Optical Coherence Tomography in Neuro-Ophthalmology
Optical coherence tomography (OCT) is an imaging technique used widely in the management of macular diseases and glaucoma  but it is gaining popularity in neurologic and neuro-ophthalmologic conditions as well . The major achievement of OCT is by documentation of quantitative changes in the optic nerve head ( atrophy, edema or normal)
The most useful OCT parameters in the management of neuro-ophthalmologic conditions
The most valuable structural parameter when examining patients with neurologic diseases is peripapillary retinal nerve fiber layer (RNFL) thickness. The RNFL is made up of retinal ganglion cell axons . Thickening of the RNFL is caused by the axonal edema and is generally present in acute processes: optic neuritis, acute ischemia and short term intracranial hypertension. The reason for RNFL thinning is loss of ganglion cell axons, a process which eventually leads to the optic atrophy. We see RNFL thinning in neurodegenerative diseases, toxic and nutritional neuropathies, in inflammatory and ischemic processes after acute period has subsided  . As we can evaluate both average RNFL thickness and its thickness in different sectors, we have a valuable tool for evaluating the correlation between clinical findings and morphological parameters (atrophy of inferotemporal RNFL sector results in upper temporal field arcuate defect). With the development of OCT technology and high resolution devices it is now possible to measure also macular ganglion cell layer thickness. Since a significant portion of retinal ganglion cell bodies reside in the macula, a loss of tissue there helps to identify optic nerve damage .
OCT in multiple sclerosis and demyelinating optic neuritis
The use of OCT in examining people with multiple sclerosis (MS) and demyelinating optic neuritis is thoroughly researched and has become an essential biomarker in the progression the disease and response to therapeutic regimens in MS patients and other neurological disorders MS can affect sight in several ways causing uveitis, optic neuritis, chronic optic neuropathy, retrochiasmatic visual field defects, double vision, nystagmus and affect information processing in the brain cortex . According to several studies the peripapillary RNFL thickness in MS patients is diminished. This is seen also in those MS patients, who have never had any ocular complaints . Post mortem studies have shown that 99% of MS patients have demyelinisation loci on the optic nerves . In the acute period of retrobulbar optic neuritis we can see either normal, diminished or increased RNFL thickness, the latter being due to subclinical axonal edema. Approximately 6 months after the acute attack of neuritis it is possible to detect peripapillary RNFL thinning. Peripapillary RNFL thickness correlates with best corrected visual acuity and is also related to contrast sensitivity, color vision, EDSS (expanded disability status scale) and brain atrophy. Furthermore, we can observe different damage pattern in different MS forms .
OCT in Alzheimer's and Parkinson`s disease
Lu et al used OCT and found that patients suffering from Alzheimer's disease have significant RNFL thinning compared to healthy individuals at the same age. The difference was present in the upper and lower quadrant, but not in the nasal and temporal quadrant . Additionally, Alzheimer`s patients are reported to have three times larger cup-to-disc ratios when compared to controls without the disease . RNFL thickness is also shown to be a useful biomarker in Parkinson`s disease, where the amount of thinning correlates with disease severity and is an indicator for progression. A meta analysis of 13 studies concluded that the RNFL thickness of Parkinson`s patients is remarkably decreased compared to control group . Today OCT is used as an auxiliary test in the management of Alzheimer and Parkinson's disease, but in may become a tool for diagnosing and monitoring these diseases in the future .
OCT in differential diagnosis of papilledema and pseudopapilledema
Papilledema is defined as bilateral optic disc edema resulting from intracranial hypertension. Therefore when papilledema is diagnosed, a careful and often expensive follow up should ensue to exclude disorders needing prompt intervention (e.g. intracranial tumors). It is sometimes difficult to differentiate whether we are dealing with true papilledema or pseudopapilledema. Obscuration of disc margins may be present in several conditions that need no treatment, for instance, in optic nerve head drusen or in narrow scleral channels with crowded discs (a condition more often present in hyperopic eyes). Traditionally ultrasonography, autofluorescent photography and sometimes CT have been the main imaging methods in the workup of blurred disc margins, but recently the OCT has gained popularity as well. When scanning the optic nerve head, the OCT device measures dimensions of neuroretinal rim and peripapillary RNFL thickness and visualizes the axial tomogram. Taking into account all these characteristics we can evaluate the probability of papilledema versus pseudopapilledema. OCT characteristics of optic nerve head drusen and papilledema are compared in the image below    .
Differentiating crowded disc and mild papilledema can be more difficult because the peripapillary RNFL thickness can be elevated in both cases. It is usually of less extent in the case of crowed disc. OCT can be used in observing patients having papilledema due to raised intracranial pressure over long periods of time. It is possible to quantify the extent of the edema in every visit and to compare it with previous data. While OCT gives us hints whether the edema is worsening or subsiding, it is very important to keep in mind that we have to compare nerve structure with its function. Decreasing RNFL thickness can both mean a good response to treatment and also neural atrophy due to long term axonal damage.
OCT in pituitary adenoma
OCT can be used as a tool for determining visual prognosis in the case of pituitary adenoma compressing the anterior visual pathways. Studies have shown that patients with normal RNFL thickness have greater potential for visual field improvement after adenoma removal whereas RNFL thinning (less than 5th percentile) predicts incomplete recovery after treatment .
The limitations of OCT
Although OCT gives useful and quantitative information about the RNFL thickness, it has its limitations. The normative database used in the OCT software is derived from subjects over 18 years old and there are no references for children. Furthermore, anatomical variations should be taken into account, for example myopia, which can accompany RNFL thinning. Optic media opacities (corneal diseases, cataract, vitreous floaters) can alter image quality and therefore reliability. Low image quality can lead to mistakes in retinal layer detection and segmentation errors may lead to false readings. Nowadays the OCT devices enable fast image acquisition, but patient cooperation and fixation is still needed . Elderly patients may have co-morbidities that can affect RNFL (e.g. glaucoma and Parkinson`s disease). In such cases it is difficult to interpret the underlying causes of the RNFL thinning.
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