Occipital Lobe Seizures
Patient 31 presented with diplopia, nystagmus, right hemianopsia progressing to transient bilateral blindness, and transient confusion. This was suggestive of an ischemic event that possibly initially involved the upper basilar artery territory of the midbrain, producing the transient symptoms of diplopia, confusion and nystagmoid eye movements. An embolus, that initially got lodged in the rostral part of the basilar artery, could have moved up, saddling into both the posterior cerebral arteries. This would produce ischemia in both the occipital lobes, which could cause bilateral blindness because of ischemia or precipitate seizures.
Although the patient had diabetes and hypertension as risk factors for occulsive cerebrovascular disease, imaging and cardiac studies, as well as subsequent angiography did not confirm a thrombo-embolic etiology of his symptoms. As far as the abnormal CSF studies are concerned, transient elevation in the IgG synthesis can occur after ischemic injury in the nervous system. The patient, therefore, may still have had an ischemic event that could not be documented with studies.
The patient's fluctuating level of alertness, as well as the episodic conjugate deviation of his eyes inferiorly to the right, along with jerky eye movements, had suggested the possibility of seizures. An EEG confirmed this, demonstrating an epileptic focus originating in the left occipital area, that was producing focal status epilepticus and intermittent non-convulsive generalization. The patient had thus developed ictal blindness, a rare, but well-recognized cause of bilateral loss of vision.
Another possible cause of the occipital seizures was a focal cerebritis due to a parainfectious process. Although the patient's clinical history was not suggestive of an overt infectious process that had affected the nervous system, his CSF studies had shown a markedly increased rate of IgG synthesis, that become normal spontaneously within 10 days. There was no serological or angiographic evidence of vasculitis, and microbiological studies on the CSF had been negative. Hyperglycemia, also, is known to produce focal seizures, but much higher blood sugars are seen to do this than the 375 mg/dl our patient presented with.
In conclusion, therefore, this is a case of ictal blindness secondary to possible transient ischemic in the upper basilar and posterior cerebral artery territories. A monophasic parainfectious cerebritis that involved the occipital lobes, was another possibility, that could have accounted for his clinical picture.
As early as 1885, Gowers described patients with "ocular and visual warnings" consisting of elementary hallucinations, blindness, and a sensation in the eyeball itself. Patients who suffered gunshot wounds to the occipital regions during World War I experienced visual aurae consisting of temporary blindness or positive phenomena like lights or stars.
Von Economo divided the occipital lobes into striate, peristriate and parastriate regions. The striate cortex contained within the walls of the calcarine fissure, constitutes Brodmann's area 17 and is recognized by the thick strip of granular cells in layer IV that is split by the geniculocalcarine band of Gennari. All layers of this cortex show marked granularization. Ablation studies in monkeys have shown that the striate calcarine cortex projects to areas 18 and 19. These areas in turn project to area 8 in the frontal lobe and to adjacent associated parietal and temporal cortices.
Cortical stimulation studies in humans and animals have shown that eye movements can be initiated by occipital mechanisms. Subcortical stimulation below the calcarine fissure produces upward contralateral eye movement; downward movement occurs when stimulation is applied above the fissure. Elementary visual hallucinations follow stimulation of the occipital cortex largely contralateral to the side of stimulation, and at times include a sensation of movement. Complex visual hallucinations are elicited on stimulation of the occipital temporal junction.
Seizures originating in the occipital lobe are relatively uncommon. Careful attention to the early signs and symptoms provide clues for the diagnosis.
The following features suggest the occipital lobe as a origin of a complex partial seizure:
Visual auras have been reported in 47-73% of patients with occipital lobe epilepsy. These are usually elementary visual sensations such as white or colored flashing lights, often in the part of the visual field corresponding to the focus. Fading, blurring, or loss of part of all of vision may occur at the start of the occipital seizure. Scotoma, hemianopsia or amaurosis may also occur; these symptoms may be isolated to one portion of the visual field, or may spread.
Ictal blindness in the form of a whiteout or blackout has been reported, including a prolonged form, described as status epilepticus amauroticus. The association between blindness and epilepsy was described by Gower in 1881. He reported 26 cases out of a 1000 epileptics in which loss of sight preceded loss of consciousness. Ashby and Stephenson, in 1903, described five cases with prolonged but reversible total cortical blindness following febrile convulsions. The first documented case of "status epilepticus amauroticus" was described by Ayala in 1929. This was a 34 year old man who had attacks of complete blindness lasting 1-2 minutes, without loss of consciousness. The attacks became more frequent, culminating in continuos blindness lasting 15 days. The status was punctuated by focal motor and grand mal seizures. Four days after the cessation of seizure activity, light perception and then his sight returned.
Ocular and head movements: Version of the head and eyes to the opposite side is common and often, but not always, a lateralizing sign. Patients may report a sensation of eye pulling to the opposite side even in the absence of eye deviation. Rapid forced blinking and oculoclonic activity may also be seen. These may take the from of tonic or clonic turning of the eyes or clonic palpebral jerks. Eye movement sensations without detectable movement have also been described. Other symptoms may result from spread to the temporal or parietal lobes. Suprasylvian spread to the mesial or parietal cortex produces symptomatology similar to that in the supplementary motor seizures, whereas spread to the lateral parietal convexity gives rise to sensorimotor phenomena. Spread to the lateral temporal cortex followed by involvement of the mesial structures may produce formed visual hallucinations and imagery. Objects may be distorted in size (micropsia or macropsia), shape (metamorphopsia), or perceived distant from the individual. Visual hallucinations usually consist of previously experienced imagery, sometimes distorted, often complex and colorful. These may be followed by complex automatisms, ictal laughter and loss of consciousness. Direst spread to the mesial temporal cortex may mimic mesial temporal epilepsy. The visual auras may be the only clue to recognizing the occipital lobe onset of these seizures. The patient, however, may not recall them because of retrograde anmesia, if the aura was fleeting, or if the seizure is no longer preceded by the aura as it was in the past.
Symptomatic occipital lobe epilepsy should be distinguished from the benign form of childhood. In about 25% of patients with an apparently symptomatic disorder, the etiology remains unknown. The remainder have tumoral epileptogenic lesions, history of severe head trauma, vascular lesions, birth injury, cortical dysplasia, history of meningitis or encephalitis, pial angiomatosis, and porencephalic lesions; a few may have unrecognized mitochondrial encephalopathy with lactic acidosis syndrome and stroke-like episodes (MELAS syndrome).
Radiologic abnormalities are detected in 37-72% of patients with symptomatic occipital lobe seizures. MRI of the head has been found to be superior to CT. Cortical dysplasia is often missed in patients with neocortical focal epilepsy.
Most patients display widespread ictal onset over the posterior head. Infrequently, patients may exhibit interictal epileptiform discharges from the posterior occipto-temporal region.
The case presented here adds to the evidence that cortical blindness may be an epileptic phenomenon. Even persistent blindness, lasting days, may represent status epilepticus amauroticus. EEG evaluation should be included in the work-up of patients who present with acute blindness, and in whom the cause remains obscure, even in the absence of obvious clinical symptoms of epilepsy.
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