The disorder

Pathophysiology & hydrops

The hallmark is endolymphatic hydrops — a swelling of the inner ear’s endolymph compartment. But the relationship between the swelling and the symptoms is more subtle than it first appears.

Endolymphatic hydrops

Trainee

Endolymph volume is normally regulated by the endolymphatic sac. An imbalance of production and resorption distends the scala media — endolymphatic hydrops — bulging Reissner’s membrane into the perilymph-filled scala vestibuli.2 Toggle the cross-section below from normal to hydrops:

In hydrops, endolymph accumulates and the scala media distends, ballooning Reissner’s membrane into the scala vestibuli. Membrane stretch — and, on one theory, periodic rupture mixing potassium-rich endolymph with perilymph — is linked to the attacks, though hydrops is now seen as a marker rather than the sole cause.

Hydrops: marker, not the whole story

The neat hydrops-causes-symptoms story is incomplete. Temporal-bone studies have found hydrops in ears that were asymptomatic in life, and the degree of hydrops does not track symptom severity.1 Hydrops is best regarded as a histological marker of the disease process rather than the direct cause of every attack — a humility worth carrying into how we counsel patients.

Why the hearing fluctuates then fixes

Early in the disease the cochlea recovers between attacks, giving the characteristic fluctuating low-frequency loss. With repeated insults the hair-cell and neural damage accumulates, and the loss becomes fixed, flatter and progressive — the basis of the audiometric staging we use to track the disease.

Key points

Endolymphatic hydrops — distension of the scala media — is the pathological hallmark.
Membrane rupture mixing endolymph and perilymph is one model of an attack; others invoke pressure and ionic mechanisms.
Hydrops is a marker of the process, not a one-to-one cause of symptoms.
Recoverable early damage explains the fluctuation; cumulative damage explains the later fixed loss.