Epidemiology

Infection is spread mostly through sexual transmission. Until the mid-1990s homosexual and bisexual activity was responsible for most of the transmission. Now, heterosexual activity is the major route of transmission in developed countries. Intravenous drug abuse is another common cause of disease transmission. Perinatal transmission from an infected mother to her offspring can occur in utero. Transmission can also occur during delivery or by breastfeeding. Finally, transmission to healthcare workers can also occur, usually as the result of a needlestick injury. Seroconversion to HIV after a needlestick injury is about 0.3% depending on viral load; this is about 10–100 times less than that with hepatitis C or B. Before the availability of a serologic test for HIV antibody, large numbers of patients, including those with hemophilia, were exposed to the virus through transfusion of blood products.

Although the number of patients with newly diagnosed HIV infection appears to be declining in the United States, HIV disproportionately affects certain segments of the population, including injection drug users, commercial sex workers, people living in poverty, and men who have sex with men. According to Morbidity and Mortality Weekly Report 2001, HIV infection has caused approximately 20 million deaths, and about 30 million people are infected. In 2006, an estimated 39 400 persons were diagnosed with HIV in 22 states. Extrapolation from these data suggests 56 300 new infections and an annual incidence rate of 22.8 per 100 000 population.

Diagnosis

HIV infection can be detected by the presence of antibody to viral antigens, 2–8 weeks after infection. The antibodies do not control the infection nor prevent its sequelae. Diagnosis of HIV infection is usually made by an enzyme-linked immunosorbent assay (ELISA) and is then confirmed by a Western blot test. ^, Some strains of HIV are not detected by some ELISA tests. Nevertheless, these tests are almost 100% sensitive, albeit not 100% specific, so false-positive results can occur. The virus can also be cultured from blood, semen, and solid tissues, but only rarely from saliva and tears. In the eye, the virus has been found in the cornea, vitreous, and retina. Rapid tests are now commercially available for HIV, although testing algorithms for these tests are still being assessed.

HIV disease

An acute retroviral syndrome usually occurs within 1–6 weeks after HIV infection. Patients typically develop fever, rash, myalgias, headache, or gastrointestinal symptoms. The CD4+ count is reduced, and many patients have elevated liver enzyme levels. Without treatment, CD4+ counts decline by about 75 cells/µL/year. The time from initial infection to the development of a disease that meets the definition of AIDS is about 10 years. AIDS is the most severe manifestation of HIV infection and occurs at a point at which the immune system is so damaged by the infection that opportunistic infections such as Pneumocystis jiroveci , Cryptococcus neoformans , cytomegalovirus (CMV), oral candidiasis, or unusual malignant processes such as Kaposi’s sarcoma can emerge. ^, There is also an encephalopathy caused by direct HIV infection of the brain.

Progression of HIV disease is related to the CD4+ lymphocyte count. The amount of HIV-1 viral RNA predicts the course of HIV disease, specifically, how rapidly the disease is likely to progress. Persons with HIV loads >30 000 copies/mL have an 80% likelihood of developing AIDS within 6 years. In contrast, those with HIV loads <500 copies/mL have a 5.4% chance of developing AIDS. CD4+ lymphocyte counts are good predictors for the development of specific clinical manifestations of the disease, particularly opportunistic infections. For example, most cases of P. jiroveci pneumonia occur when CD4+ counts fall to <200 cells/µL. Typically, CMV retinitis occurs when CD4+ counts are <50 cells/µL.

HIV therapy

There is still some debate on when to start antiretroviral therapy for HIV disease. The decision regarding initiation of treatment should be individualized but can be based on symptoms, HIV-1 RNA level and CD4+ count. The potential benefit of antiretroviral therapy must be based on the risks of therapy and the impact on quality of life of adhering to a strict therapeutic regimen which is required to avoid drug resistance. Some clinicians start treatment as soon as HIV infection is diagnosed, although many begin treatment when the CD4+ count drops to <350 cells/µL or if symptoms are present. The rapidity of the decline in CD4+ count and HIV load are other factors in the decision of when to start therapy. Others wait until there is a significant risk of the patient developing HIV disease in the near future.

