Posterior Ischemic Optic Neuropathy, often referred to as Pion Eye, is a serious condition affecting vision due to insufficient blood supply to the optic nerve, specifically the posterior portion. This article delves into the different types of PION, their causes, and associated risk factors, providing a comprehensive understanding of this complex condition.
Arteritic PION: Giant Cell Arteritis Connection
Arteritic PION is a type directly linked to Giant Cell Arteritis (GCA), a condition where inflammation damages the arteries, most commonly affecting individuals over 50. Numerous studies have documented cases of arteritic PION in patients with GCA. Historically, due to its association with GCA, arteritic PION was sometimes mislabeled as ‘retrobulbar optic neuritis’ in older medical literature.
While GCA frequently impacts the posterior ciliary artery (PCA), leading to arteritic Anterior Ischemic Optic Neuropathy (AION), it can also affect other orbital arteries. Arteritic PION arises from the involvement of collateral branches that nourish the posterior part of the optic nerve (See Figure 8). It’s crucial to note that arteritic PION is significantly less prevalent than arteritic AION. Research indicates that in cases of vision loss due to GCA, arteritic AION is far more common, with arteritic PION occurring much less frequently. For instance, a study analyzing 123 eyes with vision loss from GCA found arteritic AION in 94 eyes, while arteritic PION was diagnosed in only 7.
Nonarteritic PION: Systemic Diseases and Risk Factors
Nonarteritic PION, another category of PION eye, is associated with a range of systemic diseases. Medical literature highlights links between nonarteritic PION and conditions such as diabetes mellitus, arterial hypertension, arteriosclerosis, atherosclerosis, and significant arterial hypotension. Beyond these, anecdotal reports suggest connections to migraine, systemic lupus erythematosus, polyarteritis nodosa, carotid artery stenosis or occlusion, carotid artery dissection, pulseless disease, rupture of aneurysm of the anterior cerebral artery, extradural hematoma, head injury, emboli, aplastic anemia, sickle cell SS disease, hemodialysis, and infections like Aspergillus fumigatus.
Research comparing patients with nonarteritic PION to control groups reveals a notably higher occurrence of arterial hypertension, ischemic heart disease, cerebrovascular disease, carotid artery and peripheral vascular disease, migraine, and gastrointestinal ulcers among PION patients. While these associations don’t definitively establish a direct cause-and-effect relationship, they strongly suggest these conditions may act as risk factors, particularly for nonarteritic PION. In some instances, a clear causal link can be observed. For example, some patients have developed PION following migraine attacks, or experienced vision loss due to PION after episodes of severe nocturnal arterial hypotension. Similarly, severe arterial hypotension during surgery or hemodialysis, and even internal carotid artery occlusion, have been linked to PION development.
The development of nonarteritic PION, much like nonarteritic AION, is considered multifactorial. It involves a combination of systemic diseases, general vascular risk factors, local optic nerve vulnerabilities, and potentially, impaired autoregulation of the optic nerve. A triggering event or risk factor often acts as the final contributor, leading to the onset of PION in susceptible individuals.
Surgical PION: Postoperative Visual Loss
Surgical PION, sometimes referred to as postoperative or perioperative PION, is a particularly concerning form of PION eye. The term ‘surgical PION’ is preferred as it encompasses a broader range of surgical contexts. Surgical PION is known for causing severe bilateral vision loss, often leading to permanent blindness, making it a significant medicolegal issue. Numerous case reports, largely anecdotal, document surgical PION following various prolonged systemic surgical procedures. These procedures include spinal and other orthopedic surgeries, radical neck dissection, venous grafts in extremities, coronary artery bypass, hip surgery, nasal surgery, thoracotomy for hemothorax, penetrating thoracoabdominal injury, cataract surgery, and strabismus surgery. One study reported 28 patients developing PION after diverse surgical procedures. While relatively rare, surgical PION is a recognized complication, and cases related to spinal surgery frequently lead to requests for medicolegal consultation.
Figure 8: Visual representation of optic nerve blood supply to clarify the posterior segment relevance in PION.
The pathogenesis of surgical PION is complex and multifactorial. Key contributing factors include significant and prolonged arterial hypotension (caused by extended general anesthesia, surgical trauma, and substantial blood loss), hemodilution from extensive intravenous fluid administration to compensate for blood loss, orbital and periorbital edema, chemosis, anemia, and in rare cases, direct orbital compression due to prone positioning. Studies have shown that the prone position during anesthesia can elevate intraocular pressure (IOP), likely due to increased orbital venous pressure. The Trendelenburg position, often used in spinal surgery, can further exacerbate orbital venous pressure and edema. Increased orbital venous pressure can also result from radical neck dissection. One reported case of surgical PION occurred after significant orbital edema following surgical repair of an orbital floor fracture. Pre-existing systemic cardiovascular disease and autoregulatory dysfunction can increase a patient’s susceptibility to surgical PION. Some orthopedic surgeons intentionally induce arterial hypotension during surgery to minimize bleeding, a practice that can inadvertently increase the risk of PION. The reduced blood flow to the posterior optic nerve in surgical PION is often attributed to a combination of factors:
- Marked and prolonged arterial hypotension: Almost universally observed in surgical PION cases during surgery.
- Elevated venous and tissue pressure in orbital tissues and the optic nerve: Resulting from orbital edema, increased intraorbital pressure, and increased orbital venous pressure. The Trendelenburg position can further contribute to this. In radical neck dissection, jugular vein ligation can also elevate orbital venous pressure.
- Direct compression of the optic nerve’s capillary pial plexus: In some instances, raised orbital pressure may directly compress these delicate vessels.
Normally, tissue perfusion pressure depends on the difference between arterial and venous pressures. In surgical PION, the combination of increased orbital venous pressure and simultaneous arterial hypotension creates a dangerous scenario, significantly reducing blood flow to the optic nerve and causing ischemia. The use of vasopressor agents to raise blood pressure in hypotensive surgical patients can also pose risks. While these agents increase proximal blood pressure, they can constrict terminal arterioles, paradoxically reducing capillary bed blood flow and potentially exacerbating ischemic damage. Furthermore, these agents may cause vasoconstriction in the optic nerve itself, increasing its vulnerability to ischemia.
Some researchers have drawn parallels between visual loss in surgical PION and post-hemorrhagic amaurosis, visual loss following recurrent systemic hemorrhages. However, these conditions are distinct. Post-hemorrhagic amaurosis typically develops hours to weeks after bleeding, often after multiple hemorrhages, and can occur even when blood pressure and hemoglobin levels are normal. It frequently occurs or worsens during sleep. Post-hemorrhagic amaurosis is theorized to involve the release of angiotensin and other vasoconstrictor agents secondary to recurrent systemic hemorrhage, with arterial hypotension and increased platelet aggregation as contributing risk factors. While surgical PION and post-hemorrhagic amaurosis are clinically different, they may share some pathogenic mechanisms.
Conclusion
PION eye, or Posterior Ischemic Optic Neuropathy, encompasses several distinct types, each with unique associations and risk factors. Arteritic PION is closely linked to Giant Cell Arteritis, while nonarteritic PION is associated with a range of systemic vascular diseases. Surgical PION, a severe postoperative complication, arises from a complex interplay of factors during surgery. Understanding the different types of PION and their underlying mechanisms is crucial for diagnosis, risk assessment, and potentially, preventative strategies in susceptible individuals. Anyone experiencing sudden or unexplained vision changes should seek prompt medical attention to determine the cause and receive appropriate care.
