What Is a Mastoid Dehiscence?

Mastoid dehiscence describes a gap or defect in the bone surrounding the mastoid air cells of the temporal bone. In normal anatomy, these air cells form a honeycomb of spaces contained within a shell of dense mastoid bone. When a portion of this shell fails to form, is eroded, or is surgically removed, the underlying soft tissue structures are exposed. The area most commonly affected is the thin bony plate called the tegmen tympani.

The defect permits direct contact between the air-filled mastoid spaces and structures such as the dura mater, a venous sinus, or the inner ear. While some individuals with mastoid dehiscence remain without symptoms, others experience auditory, vestibular, or intracranial complications. A dehiscent mastoid is often recognised only after imaging is performed for an unrelated complaint, but it may also be the clear cause of disabling symptoms.

Anatomy Underpinning the Condition

The mastoid portion of the temporal bone lies behind the ear canal. It contains pneumatic air cell tracts that connect to the middle ear. Important anatomical boundaries make the region relevant to disease:

• The tegmen mastoideum and tegmen tympani form the roof of the mastoid and middle ear, separate them from the middle cranial fossa and temporal lobe.
• The sigmoid sinus groove runs along the posterior surface of the mastoid bone and holds the sigmoid venous sinus.
• The posterior fossa dura descends along the medial side of the mastoid process.
• The facial nerve descends within the bony facial canal adjacent to the mastoid air cells.
• The inner ear, including the posterior semicircular canal, lies in close relation to the medial mastoid wall.

Mastoid dehiscence can occur at any of these boundaries. The term mastoid dehiscence most often points to an absence of the normal bone separating the mastoid air cells from a vascular sinus, the dura, or a neural structure.

Causes and Contributing Factors

Several distinct mechanisms produce a dehiscent mastoid:

• Congenital factors. During development, pneumatisation of the temporal bone may proceed irregularly, leaving focal areas of thin or absent bone.
• Chronic otitis media. Long-standing inflammation and infection of the middle ear and mastoid can erode the bony partitions, especially when a cholesteatoma is present. Cholesteatoma is an abnormal collection of skin cells that expands and destroys adjacent bone.
• Previous mastoid surgery. A mastoidectomy performed for infection or cholesteatoma removal removes the bony plate that previously shielded the dura or sigmoid sinus. Surgeons may purposely leave the area dehiscent to ventilate the ear, and the bone does not regenerate.
• Trauma. Temporal bone fractures can create dehiscence along the tegmen tympani or near the sigmoid sinus.
• Erosion by pressure. Long-standing increased intracranial pressure may slowly erode the thin bone of the tegmen, causing a cerebral herniation or meningocele into the mastoid air cells.
• Age-related thinning. Some progressive resorption of the mastoid cortex occurs in older adults, occasionally producing asymptomatic dehiscence.

A notable variant is the dehiscent jugular bulb, in which a high-riding jugular bulb lacks its normal bony covering and bulges into the middle ear or mastoid cavity. Although anatomically different, it shares clinical features with mastoid dehiscence, notably pulsatile tinnitus.

Symptoms Associated with Mastoid Dehiscence

A dehiscent mastoid does not always cause symptoms. Findings may be incidental on a computed tomography scan requested after a head injury or for the evaluation of hearing loss. When symptoms arise, they reflect the structure exposed by the missing bone:

• Pulsatile tinnitus. This is a rhythmical noise, often described as a swooshing sound, which is synchronised with the heartbeat. It occurs when the sigmoid sinus lacks its bony covering. The pulsing blood flow of the sinus transmits vibration through the overlying soft tissue into the middle ear and cochlea.
• Valsalva-induced vertigo. A defect over the sigmoid sinus or the tegmen can transmit sudden changes in intracranial pressure to the inner ear. Activities such as coughing, sneezing, heavy lifting, or straining may trigger brief dizziness, disequilibrium, or a sense of motion.
• Sound-induced dizziness. When the dehiscence lies at the level of the inner ear, especially the posterior or superior semicircular canal, loud sounds can set off a reflex eye movement and imbalance. This phenomenon overlaps with canal dehiscence syndromes and is explained by a third mobile window within the inner ear.
• Hearing symptoms. The person may report a muffled sensation of fullness in the ear. Autophony, hearing one's own voice or pulse unusually loudly, can also be present.
• Cerebrospinal fluid leak. A defect involving the tegmen tympani allows cerebrospinal fluid to escape from the cranial cavity into the mastoid air cells and from there through the eardrum or down the eustachian tube. Clear fluid may drain from the nose or ear, and the person may describe a salty taste.
• Recurrent meningitis. A pathway connecting the middle ear and mastoid to the dura provides a route for bacteria from ear infection to enter the subarachnoid space.
• Headache and pressure. Exposure of the dura to mastoid pressure may provoke a localised headache or a persistent feeling of pressure behind the ear.

Many clinicians suspect a diagnosis when a patient reports vertigo triggered by straining and also has had previous ear surgery or imaging showing erosion of mastoid bone.

