Cortical Blindness: Loss of Vision Without Any Ophthalmological Cause

Swati Jha, B. Optom

Optometrist, Dr. R.P.C. AIIMS, New Delhi

 

Cortical blindness is defined as the partial or total loss of vision because of lesion in brain. It is a permanent loss of vision following damage to the primary visual cortex or retrogeniculate pathways. ⁽¹⁾

The optic disc represents as the commencement of the optic nerve. There are no visual receptors is found in optic disc, thus forms the physiological blind spot. Fibres of the temporal hemiretina are located in the lateral side, i.e., the temporal half side of the optic nerve, and fibbers of the nasal hemiretina are located in the medial side, i.e., the nasal half side of the optic nerve. And so, upper retinal fibres are located superiorly, and lower fibres are located inferiorly in the optic nerve. The optic nerve extends from the retina to the optic chiasma, and the length is about 5 cm.

The optic nerve is enveloped in meninges. The intraocular part is the optic disc from where the intraorbital portion starts leading to the intracanalicular portion as the nerve traverses the optic canal. Then the two optic nerves outlets from the optic canals and form the optic chiasm where the temporal hemiretinal fibbers remains on the same side, and nasal hemiretinal fibres intersect and join the antagonistic optic tract. The optic tracts expands dorsolateral corners of the chiasm to the lateral geniculate body. Afferent fibres from the pupil leaves the optic tract anterior to the Lateral Geniculate Body. The visual afferents gets connected in the Lateral Geniculate Body and second-order neuron starts as optic radiation and ends in the calcarine cortex of the occipital lobe.

Our eyeball acts as a camera, eyes perceive the image and sends the signal to the brain via the visual pathway which consists of optic nerve, optic chiasma, optic tract, geniculate body, and optic radiations. If lesion is found in any area of the visual pathway, the image that is captured by our eyes will not reach to the brain and we will not be able to see the image as the lesion will cause defect in visual field.

Figure 1: Depicting the Visual Pathway
(Image source: scienceDirect.com)

Cortical visual impairment is emerging as the one of the most major cause of blindness in India.⁽²⁾ The major cause of cortical blindness is stroke, hypoxia, verve epilepsy, brain injury, brain tumor and its surgery.⁽³⁾ Depending on the expanse of the lesion, visual field loss can occur leading to scotoma, quadranopsia or hemianopsia.

Cortical blindness can be diagnosed when eye care practitioner reports normal ocular findings but patient is exhibiting signs of vision loss.

Symptoms of cortical blindness are:⁽⁴⁾

1. Hemineglect: Patients fail to recognize items to one side of the field.
2. Prosopagnosia: Patients fail to recognize the faces of familiar people.
3. Simultagnosia: Patients fail to perceive more than one objects at a time.

Cortical blindness can be confirmed by an MRI (fig 2) of the brain which reports neurological and anatomical irregularities.

Figure 2: MRI of brain
(Source: American journal of neuroradiology; February 2004 25 (2) 252-256)

Treatment of cortical blindness is visual training and vision rehabilitation. Management can be given by restitution therapy i.e., behavioural improvement by increasing integrity of the injured functional area. Compensation therapy is another useful mode of treatment as it compensates the visual loss by saccadic eye movements. ⁽⁴⁾ It helps to perceive the image that falls in the field deficit area. Substitution therapy is the most used therapy as it uses prismatic devices to perceive the image of visual deficit area in the normal visual field area.

Patients undergoing cortical blindness needs proper treatment and management. Many a times professional misdiagnose cortical blindness and consider it as malingering. Education should be given to eye care professionals. Like other visually impaired, cortical blind patients are not able to do their daily activities independently and it creates disappointment and depression. Proper counselling of the patient and family members should be done and moral support should be given. It is our duty to make them feel independent by giving them perception of vision.

 

References:

1. Das, A., & Huxlin, K. R. (2010). New approaches to visual rehabilitation for cortical blindness: outcomes and putative mechanisms. The Neuroscientist, 16(4), 374-387.
2. Kemmanu, V., Hegde, K., Giliyar, S. K., Shetty, B. K., Kumaramanickavel, G., & McCarty, C. A. (2016). Prevalence of childhood blindness and ocular morbidity in a rural pediatric population in Southern India: the Pavagada Pediatric Eye Disease Study-1. Ophthalmic epidemiology, 23(3), 185-192.
3. Melnick, M. D., Tadin, D., & Huxlin, K. R. (2016). Relearning to see in cortical blindness. The Neuroscientist, 22(2), 199-212.
4. Sarkar, S., & Tripathy, K. (2020). Cortical blindness.