The choroid: what is it?

This is the pigmented, highly vascular layer of the globe lying between the sclera (on the outside) and the retina (on the inside).^[1] It is one of the three components of the uveal tract and is shaped a little like the body of a rounded wine glass. The optic nerve emerges at its base and the other two components of the uveal tract sit anteriorly: the ciliary body lies around the rim and the iris stretches over the opening. It is made up of three layers, each of which can be affected by disease processes. There is the external vessel layer, the capillary layer and the internal sheet-like Bruch's membrane.

The main function of the choroid is to nourish the outer layers of the retina but it is also thought to regulate retinal heat, to assist in the control of intraocular pressure and the pigment absorbs excess light so avoiding reflection.

The choroid can be subject to inflammatory disorders (often in conjunction with the retina - see separate article Chorioretinal Inflammation), tumour formation (see separate articles Tumours of the Choroid and Choroidal Melanoma) and a number of other disorders outlined below.

Hypertensive choroidopathy

Systemic hypertension affects ocular vasculature at various levels. Within the choroid, certain anatomical and histological characteristics make it particularly susceptible to the effects of severe systemic hypertension (chronic or acute). See separate article Non-diabetic Retinal Vascular Disease for more detail on hypertensive retinal disease.

Choroidal detachment and haemorrhage^[2]

• Nature - the choroid detaches from the underlying sclera with an associated accumulation of serum-like fluid or blood. It can occur as a result of trauma (including eye surgery - the most common association and causal factor), globe hypotony of any aetiology, inflammation or an increase in transmural pressure. More unusually, a choroidal detachment can occur on doing the Valsalva manoeuvre or spontaneously, particularly in older patients (>65 years) on anticoagulants.^[3]
• Presentation - photopsia and floaters. Where there is a large detachment, there may be a visual field defect. Pain is not generally a feature unless it is a postoperative haemorrhagic detachment, in which case there is sudden, excruciating pain with loss of vision (these are almost pathognomonic symptoms).
• Investigations - it is diagnosed on examination of the fundus. Occasionally, this needs to be confirmed with ultrasonography.
• Treatment - as soon as the diagnosis is made, topical steroids, cycloplegics and mydriatics are given. Intraocular pressure-lowering drugs are also given. Surgery may follow depending on the nature of the detachment. Operative choroidal haemorrhage tends to occur during surgery and is managed there and then in theatre.
• Prognosis - this depends on the cause and extent of the detachment. Damage can occur anteriorly (corneal endothelial damage, anterior synechiae, lens opacities) and posteriorly (maculopathy). Chronic detachment can also lead to globe phthisis (where it wastes away). Haemorrhagic choroidal detachment is associated with greater morbidity and up to 40% of these patients experience loss of useful vision.

Choroidal rupture^[4]

• Nature - this involves a break in the choroid, in Bruch's membrane and in the overlying pigment retinal epithelium. It typically occurs after blunt trauma (in 5-10% of these patients) and is four times more likely to occur as a result of a contrecoup injury than from direct trauma. There may be a single (25% of cases) or multiple areas involved and there will typically be haemorrhage and oedema initially which settle with time. A rupture can occur many years after the original insult.
• Presentation - men present more often than women (5:1), often between the ages of 20 and 40, with recent or old trauma. Patients may be asymptomatic but they may equally complain of decreased vision or of a paracentral scotoma.
• Investigations - the diagnosis is made on examination but there may be a need for ultrasound or CT/MRI investigations if additional injury is suspected. There may be a place for fluorescein angiography where the patient presents years later with complications such as the formation of a neovascular membrane.
• Treatment - most patients do well with conservative management. They will be monitored for about five years to assess for the formation of a neovascular membrane ± associated delayed bleeding. Should this occur, laser treatment may be needed.
• Prognosis - 15-30% of patients develop a recurring neovascular membrane that may lead to retinal detachment, particularly around the macula. However, if the fovea is not involved, patients with uncomplicated choroidal ruptures have a good chance of full recovery.

Choroidal folds

• Nature - these are parallel grooves involving the Bruch's membrane. When looking at them on fundoscopy, they give the impression of a sheet of clingfilm that is being pulled taught centrally around the disc. There are a number of causes including orbital disease (such as retrobulbar tumours), choroidal tumours, ocular hypotony (following significant surgery) and a number of other miscellaneous causes (idiopathic, chronic papilloedema and posterior scleritis).
• Presentation - many patients are asymptomatic but they may complain of metamorphopsia or impaired vision over time.
• Investigations - fluorescein angiogram.
• Treatment - this depends on the underlying cause but asymptomatic folds do not require treatment.
• Prognosis - this depends on the underlying cause.