How the Diagnosis Is Established

Physical examination alone rarely confirms mastoid dehiscence. The eardrum may look entirely healthy, especially when the defect resides within the mastoid cavity rather than the ear canal. Diagnosis relies on a combination of:

• High-resolution computed tomography. CT scanning of the temporal bone is the definitive imaging method. Imaging in coronal and axial planes displays the extremely thin tegmen tympani and permits detection of missing bone over the sigmoid sinus. A radiologist describing a focus of absent bone with soft tissue or dura resting against the air cells will flag radiologic mastoid dehiscence.
• Magnetic resonance imaging. When cerebrospinal fluid leak or meningocele is suspected, MRI shows the fluid track and any herniation of the meninges into the mastoid.
• Audiometry. Hearing tests identify the conductive or sensorineural component and can document sound intolerance or hyperacusis.
• Vestibular testing. Tests of balance, eye movements, and the vestibulo-ocular reflex may point toward a mobile loss of ear bone, but they are nonspecific.
• Clinical provocative manoeuvres. Reproduction of vertigo with straining, pressurising the ear canal, or applying a vibratory sound to the ear supports the diagnosis when other causes have been excluded.

The term mastoid dehiscence is sometimes reserved for the imaging finding, while the term symptomatic mastoid dehiscence is used when related vertigo or tinnitus is present and cannot be otherwise explained.

Treatment Options

Management begins by distinguishing those who need intervention from those who do not. An asymptomatic dehiscent mastoid found by chance does not require surgery. The clinician may simply provide the patient with an explanation of the finding and arrange follow-up if symptoms develop.

When symptoms are troublesome, a cautious stepwise plan is appropriate:

• Counselling and reassurance. Avoiding specific positions, heavy lifting, or straining may suffice to reduce Valsalva-induced vertigo. Some patients find this knowledge alone markedly reduces anxiety.
• Medical therapy for the underlying cause. If chronic ear infection contributed to the dehiscence, the ear is dried and treated with topical or systemic antibiotics. Cholesteatoma requires surgical removal rather than medical management alone.
• Treating vascular contributions. For symptomatic pulsatile tinnitus related to a dehiscent sinus, management may include reducing elevated intracranial pressure, treating anaemia, or ensuring reasonably controlled blood pressure. The tinnitus can also be improved by surgery on the sinus, but this is reserved for severe, refractory cases.
• Surgical repair of the dehiscence. When the symptoms are disabling, candidacy for surgery is considered. Depending on the location of the defect, a surgeon closes the hole through a postauricular or middle fossa approach. Common surgical techniques include: - Repair of the bony defect with cartilage, bone pate, conchal cartilage, or a composite graft. - A covering layer of autologous fascia laid over the defect to recreate the barrier between the air cells and the dura. - Bone cement or particulate bone chips packed into the defect for a rigid repair.

In the setting of a cerebrospinal fluid leak, surgical repair is strongly advised. If the defect is complicated by an encephalocele, the herniated brain tissue is trimmed, the dural tear is closed, and the bone is reconstructed.

Surgical Outcomes and Postoperative Care

Surgery for mastoid dehiscence carries a high rate of symptom resolution when the diagnosis is precise. Pulsatile tinnitus typically improves when a sigmoid sinus dehiscence is patched, and pressure-induced vertigo tends to disappear once the mobile window is closed. Hearing, however, may not change and may rarely worsen, which is why the intervention is not offered lightly.

Recovery involves keeping the ear and surgical site dry, avoiding heavy exertion and vigorous nose-blowing during the early weeks, and using antibiotic ear drops if the external canal was involved. The patient is followed with serial examinations and imaging to confirm that no cerebrospinal fluid leak occurs.

Complications and Prognosis

Untreated mastoid dehiscence can predispose a person to bacterial meningitis, especially if an infection, such as acute otitis media, develops within the affected ear. The dehiscence also opens a route for repeated cerebrospinal fluid leakage, which may present with watery nasal discharge months after the original insult. A neglected cholesteatoma in contact with exposed mastoid bone can expand further into the cranial cavity, potentially producing a subdural empyema or brain abscess.

Still, the natural history of an isolated, incidentally discovered defect remains favourable. Most patients do not develop these severe complications. When a real symptom such as pulsatile tinnitus or straining-induced dizziness is linked to the finding, treatment is effective in the majority of properly selected cases. The overall prognosis is good once the cause of the dehiscence is removed and inflamed bone is controlled.

Key Points for the Patient

• Mastoid dehiscence is a missing piece of bone that normally rests between the mastoid air cells and structures such as the brain lining or a blood vessel.
• It is more common in ears previously exposed to mastoid surgery or chronic infection, but it may arise spontaneously.
• The two leading symptoms are pulsatile tinnitus and vertigo brought on by straining, coughing, or loud sound.
• An imaging scan of the temporal bone is required to establish the diagnosis reliably.
• Treatment is not necessary for an asymptomatic defect. Interventions are reserved for patients with significant symptoms or those with fluid leaking from the ear.
• Surgical repair is safe and effective in carefully selected cases, but an individualised discussion of risks and benefits is always needed.

Summary

A mastoid dehiscence is an anatomical defect in the bony walls of the mastoid air cells. Because the mastoid sits immediately beside the meninges, veins, inner ear, and facial nerve, a loss of this bone can lead to curious and previously unexplained symptoms. The most common presentations are a pulse-synchronous ringing in the ear and episodes of imbalance when pressure in the chest rises. Recent advances in temporal bone imaging mean that many specialists no longer consider the finding ordinary or untreatable. Instead, careful correlation of symptoms with an image finding leads to patient-centred management, from simple reassurance to refined surgical reconstruction. Anyone with intrusive tinnitus, Valsalva-related vertigo, or clear fluid from the ear that occurs after ear surgery should raise the possibility of mastoid dehiscence with an otolaryngologist, as a high-resolution scan can provide a rapid answer.