chronic uveitis and retinal detachment.
TREATMENT
The treatment of luxation may be conservative or surgical depending on the position of the lens, the duration of the pathology, the visual capacity of the animal, as well as its general condition and age. The use of miotic drugs is indicated when the lens is luxated or subluxated posteriorly or is in the vitreous chamber in order to prevent the lens from shifting into the anterior chamber or to prevent the so-called papillary block (entrapment of the lens in the papillary hole). These two events often create the conditions for triggering the development of acute secondary glaucoma (Fig. 3). In the case that the lens luxates into the anterior chamber, the recommended treatment is surgical removal of the lens through an incision in the cornea and asportation of the lens itself (intracapsular technique) or through a process of phacoemulsification (extracapsular technique). After this operation the subject may be left without a lens (aphakic) or an intraocular lens can be implanted and fixed in the sulcus. In some cases, an attempt can be made to resolve the situation without any surgery by exerting gentle pressure with the fingers on the cornea, trying to move the lens again from the anterior chamber to the vitreous humour, taking care to keep the head tilted backwards. This manoeuvre is performed more easily with the animal sedated and after the administration of hyperosmotic drugs in order to reduce the volume of the vitreous humour.
LUXATION OF THE LENS IN THE CAT
Luxation of the lens is usually a consequence of anterior uveitis (pars planitis), trauma or glaucoma. In this species the incidence is higher in subjects with toxoplasmosis or feline immunodeficiency syndrome. The pathogenesis of the secondary forms is the same as that in the dog. In the cat, primary luxation of the lens has been described in association with microphakia. Surgical treatment involves asportation of the luxated lens if this has been displaced into the anterior chamber.
Suggested readings
- Curtis R, Barnett KC.Primary lens luxation in the dog. J Small Anim Pract. 1980. Dec;21(12):657-68.
- Curtis R.Lens luxation in the dog and cat. Vet Clin North Am Small Anim Pract. 1990 May;20(3):755-73. Review.
- Gwin R,Samuelson D,Powell N,et al.Primary lens luxation in the dog associated with lenticular zonule degeneration and its relationship to glaucoma. J Am Anim Hosp Assoc 1982;18:485-491.
- Slatter D. Disease of the canine lens and cataract formation. In: Slatter D, ed. Fundamentals of Veterinary Ophthalmology. 4rd ed. Philadelphia: Saunders WB, 2007: 859-887.
- Slatter D. Surgery of the canine lens. In: Slatter D, ed. Fundamentals of Veterinary Ophthalmology. 4rd ed. Philadelphia: Saunders WB, 2007: 888-931.