Currently, gadolinium imaging of the inner ear provides the basis for visualizing endolymphatic hydrops in vivo. In the last decade or so, the use of magnetic resonance imaging (MRI) for the diagnosis of EH has become increasingly widespread (1).

Endolymphatic hydrops scoring systems and methods are also increasingly available.

Grading of endolymphatic hydrops based on the 3D-real- IR sequence of inner ear gadolinium-enhanced MRI: (2)

• Cochlear hydrops (CL) was classified into four grades according to the criteria described by Gürkov et al. (3): (Fig 1)
• Grade 0: No expansion of the endolymphatic dark areas (1A)
• Grade I: The endolymphatic dark areas are round (1B)
• Grade II: The endolymphatic dark areas are semicircular (1C)
• Grade III: The endolymphatic dark areas are expanded and flattened, with the disappearance of the vestibular scale (1D).
• Vestibular hydrops (VL), as the criteria proposed by Bernaerts et al. was also classified into four grades (4) : (Fig 2)
• Grade 0: The saccule and utricle are normal, their combined area is less than half of the vestibular region (2A)
• Grade I: The saccule is expanded with the saccule area ≥ utricle area, and the saccule/utricle ratio is inverted (2B)
• Grade II: The endolymphatic areas of the saccule and utricle are expanded, with their delineation becoming blurred or disappearing, yet the peripheral perilymphatic high signal areas remain visible (2C)
• Grade III: The peripheral perilymphatic high signal areas are no longer visible, leaving only the endolymphatic dark areas (2D).

  A) Mechanisms of Fluid Accumulation

  1. Pressure Effects on Sensory Cells: (Fig 3)

   

  Both acute endolymphatic hydrops and in the early phase (up to 5 weeks) (6) of chronic hydrops generally occur with negligible (less than 0.5 mm Hg) change in pressure between endolymph and perilymph (5-7)

  This is because the endolymphatic boundaries are very compliant, so that endolymph volume increase can occur with very small pressure changes, below those occurring with respiration and cardiac pulsations. (8)

  Furthermore, perilymphatic pressure is not elevated in the hydropic cochleaand is not elevated in patients with Meniere’s disease,so there is no perilymph pressure abnormality to influence blood flow. (9)

   2. Ionic transports

• A disturbance of any ion transport system at any location in the ear could potentially contribute to volume disturbance (10).

  Although initial studies suggested that endolymph Na+ was elevated in the hydropic cochlea, several subsequent studies failed to confirm this finding (11-12). (Fig 4)

  In the early stages as hydrops develops there are no significant composition changes of the major ions (K+, Na+, Cl-) in either endolymph and perilymph of the hydropic ear.

  As hydrops became more pronounced with time, a small decline in endolymph K+, Cl-, and osmolarity occurred in the basal turn (11). (Fig 5)

  In the normal ear, basal turn endolymph is slightly hypertonic with respect to perilymph and with respect to endolymph of the apical turns. This difference occurs even though the water permeability between endolymph and perilymph is high (equilibration half-time 7.6 minutes) (13-15).

  B )Functional Consequences
• 1) Cochlear dysfunction
• Increased endolymph pressure can cause the Reissner’s membrane to stretch or shift, further exacerbating the imbalance of ionic gradients that are crucial for cochlear function. Endolymphatic Hydrops Visualizationin 3D-FLAIR will show a dilated scala media(the cochlear duct), which is indicative of abnormal endolymph accumulation (16). ( Fig 6)
• In advanced stages of endolymphatic hydrops, when cochlear membrane distortion becomes more significant, you may see signal abnormalities in the cochlear structures. The affected areas might show changes in intensity on T1- and T2-weighted images due to altered fluid characteristics or tissue compression (17).
• 2.Vestibular dysfunction
• In EH, the excess endolymph alters the function of these structures, leading to vestibular symptoms.On high-resolution contrast-enhanced MRI, it shows increased endolymphatic fluidis visualized as hyperintense areas within the cochlea and vestibular system (18). (Fig 7)
• The hallmark of EH on MRI is the distention of the endolymphatic spacewithin the vestibular labyrinth. Semicircular canals often show enlargement or dilatation, which is interpreted as the mechanical distortion of the canals due to excess fluid (19). (Fig 8)
• MRI may show swelling or displacementof the normal anatomical structures of these structures, further confirming vestibular system involvement (16). ( Fig 9)
• MRI findings often show a larger endolymphatic space on the affected side, reflecting the severity of hydrops. This asymmetrymay correlate with clinical symptoms such as unilateral vertigo or imbalance (20). (Fig 10)

  C) Inflammatory and Cellular Changes (21)

  EH is ubiquitous in Meniere disease and is found in the affected ear in unilateral cases, and is sometimes bilateral .Normal hair cell populations may be found in the cristae and cochlea until the disease is advanced.

  With progression, atrophy of dendrites and of hair cells may be found. Neuroepithelial degeneration with thickening of the basement membrane and loss of stereocilia have also been reported

  D.Chronic and Progressive Nature

  1. Surgical ablation of the endolymphatic sac

  The most widely studied model of endolymphatic hydrops was created by surgical ablation of the endolymphatic duct and sac in guinea pigs (22).Within a few days of sac ablation a mild hydrops developed, primarily affecting the membranes bounding the saccule, endolymphatic sinus, Reissner membrane in the cochlea and, to a lesser degree, the utricle.

  Initially the semicircular canals were unaffected.Over the course of a few months, the hydrops became increasingly severe, with the Reissner membrane severely distended into SV and the space almost filled by endolymph after 3 to 4 months (22).

• 2.Hormonal-induced endolymphatic hydrops

• Experimentally, Aldosterone is known to play some part in the development of EH under certain circumstances (23-24).

  Vasopressin (V2) receptors have been demonstrated in inner ear tissues, including the lateral wall and the endolymphatic sac and may regulate water movements through cochlear tissues by control of aquaporin expression (25).

  E) Genetic and Environmental Factors

  The exact cause of endolymphatic hydrops and Meniere's disease remains largely idiopathic, but several factors may contribute to its development.

  Epidemiologic characteristics have also been identified. EH is found in all cases of Meniere disease, but many cases of EH are asymptomatic or show only hearing loss.

  There is also an association with migraine in about half of MD cases. MD is rare in children and increases in prevalence over the lifespan. Identical attacks associated with EH occur in other disorders such as otosyphilis and neuroborreliosis (26).

  The genetic etiology of MD is supported by a prevalence of familial cases , an over-representation of MD in people of Caucasian ancestry, and candidate gene studies (27).

  Viral infections can potentially damage the inner ear, disrupting fluid regulation or leading to the development of hydrops.