Cortical Blindness

Etiology

Cortical blindness can affect both children and adults. In children, common causes include:[5][6][7][8]

• Traumatic brain injury to the occipital lobe of the brain
• Congenital abnormalities of the occipital lobe
• Perinatal ischemia

In adults, it is seen in lesions of the primary visual cortex of the occipital lobes secondary to multiple disorders, including:

• Stroke
• Cardiac embolism
• Head trauma
• Occipital lobe epilepsy
• Hyponatremia
• Severe hypoglycemia
• Creutzfeldt-Jacob disease
• Infection e.g., HIV
• Eclampsia
• MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes)
• Rarely, transient cortical blindness can be caused by
  • Infective endocarditis or
  • Hypertensive encephalopathy,
  • Posterior reversible encephalopathy syndrome (PRES)

Epidemiology

Exact epidemiological data is not available. Studies have shown a high incidence of CB in patients with cerebral stroke in the range of 20 to 57%.[9]

Pathophysiology

For the localization of lesions in cortical blindness, knowledge of the visual pathway is important. In cortical blindness, lesion lies in the striate cortex, but often the nearby areas of the brain are also involved in giving rise to different manifestations.

Visual Pathway

The optic disc is the starting point of the optic nerve. The optic disc does not contain any visual receptors. It forms the physiological blind spot. Fibers from the temporal hemiretina are situated in the lateral half, i.e., the temporal half of the optic nerve, while fibers from the nasal hemiretina are situated in the medial half, i.e., the nasal half. Similarly, upper retinal fibers are present superiorly, and lower fibers are present inferiorly in the optic nerve. The optic nerve extends from the retina to the optic chiasm, and the length is about 5 cm. It is conventionally divided into four portions- intraocular, intraorbital, intracanalicular, and intracranial.

The optic nerve is covered by the layers of meninges. The intraocular part is the optic disc wherefrom the intraorbital portion starts leading to the intracanalicular portion as the nerve traverses the optic canal. Then the two optic nerves exit from the optic canals and form the optic chiasm where temporal hemiretinal fibers continue ipsilaterally, and nasal hemiretinal fibers decussate and join the opposite optic tract. The tracts extend from the chiasm to the lateral geniculate body (LGB). Afferent fibers from the pupil leave the tract just anterior to the LGB. The visual afferents synapse in the LGB and second-order neuron starts as geniculocalcarine pathways (optic radiation) and terminate in the calcarine cortex of the occipital lobe. The primary visual cortex (V1) lies in Brodmann’s area 17. Area 18 (V2), also known as the parastriate or parareceptive area, receives and interprets impulses from area 17. The peristriate or perireceptive cortex, area 19 (V3, V4, V5), has connections with areas 17, 18, and with other portions of the cortex.

The anterior choroidal artery and thalamoperforating vessels from the posterior cerebral artery (PCA) supply the optic tract. Optic radiation is supplied by the middle cerebral artery (MCA), and the occipital lobe is primarily supplied by the PCA. The occipital lobe also receives supply from the MCA.

History and Physical

The patient may present with visual loss, dimness of vision, or visual field defect. To reach a proper diagnosis and the cause of CB, proper history should be taken regarding any risk factors like birth history where applicable, addiction, hypertension, diabetes mellitus, palpitation, and fever. Complete physical examination, including neurological and ophthalmological examination, is required. Few important things in the general physical examination must be noted, which include pulse (to rule out arrhythmia), blood pressure, and temperature. A complete neurological examination should be done. An important thing to remember is that pupillary light reflex remains intact in cortical blindness, so do the extraocular movements. There is no relative afferent pupil defect (RAPD) in cortical blindness. The cardiovascular system is another important system to be examined to rule out a cardioembolic stroke.

Ophthalmic examination or referral is needed in such cases. A visual field defect may be noted on confrontation perimetry. The anterior segment and posterior segment findings are usually unremarkable.

The clinical features vary according to the location of the lesion.

Associated damage to the temporal lobe causes acute disturbance in memory, particularly if it affects the dominant lobe. The defect usually reverses because of the bilateral representation of memory. Occlusion of the posterior cerebral artery can produce visual hallucinations of brightly colored scenes and objects (peduncular hallucinosis) due to damage to the thalami brainstem.[10] Left-sided large PCA stroke can produce visual agnosia due to disconnection between language and visual systems, whereas right-sided stroke can produce prosopagnosia due to involvement of the inferior occipital areas, fusiform gyrus, and the anterior temporal cortex.[11]

Optokinetic nystagmus (OKN) is elicited by a rotating drum with alternating stripes. The eye smoothly follows a line (pursuit), and then it suddenly returns to fix (saccade) on the next line. Pursuit is controlled by the ipsilateral parietal lobe, and the contralateral frontal lobe is involved in the control of the saccade. Thus in parietal lobe disease, ipsilateral pursuit (and OKN during drum rotating to the side of parietal lobe lesion) is affected/asymmetric, but contralateral pursuit (and OKN) is normal.

Cogan dictum states that for homonymous hemianopia with:

1. Asymmetric OKN indicates parietal lobe lesion (likely a tumor)
2. Symmetric OKN suggests occipital lobe lesion (commonly due to stroke- infarction)

Incomplete cortical blindness is much more common than a complete one. Unilateral V1 damage causes a homonymous visual field defect. The homonymous visual field defect is characterized by a similar visual field defect on the same side (either the left side or right side) in both the eye (e.g., the right eye has a left visual field defect, and the left eye also has a left visual field defect, making it difficult for the patient to see objects on the left side without moving the eye).

