Chiasmitis, or chiasmal optic neuritis, is characterized by acute vision loss with a visual field defect and/or radiographic findings consistent with a lesion of the optic chiasm. Chiasmitis can be caused by inflammatory, infectious, toxic, or autoimmune processes. Many cases of chiasmitis are idiopathic, but some cases are associated with multiple sclerosis (MS), MS mimics, or MS like illnesses (e.g., neuromyelitis optica, anti-myelin oligodendrocytic glycoprotein disease). In idiopathic chiasmitis, the clinical course and features are similar to demyelinating optic neuritis (ON). Likely, chiasmitis and demyelinating optic neuritis are two different manifestations of the same pathologic process rather than two separate disease entities.
The pathophysiology of chiasmitis has not been well described. Whereas the cause is infectious/inflammatory, there may be direct damage to the optic chiasm via ischemia and degenerative changes. However, immune-mediated parainfectious and/or postinfectious sequalae have also been reported. In cases where no cause is initially found, chiasmitis is thought to be caused by demyelination, and multiple sclerosis may be later diagnosed.
Reported causes of chiasmitis include:
- Infectious: Schistosomiasis, Epstein-Barr virus infection, Lyme disease, Mumps virus infection, Varicella Zoster virus infection, Tuberculosis, Mycobacterium haemophilum infection, Cryptococcus, Cysticercosis.
- Inflammatory: Sarcoidosis
- Ischemic: Moyamoya disease
- Vasculitic: Giant cell arteritis
- Autoimmune: Multiple sclerosis, Systemic lupus erythematosus
- Antibody mediated: Neuromyelitis optica, Anti-myelin oligodendrocytic glycoprotein disease
There are no definitive diagnostic criteria for chiasmitis. The diagnosis can be made clinically with the presence of visual loss in a pattern consistent with a lesion of the chiasm.
- Blurry vision or loss of vision
- Photopsias, typically transient and described as sparkles, glare, or too much light
- Dimming of the temporal visual fields
- Loss of depth perception at fixation
- “Hemifield slide” or double vision in the setting of normal eye movements (Kidd, Acheson)
Of note, unlike ON, idiopathic chiasmitis does not typically present with pain. Kawasaki et al. reported that in their series of 20 patients, only 20% reported associated eye pain.
- Loss of central visual acuity (Kidd, Acheson)
- Lesions that involve the anterior chiasm will demonstrate a junctional scotoma or junctional scotoma of Traquair.
- Lesions that involve the body of the chiasm will demonstrate a bitemporal hemianopsia. This is the typical sign of chiasmal involvement.
- Lesions that involve the posterior chiasm will demonstrate bitemporal hemianopic scotomas.
- Lesions that involve the lateral chiasm will demonstrate a homonymous hemianopsia.
- Lesions of the optic tract may have a homonymous hemianopsia, relative afferent pupillary defect, and band atrophy in the eye with the temporal visual field loss.
- Optic nerve fiber atrophy will demonstrate a “band” or “bow-tie” pattern, as the atrophy typically affects the nasal and temporal aspects of the optic disc, sparing the superior and inferior aspects.
Magnetic resonance imaging (MRI) is useful in making the diagnosis of chiasmitis, with many cases becoming apparent with the increased use of MRI. Patients may have enlargement and/or enhancement of the optic chiasm. There may be signal abnormalities on T2-weighted images, and lesions may enhance with gadolinium. Kawasaki et al. reported 12 out of 15 patients (80%) with idiopathic chiasmitis who underwent MRI had chiasmal enlargement and/or enhancement. Newman et al. reported 4 out of 6 patients (67%) with chiasmitis who underwent MRI had chiasmal enlargement. Two patients had contrast-enhancing lesions, and three patients had signal abnormalities on T2-weighted images.
There is no definitive treatment for chiasmitis, and treatment should be tailored to the individual patient and possible underlying etiology. Due to the similarities between idiopathic chiasmitis and ON, there have been recommendations to follow the regimen set by the Optic Neuritis Treatment Trial. That treatment would entail 1000 mg per day of IV methylprednisolone for 3 days, followed by 1 mg/kg per day of oral prednisone for 11 days. Depending on the other presenting features, a prolonged wean or other disease modifying / immunosuppressive therapy may be required.
Kawasaki et al. followed 20 patients in the largest and longest study to assess outcomes of idiopathic chiasmitis. They believed that the prognosis of idiopathic chiasmitis resembles that of ON. Kawasaki et al. reported that patients experience vision loss that worsened over days to weeks, stabilized, then improved. There was only one patient with progressive visual loss beyond 1 month. With varied treatment, 97% of affected eyes improved to 20/40 or better. All visual fields stabilized or improved.
15 of their patients presented with idiopathic chiasmitis as their first presumed demyelinating event. Within three years, six patients (40%) were diagnosed with clinically definite MS. Of the six patients, four patients had a second demyelinating even within one year. They concluded that idiopathic chiasmitis shares a similar risk to ON of development of MS.