Optical Coherence Tomography for Neuromyelitis Optica Spectrum Disorder - A Complete Guide

Introduction:

Diagnosing rare diseases early is still a major challenge in the medical field. This is because rare diseases can manifest in varied systems of the body and may have overlapping conditions. Also, the specificity of diagnostic methods for rare diseases is still in a nascent stage. For rare diseases like neuromyelitis optica spectrum disorder (NMOCD), optical coherence tomography is a great diagnostic tool for understanding the severity of the disease and separating it from other lesions similar to multiple sclerosis.

What Is Neuromyelitis Optica Spectrum Disorder?

Neuromyelitis optica spectrum disorder (NMOCD) or Devic’s disease (as it was described by Dr Devic) is a rare spectrum of autoimmune disease with core clinical manifestations in the organs of the central nervous system (bilateral optic nerve and spinal cord). The clinical manifestations of NMOSD generally involve inflammation of these organs, resulting in optic neuritis or myelitis (longitudinal transverse myelitis).

The pathophysiology or progression of the disease is attributed to the deposition of pathological antibodies against aquaporin-4 water channels in the astrocytes. Aquaporin-4 antibodies, or AQP-4 antibodies, cause excessive damage to the axonal structure and the myelin sheath (causing demyelination) and subsequent inflammation. Optic nerves and the spinal cord are affected because these areas in the body have the highest number of aquaporin channels. So, neuromyelitis optica spectrum disorder is a condition that causes astrocytopathy (damage and destruction of nerve cells called astrocytes).

What Are the Clinical Manifestations of Neuromyelitis Optica Spectrum Disorder?

The main components that form a part of neuromyelitis optica spectrum disorder are:

• Optic neuritis.

• Acute myelitis.

• Area postrema syndrome.

• Brainstem syndrome.

• Narcolepsy or acute diencephalic syndrome.

• Symptomatic cerebral syndrome.

These core clinical characteristics cause features like:

• Pain in both eyes and vision loss (bilaterally) due to optic nerve inflammation.

• Weakness in muscles.

• Paralysis of limbs.

• Numbness.

• Severe pain in the back.

• Bladder dysfunctions.

• Bowel issues.

• Nausea.

• Vomiting (intractable).

• Persistent hiccups.

• Difficulty swallowing.

• Hoarseness in voice.

• Vertigo secondary to brainstem syndrome.

• Uncoordinated movements (particularly the limbs).

• Excessive sleepiness in the daytime.

• Generalized muscular weakness.

• Neuropathic pain in back and limbs.

• Difficulty to remember.

• Difficulty concentrating.

The most important feature is the incidence of ophthalmic manifestations, which form an inherent part of the neuromyelitis optica spectrum syndrome. A significant diagnostic tool is essential to diagnose and manage these ophthalmic manifestations, and optical coherence tomography exactly fills this role.

What Is Optical Coherence Tomography?

Optical coherence tomography is a diagnostic technique that provides cross-sectional (tomographic) images of the retina. It is a non-invasive modality that uses light waves to get images of the light-sensitive tissues of the eyes the retina.

How Is Optical Coherence Tomography Useful for NMOSD?

Optical coherence tomography is used to diagnose neuromyelitis optica spectrum disorder (NMOSD).

• In NMOSD, the optic nerve is affected, and there is inflammation. When the optic nerve is affected, the retinal ganglion cells causing light transmission to the brain are also affected. This significantly reduces the thickness of the retinal nerve fiber layer (RNFL) of the eyes. The RNFL thickness is about 80 to 120 micrometers in a healthy person's eyes. In patients with neuromyelitis optica spectrum disorder, the thickness is reduced, and the reduced thickness can be measured by optical coherence tomography.

• Optical coherence tomography can also measure the thickness of the ganglion cell layer in the retina, and a reduction in thickness directly helps diagnose NMOSD.

• Optical coherence tomographic scans are also advised for patients affected by NMOSD undergoing treatment. These scans provide details about the status of the retina and optic nerve before and after therapy.

• Optical coherence tomography helps detect subclinical recurrences of optic neuritis before significant vision loss occurs.

• The most challenging part in diagnosing neuromyelitis optica spectrum disorder is to differentiate it from other similar disorders like multiple sclerosis. Optical coherence tomography is a great tool for differentiating between these conditions. This is done by analyzing specific patterns of retinal damage in optical coherence tomographic images. In NMOSD, optic neuritis in patients tends to cause more severe and extensive thinning of the inner retinal layers (retinal nerve fiber layer and ganglion cell layer). This significant thinning is often bilateral and more pronounced, reflecting the substantial axonal loss and damage to the optic nerve. In contrast, multiple sclerosis results in less severe retinal nerve fiber layer (RNFL) thinning, and the damage is usually more focal and less extensive. Moreover, in multiple sclerosis, the peripapillary RNFL thinning tends to be more prominent in the temporal quadrant, whereas NMOSD shows a more diffuse thinning pattern.

• In patients with NMOSD, optical coherence tomographic images show thinning of the ganglion cell layer in the macular region, which is not noted in patients with multiple sclerosis.

Thus, the results of optical coherence tomography will be correlated with AQP-4 antibody testing in the serum, visual evoked potentials, and magnetic resonance images to confirm the diagnosis of neuromyelitis optica spectrum disorder. The optical coherence tomographic images also help to predict the visual outcomes and guide the rehabilitation process for patients affected with NMOSD.

What Are the Limitations of Optical Coherence Tomography in NMOSD Diagnosis?

• Similar findings, such as thinning of the retinal nerve fiber layer thickness observed in optical coherence tomography (OCT), are also noted in multiple sclerosis, glaucoma, or optic neuropathies due to other causes. This can make the diagnosis of NMOSD really challenging and require additional tests to confirm it.

• OCT may not detect early or subclinical changes in NMOSD. In the initial stages, retinal damage might be minimal or absent, and OCT findings might still appear normal despite the presence of the disease.

• Optical coherence tomography cannot detect acute changes in the retinal layers or axonal loss. Thus, using OCT as the only diagnostic tool delays the diagnosis of NMOSD.

• NMOSD can sometimes affect one eye more than another, leading to inter-eye asymmetry. While OCT can measure this asymmetry, interpreting the significance of unilateral changes can be challenging without comprehensive clinical correlation.

• Opacities in the eye due to cataract or hemorrhage can diminish the quality of image provided by OCT. This can mask the accuracy of diagnosing NMOSD.

• The most important limitation is that OCT can never provide details about the visual function of patients with NMOSD. It can only detect the structural change and indicate the compromise in the structural integrity, whereas functional integrity can never be assessed with OCT.

Conclusion:

Despite the limitations, optical coherence tomography provides details about the condition of light-sensitive and nerve components of the eyes in patients with and without therapy for neuromyelitis optica spectrum disorder. Using optical coherence tomography as a part of a multimodal diagnostic approach and correlating the interpretation of OCT images with clinical features can help confirm the diagnosis of NMOSD and track the efficacy of the treatment provided for the affected patients.