Posterior Ischemic Optic Neuropathy, often referred to by its medical abbreviation PION, is a serious condition affecting vision. This article delves into the different facets of PION, aiming to provide a comprehensive understanding of its types, causes, and underlying mechanisms.
Arteritic PION: The Role of Giant Cell Arteritis
Arteritic PION is frequently linked to Giant Cell Arteritis (GCA), a condition well-documented in medical literature. Older publications sometimes referred to this condition as ‘retrobulbar optic neuritis’ when associated with GCA. While GCA most commonly impacts the Posterior Ciliary Artery (PCA), leading to arteritic Anterior Ischemic Optic Neuropathy (AION), it can also affect other arteries in the orbit. Arteritic PION arises from the blockage of collateral branches that supply blood to the posterior part of the optic nerve.
It is important to note that arteritic PION is less prevalent than arteritic AION. For instance, a study examining 123 eyes with vision loss due to GCA revealed arteritic AION in 94 eyes, while arteritic PION was observed in only 7 cases. This highlights the relative rarity of arteritic PION compared to its anterior counterpart.
Nonarteritic PION: Associations and Risk Factors
Nonarteritic PION has been associated with a range of systemic diseases. These include common conditions like diabetes mellitus, arterial hypertension, arteriosclerosis, atherosclerosis, and significant arterial hypotension. Beyond these, anecdotal reports link nonarteritic PION to migraine, systemic lupus erythematosus, polyarteritis nodosa, carotid artery stenosis or occlusion, carotid artery dissection, pulseless disease, rupture of an aneurysm of the anterior cerebral artery, extradural haematoma, head injury, emboli, aplastic anaemia, sickle cell SS disease, haemodialysis, and infections like Aspergillus fumigatus.
Research by Sadda and colleagues examining patients with nonarteritic PION identified a significantly higher prevalence of several systemic diseases compared to a control group. These included arterial hypertension, ischaemic heart disease, cerebrovascular disease, carotid artery and peripheral vascular disease, migraine, and gastrointestinal ulcers. While these associations don’t definitively establish a cause-and-effect relationship, they suggest these conditions may act as risk factors, particularly for nonarteritic PION. In some instances, a more direct causal link can be observed, such as PION following a migraine attack, severe nocturnal arterial hypotension, hypotension during surgery or haemodialysis, or in cases of carotid artery occlusion.
The development of nonarteritic PION, mirroring nonarteritic AION, is considered multifactorial. Various systemic diseases, vascular risk factors, and local predispositions can make the optic nerve vulnerable. Dysfunctional autoregulation of the optic nerve may also contribute. Ultimately, a triggering risk factor, often referred to as the ‘last straw,’ precipitates the onset of PION.
Surgical PION: Postoperative Visual Loss
Surgical PION, also known as postoperative or perioperative PION, is a distinct entity with significant medicolegal implications due to its potential to cause severe, often permanent, bilateral vision loss or even complete blindness. Numerous cases, primarily anecdotal, are reported in connection with prolonged systemic surgical procedures across various specialties. These include spinal and other orthopaedic surgeries, radical neck dissection, venous grafts in extremities, coronary artery bypass, hip surgery, nasal surgery, thoracotomy for haemothorax, penetrating thoracoabdominal injury, cataract surgery, and strabismus surgery. Sadda and colleagues documented 28 cases of surgical PION following diverse surgical procedures. While surgical PION was less frequent in some series, the medicolegal concerns remain substantial.
The pathogenesis of surgical PION is complex and multifactorial. Key contributing factors include severe and prolonged arterial hypotension (resulting from extended general anaesthesia, surgical trauma, and substantial blood loss), haemodilution due to extensive intravenous fluid administration to compensate for blood loss, orbital and periorbital oedema, chemosis, anaemia, and in rare cases, direct orbital compression from the prone position. Studies have shown that the prone position during anaesthesia elevates intraocular pressure (IOP), likely reflecting increased orbital venous pressure. The Trendelenburg position, often used in spinal surgery, can further exacerbate orbital venous pressure and oedema. Radical neck dissection can also contribute to increased orbital venous pressure. Orbital oedema following orbital floor fracture repair has also been implicated in surgical PION. Pre-existing systemic cardiovascular disease and autoregulatory dysfunction can heighten susceptibility to surgical PION. Induced arterial hypotension during surgery to minimize bleeding can also be a contributing factor. The reduced blood flow to the posterior optic nerve in surgical PION is often a result of a combination of factors:
- Marked and prolonged arterial hypotension: Almost universally observed in surgical PION cases.
- Elevated venous and tissue pressure in orbital tissues and the optic nerve: Triggered by orbital oedema leading to increased intraorbital pressure and subsequently, increased orbital venous pressure. The Trendelenburg position can worsen this. Jugular vein ligation in radical neck dissection can also raise orbital venous pressure.
- Direct compression of the optic nerve’s capillary pial plexus: In some instances, elevated orbital pressure may directly compress the delicate capillary network of the optic nerve.
Effective tissue perfusion relies on the pressure difference between arterial and venous systems. In surgical PION, the combination of increased orbital venous pressure and arterial hypotension creates a dangerous scenario, significantly reducing blood flow to the optic nerve and inducing ischaemia. The use of vasopressor agents to counteract arterial hypotension can paradoxically worsen the situation. While these agents increase proximal blood pressure, they constrict terminal arterioles, reducing blood flow in the capillary beds distal to the arterioles, thus exacerbating ischaemic damage. Furthermore, vasopressors can cause vasoconstriction within the optic nerve itself, increasing its vulnerability to ischaemia.
Some researchers have drawn parallels between visual loss in surgical PION and post-haemorrhagic amaurosis, which is visual loss following recurrent systemic haemorrhages. However, these conditions are distinct. Post-haemorrhagic amaurosis typically develops hours, days, or weeks after bleeding, often after multiple haemorrhages, even when haemoglobin and blood pressure are within normal ranges, and frequently worsens during sleep. Post-haemorrhagic amaurosis pathogenesis is thought to involve the release of angiotensin and other vasoconstrictors secondary to recurrent haemorrhage, with arterial hypotension and increased platelet aggregation as contributing risk factors. While surgical PION and post-haemorrhagic amaurosis are clinically different, they may share some pathogenic mechanisms.
