What Are Onodi Cells? Why Do They Matter in Sinus Surgery?

An Onodi cell (also known as a sphenoethmoidal cell) is a common anatomical variation in your sinuses where a small air-filled space—a posterior ethmoid cell—grows further back than usual, extending over or around your sphenoid sinus (located deep behind your nose) ^[1][2]. Seeing an Onodi cell mentioned on your CT scan is not a diagnosis of disease, but rather a description of your unique internal anatomy. Having an Onodi cell alone is not a reason to need sinus surgery ^[1].

The main reason it matters clinically is if you are planning to have surgery or if that specific cell becomes inflamed. It is incredibly important for your surgeon to know about this variation before performing sphenoid sinus surgery, because the optic nerve (which connects your eye to your brain) and the internal carotid artery (a major blood vessel) often run closely alongside this extended sinus pocket ^[3][4][5]. By studying your CT scan to map your anatomy, your surgeon can plan the safest surgical route to avoid these critical structures ^[6].

Your Unique Sinus Anatomy and Sinusitis

Everyone’s internal sinus structure is a little bit different. Usually, the sphenoid sinuses sit neatly at the very back of your nasal passages, with the ethmoid sinuses positioned in front of them. However, in people with an Onodi cell, a posterior ethmoid cell has grown backward, sometimes sitting superior (above) or lateral (to the side) of the sphenoid sinus ^[1][2].

If you have chronic sphenoidal sinusitis, your doctor will need to determine exactly which sinus is inflamed. Sometimes the sphenoid sinus is blocked, but other times the Onodi cell itself can become inflamed, infected, or develop a cyst called a mucocele ^[7][8]. A mucocele is a benign, fluid-filled sac that can slowly expand and press on nearby structures ^[7]. Knowing exactly which air-filled spaces are diseased dictates where the surgeon needs to operate.

The Relationship to Your Optic Nerve and Blood Vessels

The primary reason surgeons care so deeply about Onodi cells is their variable relationship to the optic canal (the bony tube that protects your optic nerve) and the carotid canal (which protects your internal carotid artery) ^[3][5]. In a typical sinus layout, these structures run near the sphenoid sinus. But when an Onodi cell is present, the optic nerve or carotid artery may sit right against its wall or even protrude into this cell space ^[3][4]. In some patients, the bone covering these structures can be exceptionally thin or even missing entirely ^[5].

If a surgeon were to perform surgery without knowing the Onodi cell was there, they might assume they are operating in a standard sphenoid sinus space and accidentally injure these structures ^[1][9]. While such injuries are rare, damage to the optic nerve or carotid artery is a serious complication that can cause severe bleeding or permanent vision loss ^[9].

How Your Surgeon Protects You

To reduce surgical risk, your surgeon will carefully examine your preoperative CT scan from multiple angles (axial, coronal, and sagittal planes, which refer to looking at the body from the top-down, front-to-back, and side-to-side) to map out a 3D picture of your anatomy ^[10][11]. They will specifically look for:

• The exact location and size of any Onodi cells ^[12].
• How close your optic nerve and internal carotid artery are to the sinus walls, and whether the bone covering them is fully intact ^[3][5].
• The safest pathway to navigate their surgical instruments ^[6][13].

Knowing this information allows your surgeon to customize their approach. Having an Onodi cell does not mean that surgery is unsafe for you; it means your surgical team will use your CT scan as a precise roadmap to minimize risks ^[6][13]. Sometimes, surgeons will also use image-guided navigation technologies during the procedure to help confirm their location ^[6]. While no surgery is completely risk-free, careful planning significantly reduces the chance of complications.

When to Seek Urgent Care
While an Onodi cell itself is harmless, new or rapidly worsening vision changes (like blurriness, double vision, or visual-field changes), severe eye pain, eye swelling, or severe headache with fever can be signs of a serious problem. If you experience these symptoms, contact your surgical team urgently or seek emergency medical care.