What Causes Sigmoid Sinus Dehiscence?

Sigmoid sinus dehiscence is usually not something you were born with. It seems to develop as forceful blood flow in a vein behind the ear slowly wears away the bone covering it.¹ That gap matters because it lets the vein’s sound reach your ear, which is why many people find it only after they start hearing a whoosh in time with their heartbeat.² The cause is often not the bone itself but something upstream, a narrowed vein or raised pressure in the head, and that changes what fixing it can look like.

Where the Gap Forms and What It Is

The sigmoid sinus is a large S-shaped vein that carries blood from the brain down toward the neck, running just behind the ear. A thin wall of bone, which your report may call the sigmoid plate, separates this vein from the air-filled pockets of the mastoid, the bony bump behind the ear.³ When that wall has a hole, the vein sits directly against an air space, and a CT report may call this an air-on-sinus sign.⁴ Gaps most often sit in the upper part of the plate and the outer side wall, and some people have more than one gap on the affected side.⁵

Dehiscence simply means an opening where bone should be solid. Your report may say sigmoid plate dehiscence or sigmoid sinus wall dehiscence; the two mean the same thing. The term describes the bone, not a tear in the vein, so blood is not leaking. The gap is usually only a few millimetres across.

The vein keeps its own wall and the blood stays inside it; what has changed is that bone no longer covers part of that wall. A small gap is easy to miss on thick CT slices, so a radiologist reading a thin-slice scan is more likely to describe the gap precisely. A different gap, in the bone over the inner ear’s balance canal, is a separate condition with symptoms such as sound-triggered dizziness.

Fast Blood Flow Wearing Down the Bone

Just before the sigmoid sinus, blood passes through the transverse sinus. Blood forced through a narrowing speeds up, and computer models show this jet hits the upper curve of the sigmoid sinus wall harder as the narrowing gets tighter.⁶ The impact weakens the farther the narrowing sits from the sigmoid sinus.⁷ The bone is worn away right where the jet lands, and follow-up scans have tracked this erosion developing.⁸ In models built from patients’ own scans, the spot where fast blood struck the wall matched the location of the gap in 17 of 20 people.⁹ Bone responds to steady load by remodelling, and a thin plate with air on the other side has little backup, so it can thin until it opens. You would not feel this happening, since the erosion itself causes no pain, and sound often starts only once the gap opens fully.

Which side carries more blood matters too. Most people have one dominant side, where the drainage vein is larger and carries more blood. Extra blood diverted to the dominant side, often because the other side is narrowed, has been described as a cause of the erosion.¹⁰ In one group of 54 patients, about 57% had their dominant vein on the same side as the gap and only about 9% on the opposite side.¹¹ That is why doctors may image the veins, not just the bone.

The Link With Raised Pressure in the Head

Idiopathic intracranial hypertension (IIH) is raised pressure of the fluid around the brain without a clear cause, and it is most common in women of childbearing age. The gap is more common in people with IIH than in people without it.¹² Among people with the heartbeat sound, those with the gap were far more likely to have signs suggesting IIH on scans.¹³ Their scans more often showed clues such as a flattened back of the eye, a swollen sheath around the optic nerve, a partly empty pituitary space, and narrowing in the transverse sinus.¹⁴

Raised pressure around the brain can squeeze the transverse sinus and narrow it. That narrowing can produce the fast jet that erodes the bone, which links raised pressure in the skull to the gap.

Having the gap does not mean you have raised pressure, and no single scan clue confirms it. Doctors weigh your symptoms, eye exam, and imaging together.

Some symptoms suggest you should be checked: daily headaches that are worse lying down or in the morning, brief blurring or greying of vision, double vision, or an eye doctor finding swelling at the back of the eye. Tell your doctor about these, since IIH can harm sight if untreated. An eye exam is often the first step, and a lumbar puncture measures pressure directly if needed.

Born With It or Developed Later

In CT scans of people of different ages, roughly 13% to 20% had a sigmoid plate gap.¹⁵ The rate and size of gaps did not change in a clear way from younger to older age groups.¹⁶ So some gaps may be present early while others form later, and the question is not fully settled. Gaps in people without symptoms were rare and tiny compared with gaps in people who hear the sound.¹⁷

A gap on CT is not rare and is not by itself a disease. Many people with a gap hear nothing, so a gap explains the sound only if it matches your side and symptoms. A tiny gap without symptoms usually needs no treatment.