When therapy is started, a highly active regimen consisting of a combination of antiretroviral agents should be employed to minimize resistance. The goal of therapy is to suppress plasma HIV-1 RNA to below detectable limits using a sensitive assay. This highly active antiretroviral therapy (HAART) regimen has also been called the ‘AIDS cocktail’ or ‘triple therapy,’ because it usually consists of three medications. All regimens should combine drugs with synergistic antiviral activity. Regimens can include nucleoside or nucleotide analogue reverse transcriptase inhibitors with a protease inhibitor or combinations of two nucleoside reverse transcriptase inhibitors with a non-nucleoside reverse transcriptase inhibitor. Other combinations have also been used successfully. Therapy is changed based on tolerability, HIV-1 RNA levels and CD4+ T-cell counts. Box 11-1 lists currently available antiretroviral agents. HAART regimens involve multiple medications that need to be taken at specific times. Again, lack of adherence to these regimens can lead to resistance and failure of therapy, so this must be closely monitored.

Ocular manifestations of HIV infection

Ocular manifestations of HIV infection occur in every tissue of the eye, from the eyelids to the optic nerve ( Box 11-2 ). The most common findings include dry eye, a retinal microvasculopathy, and CMV retinitis. Although direct HIV infection in the brain appears to produce a severe encephalopathy, there is still no definitive evidence that HIV infection of ocular tissue leads to clinically important pathologic effects. However, subclinical infection of retinal neural and endothelial cells has been reported. The retinal microvasculopathy of HIV occurs in 25–92% of patients and includes small dot retinal hemorrhages and nerve fiber layer infarcts called cottonwool spots ( Fig. 11-2 ). This retinopathy may be caused by interactions between viral antigens and antibodies that circulate in the blood and then deposit in the eye, but this has not been clearly shown. The incidence of this retinal microvasculopathy increases with the degree of immunosuppression. The presence of the p24 antigen, a surface HIV antigen, in the blood also increases with more advanced disease, and it is possible that this antigenemia may be a cause of this retinopathy, with immune complex formation in the retinal vasculature. Superficial and deep retinal hemorrhages, retinal perivasculitis, and vascular occlusions also occur. Small peripheral blot hemorrhages are frequently noted on retinal examination and appear to be more common with decreasing CD4+ counts and anemia. Electron microscopic studies of these lesions reveal swollen endothelial cells and degenerating pericytes. Inflammatory cell infiltrates and infectious organisms are not seen in cottonwool spots; however, the microvascular abnormalities are widespread throughout the retina and may provide an entrance for opportunistic viral infections of the retina.

Multiple nerve fiber layer infarcts (cottonwool spots) in patient with AIDS.

Clinically, cottonwool spots are seen as superficial white fluffy lesions in the retina ( Fig. 11-2 ). There may be small dilations of the microvasculature nearby. Visual symptoms are rarely associated with the presence of a cottonwool spot. A cottonwool spot cannot occur in the foveal avascular zone as there is no nerve fiber layer to become infarcted in this part of the retina. Perifoveal cottonwool spots may cause symptoms. Patients may notice a relative, but not absolute, scotoma. This can be useful in distinguishing a cottonwool spot from an early area of viral retinitis in which the scotoma is absolute. In patients with AIDS these lesions can sometimes be more than 0.5 disc diameter in size. They are distinguished from infectious retinitis by the fact that they do not enlarge over time. Although individual lesions will disappear over a period of several months, new lesions often appear. The retinal hemorrhages associated with HIV infection tend to be small dot hemorrhages and rarely cause visual symptoms. Occlusive vasculopathy, such as central retinal vein occlusions, may affect vision.

