Superior Canal Dehiscence Syndrome (SCDS) is a rare inner ear condition impacting an individual’s hearing and balance. This disorder arises when a portion of the bone covering a specific inner ear structure is either missing or abnormally thin. The presence of this defect allows for unusual transmission of sound and pressure to the inner ear, leading to a range of auditory and vestibular symptoms. Understanding the underlying causes of SCDS involves examining the intricate anatomy of the inner ear and the various factors that can contribute to this bony deficiency.
Understanding the Superior Semicircular Canal
The inner ear houses a complex system responsible for both hearing and balance. Within this system are three small, fluid-filled tubes known as the semicircular canals, oriented at right angles to each other. The superior (or anterior) semicircular canal detects rotational head movements.
Fluid within this canal moves in response to head motion, stimulating tiny hair cells that send signals to the brain about spatial orientation. Normally, this delicate canal is fully encased within the dense bone of the skull, providing a protective barrier that maintains precise fluid dynamics for balance and hearing.
The Fundamental Cause: Bone Deficiency
The core anatomical defect in Superior Canal Dehiscence Syndrome is the absence or abnormal thinning of the bone covering the superior semicircular canal. This bony gap, known as a “dehiscence,” directly exposes the inner ear canal to the brain cavity.
This exposure creates an abnormal “third window” in the inner ear, in addition to the two normal openings (the oval and round windows). This third window alters the normal mechanics of inner ear fluid, allowing sound and pressure waves to travel through this new pathway. This abnormal transmission of energy can disrupt both hearing and balance functions.
Developmental and Acquired Pathways
The bony deficiency characterizing SCDS can broadly originate from two main pathways: developmental factors present from birth or acquired factors that occur over time. A predisposition to this condition can be congenital, meaning individuals are born with an unusually thin area of bone. The superior semicircular canal’s bony covering is among the last parts of the inner ear to fully develop during fetal and early childhood stages. Incomplete development can leave a segment of bone thinner than normal, potentially less than 0.5 mm thick.
Even with thin or dehiscent bone present from birth, symptoms may not appear immediately, often manifesting later in adulthood. This suggests that while a developmental predisposition may exist in approximately 1-2% of the general population, other factors often contribute to symptom onset. In contrast, acquired dehiscence involves bone that was initially normal but thins or erodes over an individual’s lifetime. This gradual process can lead to the formation of a dehiscence where none existed before, or it can worsen an already thin area.
Specific Mechanisms and Predisposing Factors
While a developmental predisposition to thin bone is often considered, specific mechanisms and predisposing factors can contribute to the formation or symptomatic onset of SCDS. Fluctuations in intracranial pressure (ICP) are recognized as a significant factor. Activities that temporarily increase pressure within the skull, such as coughing, sneezing, straining, heavy lifting, or even childbirth, can transmit these pressure changes abnormally through the dehiscent area to the inner ear. This can trigger various symptoms due to the altered inner ear fluid dynamics. Some theories propose a “wear and tear” effect, where chronic exposure to normal ICP over a congenitally thin canal can lead to gradual erosion and symptom development.
Physical head trauma can also play a role in precipitating SCDS symptoms, particularly in individuals with an underlying thin or already dehiscent bone. A study found that nearly half of symptomatic SCDS patients reported a preceding traumatic event, which could be external head injury or an internal pressure event like those experienced during scuba diving or flying. Symptoms might begin immediately after such an event or even years later, suggesting trauma can unmask a pre-existing anatomical vulnerability.
Vascular pulsations from blood vessels near the inner ear are another proposed mechanism for acquired dehiscence. Their constant rhythmic pressure on an already thin bony covering can gradually erode the bone over time. Advancing age is also associated with a general thinning of the temporal bone, which could contribute to the development or progression of a dehiscence. While a specific gene has not been identified, familial cases of SCDS suggest a potential genetic predisposition.