Choroidal dystrophies^[5]

• Choroideremia - this is a very rare condition that is inherited in an X-linked recessive manner and only affects males. Patients present in the first decade of life with an inability to see in dim conditions (nyctalopia) and visual field loss.^[3] A number of abnormalities can be seen both on examination of the fundus and on functional testing of the retina. Vision is usually usefully retained until about the sixth decade of life after which there is severe visual loss. No treatment is currently available.
• Gyrate atrophy - this is another inherited disorder (autosomal recessive) characterised by axial myopia (short-sightedness caused by a long axis between the front and the back of the eye) and nyctalopia. There will be chorioretinal atrophy present and elevated orthinine levels in the plasma, urine, cerebrospinal fluid and aqueous humour (due to the lack of the orthinine-degrading enzyme, ketoacid aminotransferase). The extraocular features are absent or subtle.^[3] Two subgroups of patients are identified based on their response to treatment with vitamin B6 (pyridoxine): those responsive to treatment have a more slowly progressing course of the disease.
• Central areolar choroidal dystrophy - autosomal dominant inherited condition presenting in the fourth to fifth decade of life with poor central vision and nyctalopia. Both eyes tend to be affected and the prognosis is poor with severe visual loss by the sixth to seventh decade of life.
• Diffuse choroidal atrophy - another autosomal dominant inherited condition similar to central areolar choroidal dystrophy. Patients tend to present a decade earlier and early macular involvement rapidly impairs vision.
• Helicoidal peripapillary chorioretinal degeneration - autosomal dominant condition presenting in childhood characterised by strips of choroidal atrophy that variably affect the individual: there may be early age visual loss or elderly mild visual impairment.
• Pigmented paravenous retinochoroidal atrophy - a rare condition usually found in young men with no clear inheritance pattern, and a good prognosis due to rarity of macular involvement.

Choroidopathy: the idiopathic multifocal white dot syndromes^[5]

This is a heterogeneous group of conditions characterised by chorioretinal inflammation. Some of these conditions are associated with systemic infection and they all broadly have similar treatment options: immunosuppressive therapy, laser photocoagulation, topical or systemic steroid therapy, photodynamic therapy and, most recently, anti-vascular endothelial growth factor agents.^[6]

Acute multifocal posterior placoid pigment epitheliopathy (AMPPE)

• Nature - this is an uncommon, idiopathic, bilateral, self-limiting condition. Often associated with HLA-B7 and HLA-DR2.
• Presentation - third to fifth decade in life, after a 'flu-like illness in about 1/3 of the cases: progressive subacute impairment in one eye, followed by the other a few days later.
• Investigations - fluorescein angiography.
• Treatment - none.
• Prognosis - generally very good although in those patients who get recurrent episodes, a residual paracentral scotoma may remain.

Serpiginous choroidopathy

• Nature - uncommon idiopathic bilateral progressive disease with a prolonged, fluctuating course over many years. Relapses occur after several months' remission.
• Presentation - fourth to sixth decade in life. There is unilateral blurring of vision and image distortion (metamorphopsia) with the fellow eye being affected some time later (this time period varies).
• Investigations - fluorescein angiography.
• Treatment - monotherapy: ciclosporin or triple therapy: systemic steroids, azathioprine and ciclosporin.
• Prognosis - generally poor as recurrent episodes cause progressive atrophy and 50% of patients have foveal involvement, so markedly affecting vision.

Birdshot retinochoroidopathy

See separate article Birdshot Retinochoroidopathy for more detail.

Punctate inner choroidopathy (PIC)

• Nature - uncommon, idiopathic bilateral disease usually affecting myopic women.
• Presentation - fourth to fifth decade of life. Blurring of central vision ± photopsia (seeing flashing lights).
• Investigations - fluorescein angiogram.
• Treatment - in certain cases, patients may be amenable to laser photocoagulation, systemic steroids or surgery (where neovascular membranes are excised).
• Prognosis - reasonable if the lesions do not involve the macula.

Multifocal choroiditis with panuveitis (MCP)

• Nature - uncommon, recurrent disease which is usually bilateral and associated with a panuveitis. Its aetiology is not clearly identified but the Epstein-Barr virus may have a role.
• Presentation - fourth decade (female:male = 3:1) with blurring of vision, floaters and photopsia.
• Investigations - fluorescein angiogram.
• Treatment - systemic steroids have been shown to be effective in at least 50% of cases. Immunosuppressive agents are also used in order to reduce the amount of systemic steroids required long-term. Some patients may also benefit from laser photocoagulation.
• Prognosis - this condition recurs over months and frequent foveal involvement gives rise to a relatively poor prognosis.

Multiple evanescent white dot syndrome (MEWDS)

• Nature - uncommon, idiopathic and self-limiting condition. It is usually unilateral.
• Presentation - third to fifth decade, more often in females than males (4:1). The patient complains of decreased vision and photopsia, most often in the temporal visual field.
• Investigations - fluorescein angiogram.
• Treatment - none.
• Prognosis - full recovery after a few weeks.

Acute zonal occult outer retinopathy (AZOOR)^[7]

• Nature - possibly part of a spectrum of diseases comprising - among others - PIC, MCP and MEWDS. It is uncommon, may be uni/bilateral and typically affects young/middle-aged women after a 'flu-like illness.
• Presentation - acute scotoma (worse in bright light) and photopsia. There may be an associated vitritis.
• Investigations - electrodiagnostic tests are invariably abnormal.
• Treatment - many use immunosuppression but its benefit hasn't been fully proven yet.