Secondary glaucoma
Secondary glaucoma is a collection of progressive optic nerve disorders associated with a rise in intraocular pressure which results in the loss of vision. In clinical settings, it is defined as the occurrence of IOP above 21 mmHg requiring the prescription of IOP-managing drugs. It can be broadly divided into two subtypes: secondary open-angle glaucoma and secondary angle-closure glaucoma, depending on the closure of the angle between the cornea and the iris. Principal causes of secondary glaucoma include optic nerve trauma or damage, eye disease, surgery, neovascularization, tumours and use of steroid and sulfa drugs. Risk factors for secondary glaucoma include uveitis, cataract surgery and also intraocular tumours. Common treatments are designed according to the type and the underlying causative condition, in addition to the consequent rise in IOP. These include drug therapy, the use of miotics, surgery or laser therapy.
Pathophysiology
Secondary glaucoma has different forms based on the varying underlying ocular conditions. These conditions result in an increase in IOP that manifests as secondary glaucoma.Paediatric congenital cataract associated glaucoma
Based on the onset of secondary glaucoma in paediatric patients, it can be classified into early-stage and late-stage glaucoma cases. Early-stage secondary glaucoma, observed as angle-closure glaucoma, results from the blockage and inflammation of the peripheral anterior synechiae structure. However, early-stage secondary glaucoma rarely occurs with the readily available prescription of anti-inflammatory medications. On the other hand, late-stage glaucoma is commonly associated with open-angle glaucoma but the mechanisms are currently unconfirmed. Yet, it is believed to be closely related to the onset of trabeculitis or vitreous toxicity.In paediatric congenital cataract patients under the age of two, cataract surgery is considered and frequently employed as the primary treatment. There are two types of therapeutic combination, primary and secondary lens implantation. In primary IOL, cataract surgery is performed alongside immediate implantation of IOL. However, in secondary IOL implantation, the patient is prescribed aphakic glasses or contact lenses till the implantation of IOL after a varied period of time between a few months or years. Primary IOL implantation is observed to significantly reduce and avoid the occurrence of secondary glaucoma in paediatric patients under the age of two.
Herpetic anterior uveitis associated glaucoma
In patients diagnosed with herpetic anterior uveitis, elevated IOP and secondary glaucoma are often detected. This is due to two main reasons, the blockage of vitreous flow resulting from inflammation in the structures of the trabecular meshwork, and the sedimentation of inflamed cells. Specifically for viral anterior uveitis, patients with IOP levels above 30 mmHg are often suffer from secondary glaucoma caused by cytomegalovirus.Other forms of secondary glaucoma
- Pigmentary glaucoma: In pigmentary glaucoma, the obstruction of the trabecular meshwork caused by iris pigment release results in increased IOP. This release in iris pigment occurs as a result of the interaction of a flaccid iris with the zonular fibres.
- Exfoliation syndrome: Exfoliation syndrome is a classic cause of secondary open-angle glaucoma, a common symptom of exfoliation syndrome is a cloudy layer on the anterior lens capsule.
- Aphakic and pseudophakic glaucoma: Aphakic glaucoma is a common side-effect of cataract surgery which causes an increase in IOP.
- Corticosteroid-induced glaucoma: Corticosteroids is a risk factor for the development of secondary glaucoma, as there had been increased IOP observed as a drug side-effect.
- Post-traumatic glaucoma: Trauma to the eye is often observed to cause secondary glaucoma. The incidence is notably higher in populations with increased levels of physical activity.
- Ghost-cell glaucoma: Ruptured red blood cells will release haemoglobin in the form of Heinz bodies, which are potent in increasing the IOP level.
- Inflammatory glaucoma: The inflammatory reaction will affect the drainage of aqueous humour in the eye, causing an increase in IOP.
- Glaucoma associated with ocular tumours: Although each tumour subtype has its own mechanism in causing secondary glaucoma, the general cause is the restriction of the meshwork resulting in the obstruction of aqueous humour flow.
- Increased episcleral venous pressure: According to the Goldmann equation, the relationship between episcleral venous pressure is directly proportional to the IOP. Therefore, an increase in the EVP will result in an increase in IOP.
- Neovascular glaucoma: As a consequence of neovascularisation, or the formation of new blood vessels and supporting connective structures, there is blockage of the anterior chamber angle. This leads to elevation of IOP causing neovascular glaucoma.
- Schwartz–Matsuo syndrome: Schwartz–Matsuo syndrome is another cause of secondary glaucoma. The disease is characterized by rhegmatogenous retinal detachment, elevated intraocular pressure with an open anterior chamber angle with presence of aqueous cells. Schwartz hypothesized iridocyclitis as the cause of elevated intraocular pressure, but Davidorf suggested that the elevated IOP may be due to obstruction of trabecular meshwork by pigmented cells representing photoreceptor outer segments from retinal pigment epithelium that migrate anteriorly within the aqueous humor. Later, Matsuo et al. isolated photoreceptor outer segments and inflammatory cells in aqueous humor and hypothesized that photoreceptor outer segments pass through the retinal break cause aqueous outflow obstruction and elevated IOP.
Epidemiology
Varying forms of secondary glaucoma
Pigmentary glaucoma has lower incidence in Black and Asian populations, due to their characteristically thicker irises that result in a lower likelihood of pigment release, as compared to the White populations. Incidence of exfoliation syndrome-caused secondary glaucoma is estimated to be approximately 10% of the glaucoma patient population in the United States and over 20% of the patient population in Iceland and Finland.In populations above the age of 40, neovascular glaucoma has a prevalence of 0.4% worldwide. The incidence of pigmentary glaucoma decreases with age while in exfoliation syndrome the incidence increases with age. However, given the derived nature of secondary glaucoma, there may be no significant association between age, ethnicity or gender and the prevalence of the condition.
Secondary glaucoma indicated after congenital cataract surgery is found between 6 and 24% of the cases noted, whereas, secondary glaucoma caused by primary IOL implantation was observed as 9.5%. Additionally, for patients with aphakia and secondary IOL implantation, 15.1% of the cases were determined. The incidence risk in primary IOL implantation in children with cataract in both eyes is lower than secondary IOL implantation and aphakic condition. However, this difference is not observed in the general population and populations with cataract in one eye.
Due to lack of concrete and specific epidemiological evidence, further research is required to accurately estimate the prevalence of secondary glaucoma and its subtypes.