Dry eye is also a common finding in patients with HIV infection and may be due to reduced tear production associated with diminished lactoferrin and lysozyme. In most patients the condition is mild to moderately severe and can be treated with artificial tears. More severe cases of dry eye can occur. Dry eye associated with Stevens–Johnson syndrome has also been reported in patients with AIDS.

Even in the HAART era many patients with HIV infection have visual symptoms in the absence of active opportunistic infection. Visual acuity, contrast sensitivity, and visual fields are worse than expected compared to a normal age-matched control group. Although the exact pathogenesis of this neuroretinopathy is unknown, causes may include HIV vasculopathy or a direct effect of HIV on the retina and optic nerve, undiagnosed coinfection, or effects from therapy.

Progression

CMV retinitis progresses in two ways. First, new lesions that are not physically near an old one may form, probably by hematogenous spread. Second, and most commonly, an old lesion spreads at its borders to involve new, previously uninfected retina. In this latter situation the center of the old lesion eventually ceases to be white because the whiteness is due to scattering of light by edematous and necrotic retina. Once retinal cell death has occurred and only a glial scar remains, the atrophic retina is again transparent with underlying retinal pigment epithelial stippling ( Fig. 11-4 ). Therefore, when one assesses the progression and resolution of CMV retinitis it is most important to pay attention to the edge of the lesions to look for advancement of the border, and to not look at the center of the lesion and interpret a decrease in whiteness as an improvement.

CMV infection causes vision loss in several ways. Most commonly the infected retina is destroyed, and patients develop an absolute scotoma due to retinal necrosis. When the macula is involved, central vision is lost. Macular involvement can be a presenting sign of the disease, or occur late in the course. However, until the macula is involved by active retinitis, the vision can be nearly normal unless there is secondary macular edema from a nearby area of retinitis. Patients can retain normal central vision with only a few degrees of visual field in this disease, and they can be significantly disabled by the peripheral retinal destruction. The optic disc can also become involved, and the vision can be lost even when the amount of retinitis is still minimal. In some patients paramacular CMV infection will produce macular edema, which is a reversible cause of visual loss if it can be treated before the active virus progresses through the fovea. Last, CMV retinitis can cause a rhegmatogenous retinal detachment because there is vitreous traction on the thin, atrophic retina. As you will read in more detail later in this chapter, retinal detachments with CMV retinitis are difficult to treat.

CMV retinitis in the era of highly active antiretroviral therapy

The presentation and course of CMV retinitis in patients with AIDS have dramatically changed with the use of HAART: incidence has decreased, progression is slower, and the clinical manifestations have changed. Before HAART was available, untreated CMV retinitis progressed relentlessly, leading to retinal necrosis and blindness. Even with anti-CMV treatment the median time to progression of CMV retinitis was approximately 2 months in patients treated with intravenous ganciclovir or foscarnet, ^, 2–4 months with intravenous cidofovir, ^, to 7 months with a sustained-release intravitreal ganciclovir implant.

HAART has led to reduced HIV replication, elevations in CD4+ cell counts, and reduced mortality. Studying HIV-infected patients with CMV retinitis has allowed us to answer the important clinical question of whether the rejuvenated immune system that results from HAART can effectively control opportunistic infections. The progression of CMV retinitis can be accurately assessed using the masked grading of standard retinal photographs. We know, for example, that in untreated, immunocompromised patients CMV retinitis progresses within a median of 2–3 weeks.

A number of studies suggest that HAART-induced immune recovery has had a substantial impact on the course of CMV retinitis. The use of HAART has led to a reduction in the incidence of CMV retinitis, altered its clinical appearance, and dramatically changed the clinical course of the disease. Widespread use of HAART has led to a decrease in the annual number of new cases of CMV retinitis by >75%. ^, In a study of 1255 patients, each of whom had at least one CD4+ cell count <100 cells/µL, the incidence of any of three major opportunistic infections, including CMV retinitis, declined from 21.9 per 100 person-years in 1994 to 3.7 per 100 person-years in mid-1997.

