Clinical and pathological observations concerning the aetiology of primary lens luxation in the dog.

A series of 123 consecutive referred cases of lens dislocation in the dog were classified as primary (100), secondary (21) and congenital (two). Cases designated as primary lens luxation comprised only the terrier breeds or crossbreds and collectively appeared to represent a single clinical entity characterised by age of onset (mean four to five years), essential bilaterality and the apparent absence of antecedent ocular disease. Elevated intraocular pressures were encountered in many eyes exhibiting subluxation. However, in three Tibetan terriers, on which tonometry was performed on a daily basis for approximately one year during the period of subluxation and before the development of luxation, there was no evidence of abnormal intraocular pressure change, suggesting that ocular hypertension in primary lens luxation is a secondary and not a primary development. All but one of nine affected eyes representing three terrier breeds showed abnormalities of the suspensory apparatus of the lens (zonule) which resembled those reported previously in Tibetan terriers bred for the condition.