Sir, Hughes (BDJ 2010; 209: 57–58) described a rare but recognised ocular complication of inferior dental nerve blocks. Ocular complications including blurred vision, amaurosis (visual loss) which is most commonly transient but can be permanent, loss of accommodation (resulting in blurred near vision), mydriasis, blepharoptosis, diplopia and a Horner's like syndrome (miosis and blepharoptosis) have been reported. In the majority, the effects have been temporary, resolving within 5-45 minutes, but in a few cases permanent sight loss has resulted. Proposed mechanisms include intra-arterial injection, intravenous injection or local diffusion of the anaesthetic with a vasoconstrictive agent.1,2,3 Phosphenes is a term used to describe visual sensations such as flashing lights (photopsia) and coloured lights, produced by stimulation of the visual system by something other than light. These visual phenomena can be produced by a variety of stimuli.4

The main blood supply to the orbit is via the ophthalmic artery, a branch of the internal carotid artery. The central retinal artery, the first branch of the ophthalmic artery, is a terminal artery, supplying the inner layers of the retina. Interruption to blood flow within the central retinal artery results in phosphenes which can evolve into transient or permanent visual loss. The lateral rectus muscle receives its blood supply from both the lacrimal artery and the lateral muscular artery, both branches of the ophthalmic artery. The lateral rectus is the only recti muscle to receive part of its blood supply from the lacrimal artery. The ophthalmic artery (internal carotid circulation) anastamoses with the external carotid circulation. Retrograde flow from the external carotid circulation into the orbit has been demonstrated.5 If during the nerve block the anaesthetic with a vasoconstrictive agent is inadvertently injected into the inferior alveolar artery, it can, by retrograde flow, enter the middle meningeal artery (external carotid circulation) and then, via anastomoses, flow into the ophthalmic artery. In some patients this is more likely as the ophthalmic artery arises, not from the internal carotid artery but directly from the middle meningeal artery (external carotid circulation) and in others, although the ophthalmic artery arises from the internal carotid artery, the middle meningeal artery makes the major contribution to flow. The lacrimal artery, a branch of the ophthalmic artery, which supplies the lateral rectus muscle, also has an anastomoses with the middle menigeal artery. Again, in some patients, like the ophthalmic artery, the lacrimal artery arises directly from the middle meningeal artery.6 So in a small number of patients the orbital blood supply is not from the internal carotid circulation but from the external carotid circulation.

The patient's symptoms could be explained by inadvertent intra-arterial injection of an anaesthetic with a vasoconstrictive agent causing vasospasm, which when combined with rare arterial anatomical variations (orbital blood supplied predominately from the external carotid circulation) may result in transient ischaemia of the lateral rectus muscle, via the lacrimal artery, causing diplopia and transient ischaemia of the retina, via the central retinal artery, resulting in phosphenes or amaurosis.