Ocular Manifestations of Chiari I and II Malformations

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Article initiated by:

Bayan Al Othman, MD, Jared Raabe, MD

All contributors:

Andrew Go Lee, MD, Ashwini Kini, MD, Nagham Al-Zubidi, MD, Bayan Al Othman, MD

Assigned editor:

Bayan Al Othman, MD

Review:

Assigned status Up to Date

 by Bayan Al Othman, MD on May 17, 2024.

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Ocular Manifestations of Chiari I and II Malformations

Disease Entity

Chiari malformations occur when the volume of the cerebellum and brainstem is too large to be contained within the posterior fossa of the cranial vault. When this occurs, the increase in pressure causes part of the cerebellum and/or brainstem to prolapse through the foramen magnum. Type 1 Chiari malformations (CM-1) are usually defined by displacement of only the cerebellar tonsils at least 5 mm below the level of the foramen magnum. Type 2 Chiari malformations (CM-2) occur when not only the tonsils, but also part of the cerebellar vermis breaches the foramen magnum. Type 3 and 4 Chiari malformations also exist, but are rare and will not be discussed in this article. Neuro-ophthalmologists usually become involved in the care of these patients due to symptoms of increased intracranial pressure (ICP) or pathologic signs such as nystagmus, sixth cranial nerve palsy, or an esotropia.

Etiology

Various theories regarding the etiology of Chiari malformations have been considered, including hydrodynamic pulsion from fetal hydrocephalus, oligo-cerebral spinal fluid leading to an underdeveloped ventricular system and disorganization of growth of the posterior fossa, mechanical crowding, birth trauma, and molecular genetic abnormalities. CM-1 can be either congenital or acquired and is often associated with syringomyelia. CM-2 is classically congenital and usually associated with a spinal myelomeningocele.

Ocular Manifestations

Visual symptoms are present in up to 80% of patients with Chiari malformations. These may include blurred vision, photophobia, diplopia, or retro-orbital pain. Other ocular manifestations include nystagmus (most often down-beat, but can be of any kind), cranial nerve palsies (most often abducens palsy), transient visual disturbances, acquired comitant esotropia, or alternative hypertropia. These all occur secondary to either elevated ICP, cerebellar compression, or brainstem compression. Papilledema can occur in patients with the Chiari malformation and it is known to be associated with idiopathic intracranial hypertension (IIH).

Diagnosis

Diagnosis of CM-1 or CM-2 is made via neuroimaging, usually sagittal magnetic resonance imaging (MRI) of the brain. If MRI is contraindicated or cannot be performed, high-resolution sagittal computed tomography (CT) of the brain may also be used. CM-1 can be diagnosed if the tonsils are displaced > 5 mm through the foramen magnum, or if they are > 3 mm past the foramen magnum in the presence of other characteristics of the disease (e.g. syringomyelia). CM-2 is suspected when a myelomeningocele is detected in a fetus or newborn and confirmed when displacement of the cerebellar vermis and medulla is observed on neuroimaging.

Signs

Nystagmus, sixth cranial nerve palsy, comitant esotropia, papilledema, ataxia

Symptoms

Nausea, vomiting, pulsatile tinnitus, blurred vision, diplopia, retro-orbital pain, photophobia

Management

Management of Chiari malformations depends on the severity of the symptoms and how they affect the patient’s quality of life. Observation may be appropriate if the patient is asymptomatic or mildly symptomatic. Nystagmus can often be treated pharmacologically with GABAergic agents (e.g. clonazepam or baclofen). It has also been successfully treated with surgical decompression, however, some patients have suffered worsened outcomes due to surgical complications. A small comitant esotropia may be corrected with prisms, but significant strabismus can be treated surgically. Decompressive neurosurgery is performed first, followed by strabismus surgery several months later. Non-ocular symptoms of CM-1 and CM-2 are usually treated with decompressive neurosurgery.

Prognosis

The prognosis for patients with a Chiari malformation varies depending on the symptoms they experience. Some evidence exists demonstrating that most stable or asymptomatic patients are unlikely to experience disease progression. Patients with nystagmus may respond to pharmacotherapy. The majority of strabismus patients improve with decompressive neurosurgery with or without subsequent strabismus surgery. Most patients experiencing non-ocular symptoms from a syrinx or brainstem compression usually improve or stabilize with decompressive neurosurgery.