Congenital infection is more common later in gestation, but disease manifestations are worse if acquired earlier in gestation (Dunn et al., 1999). Classically, congenital disease is associated with bilateral macular scarring, but acquired infection can also result in macular disease and, rarely, bilateral scarring as well (Glasner et al., 1992). Other manifestations include optic neuritis, iritis, neuroretinitis, retinal vasculitis, acute retinal necrosis, recurrent iridocyclitis, and persistent vitritis. Long-term follow-up of congenitally infected children results in identification of further ocular sequelae not present at birth - for example, in one study four of six untreated congenitally infected children developed scars subsequent to birth during the next 20 years (Koppe et al., 1986). It is estimated that 85 percent of infants untreated and without ocular lesions at birth will subsequently develop ocular toxoplasmosis (Koppe et al., 1974; Wilson et al., 1980). Microphthalmos and microcornea can occur as a consequence of severe congenital eye disease (Suhardjo et al., 2003). Nystagmus and strabismus and amblyopia secondary to congenital toxoplas-mosis are more complex than even most expert ophthalmologists realize (O'Neill, 1998). There is a tendency for clinicians not to struggle with the complex care involved in trying to achieve optimal visual outcome in congenital infection. Less initially severe but still disabling disease such as anterior uveitis (Cano-Parra et al., 2000) secondary to T. gondii is likely underdiagnosed because of the limitations of current non-invasive diagnostic tests.
Exposure to Toxoplasma 6 months prior to conception is thought to eliminate the possibility of congenital transmission secondary to lifelong immunity in immunocompetent individuals. Rarely, reactivation of toxoplasmosis in previously infected immunodeficient women can result in congenital transmission of toxoplasmosis (Mitchell et al., 1990). There is one recent case report of treated acquired ocular toxoplasmosis during pregnancy occurring in the mother without any subsequent fetal disease (Ramchandani et al., 2002). The exact mechanism of transmission is not yet understood, but is thought to be secondary to transplacental transmission of the parasite. The severity of ocular manifestations parallels the severity of CNS disease in congenital infection (Roberts et al., 2001).
A recent report highlighted the ophthalmic findings of congenital Toxoplasma infection in treated and untreated individuals (Mets et al., 1996): 79 percent of children had retinochoroidal scars, 28 percent of individuals had significant unilateral vision loss, and 29 percent of children had bilateral vision loss. The presence of inactive chorioretinal lesions in congenitally infected newborns indicates that the complete cycle of infection, activation, and resolution of chorioretinal lesions may occur in utero (Guerina et al., 1994; Mets et al., 1996). The New England Regional Toxoplasma Working Group detected 100 of 635000 infants who were seropositive for IgG and IgM against Toxoplasma. Of 39 treated children observed for as long as 6 years, 4 had new postnatally developed retinal scars; a separate 9 of 48 patients had retinal lesions at birth (Guerina et al., 1994). In a different study from England, after 20 years of follow-up, 9 of 11 patients with congenital toxoplasmosis had evidence of chorioretinitis and 4 had severe impairment (Koppe et al., 1986).
The largest report of congenital toxoplasmosis in twins highlights that multiple factors beyond time of exposure during gestation influence ocular outcome in congenital infection (Peyron et al., 2003). Although there are possible confounding issues of shared placentas and mortality (as is true of any infectious congenital disease involving twins), if concordance of the disease is more common among monozygotic twin than among dizygotic ones, then genetic susceptibility is likely more important than environmental influence in disease outcome. While there is no rigorous protocol that has been published focusing on a cohort of ocular outcome in twins (Rieger, 1959), it appears there is a lack of identical outcome between eyes and between twins. It is therefore not time or inoculum alone that leads to presence or absence of ocular disease, size of lesions, or location of lesions (Couvreur et al., 1976). There are differences in specific ocular outcomes in both dizygotic and monozygotic twins. The general disease impact with respect to symptomatic involvement and eventual ocular involvement appears more concordant in dizygotic than in monozygotic twins. It is clear, in order definitively to assess patterns of ocular toxoplasmosis in twins, a long rigorous follow-up report remains to be published.
It is unclear why macular lesions commonly occur in congenital infection. Other frequently involved areas in the brain are the periaqueductal, periventricular, and basal ganglia regions. One theory is that, secondary to a high-affinity transport protein for putrescein, T. gondii thrives in the putrescein-rich fetal retina (Seabra et al., 2004). A separate theory is that the macula is the first part of the retina that is vascularized, as the vasculoge-nesis spreads perirpherally from the central posterior retina to the far periphery. The macula is therefore affected because it is the region that has been vascularized for longest and is thus more likely to be infected than the peripheral retina.
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