The loss of vision varies from a small scotoma to quadrantanopia to full hemianopsia, depending on the extent of the damage to V1. In the majority of cases, the central vision, including the foveal representation, remains intact. This is secondary to the blood supply of the occipital pole. The fovea receives its blood supply from the middle cerebral artery along with the branches of the posterior cerebral artery. It is generally represented in the occipital pole. This dual blood supply protects V1 making stroke causing the complete destruction of V1 is extremely rare. Rehabilitation is often dependent on the preservation of central vision.[12] Another peculiarity that can be seen in CB patients is 'blindsight,' where an individual can perceive coarse flickering movement in the blind field. This is due to preserved unconscious visual processing abilities in the individuals because of the heterogeneity of damage to V1. Other features of visual cortex lesion include Anton syndrome, Riddoch phenomenon, and formed visual hallucinations.

Anton syndrome: It is also known as visual anosognosia when a person cannot see but always denies the blindness, even with clear evidence of blindness. These individuals often try to walk through the closed door or wall, and in the process of denial, they take the help of confabulation. It occurs due to lesions in the V1.

Riddoch phenomenon: This is also known as statokinetic dissociation. Here patients can only perceive moving objects in the blind field, not the static ones.[13] Patients may not perceive color or details of the moving objects except the movement. This syndrome is often seen in lesions of the occipital lobe. The pathophysiology of Riddoch syndrome due to occipital lobe disease is thought to involve visual inputs reaching the V5 (motion processing cortex) bypassing the V1 area, leading to a conscious awareness of motion within a blind field.

Benson syndrome: Also known as posterior cortical atrophy, is a form of atypical Alzheimer disease.[14] It was first described by Dr. D.F. Benson in 1988.[15] The patient showed a decrease in visuospatial and visuoperceptual capabilities, but language, learning, and cognition remain intact in the early stages.[16] Here the damage lies in the occipital cortex and associated areas of the temporal and parietal lobe, which can be viewed in MRI (magnetic resonance imaging) brain.

Posterior reversible encephalopathy syndrome (PRES): PRES presents with acute onset headache, seizures, altered consciousness, and visual disturbance.[17] It is usually seen in association with malignant hypertension, eclampsia. Typical MRI brain findings are bilateral white-matter abnormalities in vascular watershed areas affecting mostly the occipital and parietal lobes.[17][18][19] With proper management of hypertension and other associated risk factors, PRES can be completely reversed, including the MRI findings.

Balint syndrome:  Austro-Hungarian neurologist and psychiatrist R Balint first described it.[20] It is characterized by simultanagnosia, oculomotor apraxia, and optic ataxia. Lesion lies in the bilateral parietal lobes and, in some cases occipital lobe.[21]

Evaluation

For evaluation of a patient with CB, complete blood count with ESR (erythrocyte sedimentation rate), metabolic profile, electrocardiogram, neuroimaging, automated perimetry, and visual evoked potential should be done. Regarding neuroimaging, a plain CT (compute tomography) scan of the brain is the initial investigation because of its easy availability, but CT scan can miss an early stroke and small strokes. MRI brain is superior to the CT of the brain in diagnosing stroke, but it is not easily available in all health care facilities.

Humphrey visual field (HVF) shows homonymous hemianopia. A more posterior lesion causes more congruity (similarity of the HVFs of both eyes) of the HVF defect. Lesion of the temporal lobe (temporal optic radiation) causes homonymous quadrantanopia of the opposite upper side (pie in the sky) (e.g., right temporal lobe damage will cause left upper visual field defect in both eyes). Lesion of the anterior parietal lobe (anterior parietal radiations) causes homonymous quadrantanopia of the opposite lower side (pie in the floor) (e.g., right parietal lobe damage will cause left lower visual field defect in both eyes).

Damage of the main optic radiation (deep inside the parietal lobe external to the trigone and occipital horn of the lateral ventricle) causes complete contralateral homonymous hemianopia. Anterior visual cortex lesions (usually due to PCA stroke) cause contralateral congruous homonymous hemianopia with macular sparing (because the tip of the occipital cortex responsible for the macular vision is supplied by MCA which is spared in PCA stroke). Lesion at the tip of the occipital cortex (usually due to trauma) causes contralateral congruous homonymous hemianopia involving the contralateral half of macular vision.

Though in most cases of cortical blindness, bilateral visual field defect is noted, unilateral visual field defect may be noted in lesions of the anteriormost part of the calcarine cortex, which is responsible for the extreme temporal visual field of the contralateral eye. Such lesions cause a temporal crescent-like visual defect in the contralateral eye alone.

Treatment / Management

Apart from standard management of the cause, which in most cases is stroke, the major part of treatment is visual training and rehabilitation. Three common modes of interventions are restitution therapy, compensation therapy, and substitution therapy. Restitution therapy is done to recover visual field deficits. It is like the perimetry.[22] Here, the patient detects multiple light spots on a black screen across blind and normal visual hemifield.

Compensation therapy acts by compensating for visual loss by saccadic eye movements.[23] It helps to capture visual stimuli that would otherwise fall onto the blind part of the visual field.

On the other hand, substitution therapy uses prism or other devices to project the visual stimulus from the blind side of the visual field to the normal one.[23]

Differential Diagnosis

Differential diagnoses of cortical blindness are:

• Hemineglect
• Prosopagnosia
• Simultagnosia
• Malingering

Prognosis

Prognosis depends on the severity of the damage to the visual cortex. Extensive bilateral occipital lesions have a worse prognosis than transient ischemic attacks. Sometimes with extensive training and tasks, patients can achieve some aspects of visual performance matching the intact hemifield vision, but full recovery of vision in all aspects doesn't occur after damage to the V1 area.[24]

Complications

Cortical blindness causes a great amount of morbidity in the patients. Their daily life gets hampered. Family members of the patient are also get affected by this. CB leads to a socioeconomic burden. The patients may be more prone to falls and fractures.