What Is Central Retinal Vein Occlusion

Defining central retinal vein occlusion

Central Retinal Vein Occlusion (otherwise known as CRVO) is a condition in which the major vein responsible for draining blood from the retina becomes partially or completely closed.¹ The retina is the area of the eye consisting of numerous receptors known as photoreceptors. These receptors are responsible for the conversion of incoming light signals into electrical and chemical signals, allowing the brain to construct a visual picture.² Therefore, any issue pertaining to the blood supply in this area will affect a patient's vision. Attributed most commonly to the development of a thrombus within the central retinal vein, there are a number of other causes which will be explored within this article.³ With patients reporting varying degrees of vision loss, central retinal vein occlusion requires rapid and prompt treatment to prevent irreversible decline in vision. Throughout this article we will be discussing the causes, predisposing factors and treatments of central retinal vein occlusion so that patients may be better informed of the condition. 

Exploring the anatomy of the central retinal vein

As we have discussed, central retinal vein occlusion refers to an issue with the major vein responsible for drainage of the retina. The central retinal vein acts as any other vein throughout the body whereby it takes oxygen-depleted blood from a tissue and returns it to the heart. This removal of oxygen-rich blood also ensures the retina is drained of excess fluid whilst ensuring there is sufficient flow for oxygen-rich blood to also reach the retina. Disruptions to this flow through occlusion can result in the accumulation of oxygen-depleted blood and excess fluid within and around the area supplied by the vein. 

Importantly, as the central retinal vein drains the vast majority of blood of the retina, excess fluid accumulation can rapidly encroach upon the macula.³ The macula is a part of the retina located at the back of the eye and is responsible for central vision and the vast majority of color vision.⁴ Therefore central retinal vein occlusion often results in blurred and distorted central vision due to fluid obscuring the macula. 

Causes of central retinal vein occlusion

Whilst the exact cause of central retinal vein occlusion remains a contested issue, it is thought to be most likely due to thrombus (blood clot) involvement.⁵ The formation of a blood clot within the central retinal vein will cause a marked reduction or complete obstruction of the blood vessel. As such, this will impede blood flow from the vein, thus causing this blood to accumulate. As the blood will have no other direction to travel, the central retinal vein will then begin to ‘leak’ its products into the retina.⁵ This accumulation of fluid will in turn lead to an increase in the pressure of the eye, thus leading to compression of the smaller blood vessels responsible for supplying oxygen-rich blood to the retina (the capillaries).⁵ Due to the retina now receiving less than adequate blood supply, the retina can sometimes become ischaemic (a condition where tissue is starved of oxygen).⁵ These processes together lead to a decline in patients’ vision. 

Important to note however, is that a thrombus often forms due to three major contributing factors:⁶ 

1. Venous stasis (slowed blood flow) 
2. Endothelial damage (damage to the cells lining the inside of blood vessels) 
3. Hypercoagulability (a tendency for blood to clot for than is expected)

If there are any conditions which contribute, or enhance any of these factors, then patients are at risk of developing central retinal vein occlusion.⁶ The risk factors and diseases predisposing patients to thrombus development are detailed later.

Important to note in the anatomy of the central retinal vein is that the retinal artery crosses over it at certain points. In the presence of atherosclerosis, compression of the central retinal vein is often observed.⁶ This, in turn, leads to obstruction of blood flow through the central retinal vein, thus resulting in the symptoms of central retinal vein occlusion. 

Risk factors in central retinal vein occlusion

There are a number of risk factors associated with the development of central retinal vein occlusion. Perhaps the most significant risk factor is that of age, with 90% of those who develop central retinal vein occlusion being over the age of 50.⁶ As we have discussed, there are three main factors which can lead to the development of a thrombus (the main cause of central retinal vein occlusion). Numerous diseases have been noted as having an influence on these factors and thus contribute to the development of central retinal vein occlusion. 

These other primary risk factors are:⁶ 

• Systemic Hypertension
  • High blood pressure can cause damage to the cells lining the inside of blood vessels (the endothelium). Damage to these cells can predispose patients to developing blood clots. 
• Diabetes Mellitus
  • Diabetes Mellitus can also damage endothelial cells, thus predisposing patients to the development of central retinal vein occlusion. 
• Hyperlipidaemia (high levels of fats in the blood)
  • High levels of fats in the blood can cause atherosclerosis in arteries. Atherosclerosis then predisposes to the development of thrombus. 