Evidence from patients with AIDS and CMV retinitis also suggests that immune recovery with HAART can effectively control the disease in the absence of specific anti-CMV therapy. In 1997, we first reported that CMV retinitis did not progress in four patients receiving HAART, despite the fact that they were not receiving anti-CMV therapy. Interestingly, one of these patients never received anti-CMV therapy. Since that time, a number of additional studies suggested that CMV retinitis may not reactivate after maintenance anti-CMV therapy is stopped in patients with HAART-induced immune recovery, and that active CMV retinitis may resolve in some patients who never receive specific anti-CMV treatment. Reactivation of CMV retinitis can occur, however, especially if CD4+ counts fall to <50 cells/µL. Furthermore, there have been some sporadic reports of CMV retinitis occurring in patients receiving HAART who had stable CD4+ counts >100 cells/µL. An observational study conducted in seven European HIV cohorts followed 358 patients taking potent antiretroviral therapy who interrupted maintenance therapy because of opportunistic pathogens at a CD4+ lymphocyte count >50 cells/µL. Of these 358 patients, 162 had CMV end-organ disease. Two relapses of CMV retinitis occurred during the study. The CD4+ counts at the time of relapse were 91 cells/µL in one patient and 247 cells/µL in the other. Nevertheless, the CD4+ count appears to be the best predictor of an effective immune response against CMV, including other laboratory parameters such as HIV load and CMV culture data. Researchers have investigated whether CMV load or CMV-specific CD4+ cell responses quantified by flow cytometry can quantify and predict an effective clinical immune response against CMV, but the incidence is currently so low that adequately powered clinical studies are difficult to conduct.

Some data suggest that patients may need to receive HAART for several months before the immune response against opportunistic infections is effectively restored. Because the proportion of naïve T cells increases after about 3 months of HAART, specific anti-CMV therapy should be continued for at least 3 months after CD4+ counts have increased with HAART.

Treatment

A number of medications have now been approved by the US Food and Drug Administration (FDA) for the treatment of CMV retinitis: ganciclovir and derivitives, foscarnet, cidofovir, and fomivirsen. There is also a sustained-release ganciclovir implant approved for the treatment of this disease. Prior to the advent of HAART patients were usually treated with induction doses of intravenous ganciclovir or foscarnet, followed by lower maintenance doses of the drug for life. Even with this therapy, recurrences were common. Use of the sustained-release ganciclovir implant more effectively treated ocular disease, but did not treat the systemic CMV infection. Now, with the use of HAART and the development of orally bioavailable drugs such as valganciclovir, intravenous ganciclovir and foscarnet are used less frequently.

Ganciclovir [9-(1,3-dihydroxy-2-propoxymethyl) guanine] was the first drug approved by the FDA for the treatment of CMV retinitis. Whereas aciclovir is not sufficiently active against CMV infection, ganciclovir is virostatic both in vitro and in vivo. Ganciclovir is triphosphorylated in the cell and then inhibits viral DNA polymerase. Therapy with 5 mg/kg intravenously twice daily for 14 days will halt the progression of CMV infection and the development of a less active lesion in more than 90% of patients ( Fig. 11-5 ). However, because the drug is virostatic, recurrences are common unless the underlying immunosuppression can be altered. In patients with AIDS the median time to progression of retinitis without treatment is 21 days. Intravenous administration of ganciclovir, 5 mg/kg/day, 7 days per week, prolongs the median time to relapse to 56 days, and many patients go significantly longer without a relapse. After treatment visual loss due to macular edema often improves, but visual loss due to retinal necrosis does not. The usual therapy for CMV retinitis in immunocompromised patients includes induction therapy with intravenous ganciclovir at a dose of 5 mg/kg twice daily for 14–21 days, followed by maintenance therapy at a dose of 5 mg/kg once daily. The main toxicity of ganciclovir therapy is neutropenia. Neutrophil counts should be maintained higher than 500/µL.

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