Clinical symptoms and signs

Cogan's syndrome is a disease of young adults characterised by acute IK with audiovestibular dysfunction, associated in close temporal proximity to flares of IK, clinically indistinguishable from episodes of Ménière's disease (acute onset of nausea, vomiting, tinnitus and vertigo, rapidly followed by bilateral loss of hearing). Atypical disease presents with a significant inflammatory eye lesion in addition to or instead of IK. Thus, patients who developed scleritis, episcleritis, retinal artery occlusion, choroiditis, retinal haemorrhages, papilloedema, exophthalmos or tenonitis with or without IK were classified as having atypical Cogan's syndrome by Haineset al.2 Cases in which conjunctivitis, iritis or subconjunctival haemorrhage was present in the absence of IK were also classified as atypical disease.2 If the audiovestibular symptoms were not similar to those of Ménière's disease, or occurred more than 2 years before or after the onset of eye symptoms, the patient was again considered to have atypical Cogan's syndrome.2 Certain entities may mimic the clinical picture of Cogan's syndrome. There is a clear association between upper respiratory tract infection and the onset of both typical and atypical Cogan's syndrome, and a number of viral infections have been associated with IK and deafness (mumps, herpes zoster, and rubella).

In atypical forms of the disease, inflammation can spread to other parts of the eye (scleritis, uveitis or conjunctivitis) and association with systemic vasculitis and related disorders occurs in 20% of cases.2 3

Diagnosis

The clinical diagnosis is based on the audiovestibular symptoms, the ocular inflammation and nonreactive serologic tests for syphilis. The diagnostic suspicion based on the clinical course must be completed with an immunological work-up. The existence of a population of deficient naive cytotoxic T cells could suggest a priori a poor response to steroid therapy. The decrease in this cell population might be implicated in a possible deficiency of the cytotoxic mechanisms in response to the antigen that triggers the process.4 This finding provides additional support for a cell-mediated type IV response.5

The observation of hyperintensity within the membranous labyrinth on precontrast T1-weighted magnetic resonance imaging (MRI) has been reported in a patient with Cogan's syndrome6 and probably represents haemorrhage and leakage through the abnormal labyrinthine membrane due to active disease (inflammation of the blood vessels of the stria vascularis).

Pathogenesis

Recent experiences strongly suggest that Cogan's syndrome is an autoimmune disease,7-10 mediated by means of a hypersensitivity response to one or more infectious agents associated with vasculitis. Several authors have noted an immediately preceding upper respiratory tract infection. Thus, it is quite probable that a virus infection prompts an antibody response that develops a cross-immunity with similar proteins in the tissues of the audiovestibular system, eye, and occasionally other organs as well.2-11 Temporal bone pathology includes endolymphatic hydrops, atrophy of the organ of Corti, plasma cell and lymphocytic infiltration of the spiral ligament, osteoneogenesis of the round window, spiral ganglion cell degeneration, cystic degeneration of the stria vascularis, middle ear effusion, demyelination of the eighth cranial nerve and vasculitis of the internal auditory artery.11-13

Lymphocyte transformation has reportedly been detected when the patient's lymphocytes are exposed to corneal antigen, scleroprotein, and inner ear antigen, suggesting the presence of cell-mediated autoimmune reactivity.7 9 14

Treatment

Therapy consists of high-dosage corticosteroids; the outcome varies from complete recovery of the hearing level, if treatment is given early during the course of illness, to no improvement whatsoever.15 When a limited vasculitis results in labyrinthine ischaemia, a beneficial response to treatment can be predicted. Over the long term, the organ of Corti can degenerate and fibrosis and osteoneogenesis can develop within the perilymphatic space; thus, significant improvement should not be expected with steroid treatment. Subepithelial keratitis usually responds to local atropine and corticosteroid therapy. Systemic vasculitis complicates Cogan's syndrome and should be treated initially with prednisone and, occasionally, cytotoxic agents. Aortic insufficiency can be controlled with the administration of prednisone and surgical replacement of the aortic valve.

Our patient's hearing did not improve. This lack of response may be directly related to the intensity of the hearing loss, similar to cases of idiopathic sudden deafness,16 and correlates with the MRI findings; a good correlation between labyrinthine enhancement and loss of cochlear and/or vestibular function will only be seen in patients with severe loss of function.16-18 Moreover, the decrease in naive cytotoxic T cells has been related to a worse prognosis for the recovery of the hearing loss.4

Although Cogan's syndrome is the prototype of immune-mediated inner ear disease, the variability of ocular and audiovestibular clinical manifestations complicates its diagnosis, which should be suspected whenever there is a close temporal association between ocular abnormalities and cochleovestibular symptoms. On the other hand, the application of a study protocol including MRI and a series of immunological tests might facilitate the establishment of the prognosis for the auditory injury, opening new lines of research focusing on the pathophysiological mechanisms of the disease.