Other associated risk factors have been identified and include:⁶ 

• Smoking 
• Open disc oedema 
• Hypercoagulable states:
  • Polycythemia 
  • Multiple myeloma 
  • Antiphospholipid syndrome 
  • Leiden Factor V 
  • Protein C and S deficiency 
• Vasculitis 
• HIV 
• Drugs such as diuretics or oral contraceptives 

Symptoms of central retinal vein occlusion

As we have briefly mentioned, the main symptom experienced by those with central retinal vein occlusion is changes to their vision. There are several visual disturbances associated with central retinal vein occlusion, which include:⁷ 

• Blurred vision or vision loss: patients may notice that this starts suddenly or gradually develops over many days. In mild cases of central retinal vein occlusion, patients may not notice any symptoms at all.⁸ 
• Floaters: patients often describe these as dark spots or ‘lines’ that float about their field of vision. 
• Pain or pressure within the eye: patients may notice a sensation of intense pressure or pain within or behind their eye. This is most often seen in severe cases of central retinal vein occlusion. 

In severe cases, patients can sometimes completely lose their vision. Whilst this is an extremely daunting and worrisome outcome, early treatment and intervention can often prevent this from occurring. 

Diagnosing central retinal vein occlusion

Before initiating any formal investigations, your healthcare provider may first ask a series of questions pertaining to the onset, duration and timing of the symptoms. In addition to this, they also ask questions about any other underlying health issues you may have so that they better narrow their scope for diagnosis. This is what is known as the ‘medical history’. 

Following this, the following tests may then be performed to confirm the diagnosis and to guide treatment: 

1. Eye Examination with an Ophthalmoscope
   1. Your healthcare provider will first use dilating eye drops to dilate the pupils. They will then use an ophthalmoscope (a mounted microscope) to assess the insides of your eyes and the retina. This will allow them to assess for the presence of any fluid accumulation (oedema) and to assess for any ischaemic areas.⁷ 
2. Fluorescein Angiogram 
   1. During this test, your healthcare provider will inject your arm with a specific dye. This dye will then travel to the blood vessels of the eye, allowing them to take pictures and get a clear image of the central retinal vein.⁸ 
3. Optical Coherence Tomography (OCT) 
   1. Your healthcare provider will take detailed pictures of your eye using a specialized machine. They may also use this to assess how treatment may be working.⁸ 

Treating central retinal vein occlusion

Whilst there is no definitive cure for the disease, your healthcare provider may initiate some therapies to keep your symptoms from progressing and worsening. Identifying central retinal vein occlusion and treating it rapidly will greatly improve your chances of not developing severe symptoms. Such treatments are:⁸ 

1. Injectable Drugs
   1. Your healthcare provider may administer injectable drugs known as anti-VEGF medications. These will help prevent any swelling caused by fluid accumulation. Whilst some patients may only require one injection, it is not uncommon to require multiple to keep symptoms at bay. 
2. Laser Treatment
   1. Your healthcare provider may also make use of a laser treatment known as ‘panretinal photocoagulation’ (PRP). This form of treatment is typically reserved for severe cases of central retinal vein occlusion. 

In severe cases, vitrectomy (a type of surgery) may also be indicated. However, this is usually quite rare. 

Summary

Central Retinal Vein Occlusion (CRVO) is a condition caused by a blockage or narrowing of the vein responsible for draining the retina. Due to this blockage, the vein can become ‘leaky’ and cause fluid accumulation around the retina and, most importantly, the macula. This accumulation of fluid can lead to raised pressures within the eye and most commonly presents with central vision loss and other visual disturbances. Thought to be most commonly caused by a thrombus, CRVO is associated with a number of primary risk factors including age, high circulating levels of fat, hypertension, and diabetes. Your healthcare provider will conduct several investigations to better visualize the disturbances within the vein and assess the damage within the eye. Whilst CRVO remains an incurable condition, treatment with anti-VEGF injectable medications and laser treatment can greatly reduce the severity of symptoms and possibly reduce vision loss. For these reasons, prompt identification and treatment of CRVO is necessary to prevent the development of severe symptoms.