Retinal arteriovenous communications

Retinal arteriovenous communications are congenital anastomoses between a vein and an artery in the retina. Such anastomoses are rare and are classified in 3 groups depending on whether or not there is an abnormal network of capillaries between the arteries and the veins. Classification also depends on the diameter of the anastomosed vessels. In a few, very rare, cases, the most severe arteriovenous communications may be associated with malformations in cerebral vascularisation.

Definition

Retinal arteriovenous communications are abnormal anastomoses between a vein and a retinal artery. These anastomoses may or may not include an interposed capillary network.

The lesions are classified in 3 groups:

• Group I: anastomosis between a vein and an artery with an interposed network of dilated capillaries. The arteries and veins in a small area of the fundus have lost their normal interdigitation and form a fan-shaped network joined by a few capillaries. These lesions are known as racemose aneurysms.
• Group II:direct anastomosis between a vein and an artery with no interposed capillary network.
• Group III: multiple large arteriovenous anastomoses. It is difficult, in these types, to distinguish the veins from the arteries because the retinal vascular network is very disordered. These lesions are also known as cricoid aneurysms.

Groups II and III may be associated with cerebral vascular abnormalities. This is known as the Bonnet-Dechaume-Blanc or Wyburn-Mason syndrome.

Epidemiology

The frequency of these abnormalities in the general population remains unknown.

Genetics

Arteriovenous retinal communications appear to be congenital abnormalities. To date, no hereditary factor has been identified to explain these lesions.

Diagnosis

Group I: the lesions are usually discovered during routine examination of the fundus. Visual acuity is usually retained, even when the lesions are located in the macula. Fluorescein angiography clearly demonstrates the vascular abnormality and its network of dilated capillaries. No abnormal leakage of the dye is observed.

Group II: the lesions are usually discovered during routine examination of the fundus. Visual acuity is usually retained, even when the lesions are located in the macula. The anastomotic vessels are dilated and are therefore clearly visible. Fluorescein angiography shows rapid blood flow through the arteriovenous anastomosis. In a few rare cases, there may be some leakage of dye in the anastomosis.

Fig 2: direct communication between an artery and a retinal vein, with no interposition of capillaries. This is therefore a Group II arteriovenous communication. It is even more clearly visible on the fluorescein angiography (right).

Group III: visual acuity is altered in most cases. The vascular abnormalities are obvious upon examination of the fundus. The anastomic vessels are dilated and located along an unusual pathway. The abnormalities may be located at one quadrant or extend right across the fundus. Fluorescein angiography shows very rapid blood flow within the abnormal vessels and it may be difficult to distinguish between arteries and veins. There may be leakage of dye and spontaneous retinal vascular occlusion (fig 3).

In the Bonnet-Blanc-Dechaume (or Wyburn-Mason) syndrome, there may be vascular malformations in the brain and jaw. A number of cases of dermal angiomas have also been described.

Fig 3: Complete disorganisation of the retinal vascular network (Group III). The lesions are spread over the whole of the retinal periphery.

Evolution

• Group I:the lesions are usually stable and visual acuity is maintained.
• Groups II and III:the patient may experience some loss of vision as the disease develops. This may be secondary to an arterial or venous occlusion, macular oedema or serous retinal detachment of the macula, an intravitreous haemorrhage or gradual compression of the optic nerve.

Treatment

There is no treatment to re-establish a normal vascular path. In patients in Groups II and III with severe loss of vision because of serous retinal detachment of the macula or a spread through the communication, laser photocoagulation may be offered. Vascular occlusions usually resolve spontaneously. Laser photocoagulation will only be proposed for patients with vascular occlusion leading to extensive retinal ischaemia. Haemorrhages of the vitreous persisting for more than 3 months are treated by vitrectomy.

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