The bone around the vein may matter as well. People with the heartbeat sound more often had mastoid air pockets right next to the sigmoid plate, about 53% versus 38%.¹⁸ Air pockets leave less bone backing up the plate, which may make it easier to wear through. In one group, lower vitamin D levels went with larger dehiscence, possibly through weaker bone.¹⁹ Women appear more prone to these gaps, at least among adults with other skull-base defects.²⁰

A simple blood test measures vitamin D. Low levels are common and easy to treat, and correcting them supports bone health generally, though no one has shown it closes an existing gap. Ask your doctor rather than taking high doses on your own.

When the Vein Wall Also Bulges Out

A sigmoid sinus diverticulum is a small pouch where the vein wall bulges out into the bone behind the ear. Most people with one also have a gap in the bony wall, 20 of 28 in one surgical group.²¹ The same forceful flow that wears the bone may let the vein push outward through it, like a small bulge in a tyre, usually in the same area as the gap because that is where the vein has lost its bony support. This explanation is reasonable, though not proven for every person. After stenting the vein, bone grew back more often when a pouch was present than when there was a gap alone.²²

Your report may list dehiscence, a pouch, or both. A pouch may be better seen with CT or MR vein imaging, and surgeons may treat it by pushing it back as well as covering the bone.

Why the Gap Makes a Sound You Can Hear

Blood flowing where the transverse and sigmoid sinuses meet makes a rhythmic sound with each heartbeat.²³ Intact bone, even thin bone, muffles that sound and its vibration well.²⁴ Once the bone opens, the sound reaches the eardrum loud enough to hear, around 57 decibels in one model, close to normal conversation.²⁵ It can appear even when the gap is small.²⁶ Thin bone alone usually does not cause the sound; an actual opening does.²⁷

It is often louder at night, lying down, or after exercise, when flow increases. Light pressure on the same-side neck vein often quiets it, and turning the head can change it by altering flow.

How Doctors Find the Cause

A thin-slice CT of the temporal bone, the bone around the ear, shows the gap and nearby air pockets. CT or MR venography then maps the veins to look for narrowing or a pouch.²⁸ Specialised MRI that measures flow, combined with signs of narrowing, predicted the heartbeat sound well in one study.²⁹ Scans also look for a dominant side and a high-sitting jugular vein, which are more common in people with the sound.³⁰

CT shows bone in fine detail, so it is the best test for finding the gap, but it does not show how blood is flowing. Venography shows the vein’s shape, including a narrowing, a pouch, or a smaller side, and so helps explain why the gap formed. Flow-measuring MRI adds how fast and in what pattern blood moves, though it is not available everywhere. Because each test answers a different question, your doctor may order more than one.

An ear, nose and throat specialist or neurotologist usually leads. Describe which ear, when it is louder, and whether neck pressure stops it. Mention headaches or vision changes so IIH is considered, and bring any earlier scans for comparison.

Treatments Aimed at the Cause or the Gap

Resurfacing surgery opens the bone behind the ear, exposes the vein, and rebuilds the wall with bone paste or bone cement.³¹ Surgeons may also place a thin layer of tissue from the scalp muscle and gently ease pressure off the vein.³² In a small group, the sound stopped in every patient, though it returned after more than a year in 1 of 7 ears.³³ Low-pitch hearing often improves slightly afterward, by about 9 decibels on average.³⁴

The operation is done under anaesthesia through a cut behind the ear. Patients typically go home within a day or two, though healing and return to normal activity take longer. As with any ear surgery, there are risks such as bleeding or infection, which surgeons discuss beforehand.

When a narrowed transverse sinus is found, a stent placed inside the vein through a blood vessel can widen it. Stenting alone stopped the sound in people with narrowing and a gap, without repairing the gap.³⁵ In people with a narrowing, stenting has a high rate of improving or ending the sound.³⁶

If IIH is present, weight loss and medicine to lower fluid pressure are often tried. Lowering pressure can ease narrowing and sometimes the sound, and your eyes are monitored throughout.

Useful questions before choosing: Is there a narrowing upstream, and would stenting treat it? Do I have raised pressure that needs its own treatment? Stenting usually means taking blood thinners for months, while surgery avoids them but involves an incision behind the ear.