PHTVL / PHPV Persistent hyperplastic tunica vasculosa lentis (PHTVL) and persistent hyperplastic primary vitreous (PHPV) refer to the persistence of the embryonic vascular system of the lens. PHTVL /PHPV is a congenital eye anomaly which has been described in many animals as well as in man. Until the late 1970's PHTVL / PHPV was only sporadically described in a litterature in animals including dogs. The first documented case in dog was in a Greyhound (Grimes and Mullaney). After that there were also some descriptions of individual cases of this anomaly in the litterature (Keller, Blanchard and Krehbiel, 1972, Gelatt, 1973, Barnett and Grimes, 1973, Rebhun, 1976, Rubin, 1974, ).
In 1978 a preliminary raport (Stades, v.d.Linde-Sipman, Gutteling) and 1980 a more detailed report (Stades) was published in The Netherlands describing the occurence of PHTVL / PHPV in the Dutch Dobermann population.Later this anomaly was reported to exist in the standard Schnauzer (Slatter, 1981), in Staffordshire Bull Terrier (Curtis, Barnett and Leon 1984, Leon, Curtis and Barnett 1986), in Bouviers des Flanders (v. Rensburg, Patrick, v.d.Lugt and Smit, 1992). For to help to better understand and discribe the diversity of the clinical signs of PHTVL / PHPV the Dutch reporters devided the signs into six grades according the severity of the findings. (Boeve, Stades, v.d. Linde-Sipman, Vrensen; Prog Vet & Comp Ophtalmol. Vol 2, No 4) (Figure 1.)
Grade1: Retrolental fibrovacsular pigmented dots (with a diameter about 0,1 mm) alone on the posterior capsule of the lens.
Grade2: Dots in combination with a retrolental tissue proliferation (plaque), attached to the posterior lens capsule.
Grade3: Plaque in combination with persistent parts of the hyaloid (-TVL) vascular system
Grade5: Plaque, lenticonus posterior, and persistent parts of the hyaloid-TVL system (a combination of grades 3 and 4)
Grade6: Abnormal lens shape due to the colobomata or mikrophakia, possibly in combination with the elongated ciliary processes and intra- or retrolental free blood, all in addition to severe (grades 2-5) anomalies.
Grade1 with retrolental dots alone do not develop into a cataract and do not interfere with vision. All other grades progressively worsen and cataract develops. Thus there is a chance of severe impairment or even total loss of a vison.
In grade1 cases the anomaly can be uni- or bilateral. In the more advanced cases it is mostly bilateral, although the severity of the problem can vary in the both eyes. This anomaly is due to an embryological defect in the prenatal ocular development. Normally an arteria (a. hyaloidea) grows from the retinal area through the primary vitreus to the posterior lens capsule. A.hyaloidea forms an vascular net behind the lens and around it (tunica vasculosa lentis). All this is needed to feed the lens and nearby structures. By day 45 of gestation the development of the lens is nearly finished and the network beginns to atrophy. In defected dog there is a metabolic defect and the regression is impaired. Pigment dots against the posterior lens capsule are remnants of the vascular structure failed to atrophy (grade1). In more severe cases there are more advanced defects seen already at the gestation day 35 onward. The problem seems to be originate from the persistence of the vascular structure, thus the name PHTVL is used primarly as a name of this defect. This makes this anomaly different from the human disease where PHPV is described as a non-hereditary mainly unilateral eye defect.
The authors described the heredity to be possibly autosomal incomplete dominant, with variation in the expression ( Stades, v.d. Linde-Sipman, Boeve ). The authors also claim that "a more complex heredity can not be excluded". In onother article the same author suggests a possible interaction of two or three genes.(Stades 1980) There was found no difference in the incidence of the affected dogs between the sexes or among coat colour combinations.
The lens and nearby structures can be examined for PHTVL as early as 7-8 weeks of age. The examination should be done by a veterinarian specialized in eye diseases and with proper slit-lamp biomicroscope equipment. This procedure is strongly courraged to be executed by the breeder. The examination is peformed after inducing total enlargement of the pupils and needs no sedation. It is a common practice to re-exam the dogs before their use in breeding. It is not very easy to discover the grade1 anomaly in a very small and lively puppy at the age of 7 weeks. There appears to be some differences in the results made in a puppyhood compared to the adult result. The anomalies grading higher than 1 are more easy to be recognized even in an early age.
The mode of inheritance of PHPV is not so clear, but it is known that it is a congenital condition (present at birth) and that it is not progressive. This means that if a puppy is born with PHPV it can be detected by ophthalmic screening from 6 weeks of age and if it is affected, whatever the condition of the problem at that stage it will not change throughout the dogs life.
Either of the above conditions can be operated on, but it is a serious operation and can be traumatic and very expensive. It is not always covered by insurance due to the hereditary nature.
Even though the genetic test is now available for Hereditary Cataracts it is still important to screen for PHPV