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|LETTER TO EDITOR
|Year : 2010 | Volume
| Issue : 2 | Page : 326-327
Reversible photoparoxysmal response at 1 Hz in a patient with posterior fossa mass lesion
Gopal Krishna Dash, Mahesh Pundlik Kate, Chaturbhuj Rathore, Ashalatha Radhakrishnan
Department of Neurology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
|Date of Acceptance||01-Feb-2010|
|Date of Web Publication||26-May-2010|
Department of Neurology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Dash GK, Kate MP, Rathore C, Radhakrishnan A. Reversible photoparoxysmal response at 1 Hz in a patient with posterior fossa mass lesion. Neurol India 2010;58:326-7
|How to cite this URL:|
Dash GK, Kate MP, Rathore C, Radhakrishnan A. Reversible photoparoxysmal response at 1 Hz in a patient with posterior fossa mass lesion. Neurol India [serial online] 2010 [cited 2020 Feb 25];58:326-7. Available from: http://www.neurologyindia.com/text.asp?2010/58/2/326/63779
A 42-year-old lady presented with progressive gait ataxia, intermittent headache, blurring of vision and impaired hearing of one year duration. In addition, relatives noticed brief stereotypic movements of both upper limbs with behavioral arrest and transient unresponsiveness lasting less than a minute occurring at a frequency of one per week since three months prior to admission. She was on phenytoin 300mg/day. No significant past or family history. Neurological examination revealed bilateral papilledema, diminished sensation in the right fifth cranial nerve distribution, bilateral sixth nerve palsy, and sensory-neural hearing loss. Deep tendon reflexes were brisk with extensor plantar response. She had tandem gait ataxia.
MRI showed a heterogeneously enhancing calcified mass lesion (3.5 × 2.7 × 4 cm) in the lateral aspect of fourth ventricle with secondary hydrocephalus [Figure 1]. A 16-channel-scalp EEG showed bilateral intermittent slowing with persistent photo paroxysmal response (PPR) at 1Hz with intermittent photic stimulation (IPS) with no other epileptiform abnormalities [Figure 2]. Following external ventricular drainage (EVD), her paroxysmal events subsided. As isolated PPR at 1 Hz is an atypical and uncommon finding, the EEG was repeated after one week, which did not reveal any PPR [Figure 3]. The patient subsequently underwent resection of the mass lesion (ependymoma) and shunting.
PPR is a highly heritable trait characterized by spikes, spike-waves or intermittent slow waves in response to visual stimulus at various frequencies.  It is a frequent feature of idiopathic generalized epilepsy (IGE), visual sensitive epilepsy(occipital epilepsy) and in certain childhood epilepsies like severe myoclonic epilepsy in infancy.  In all these instances, PPR usually becomes evident between 10-20 Hz frequency.  PPR at 1Hz is a rarity and is usually reported in progressive epileptic encephalopathies like neuronal ceroid-lipofuscinosis (NCL) and mitochondrial cytopathy. Even in normal individuals where PPR occurs as a genetic trait, it occurs at a faster frequency and to the best of our knowledge has not been reported at 1 Hz frequency.
Very few studies have looked into the pathophysiologic basis of PPR. These studies noted " wave-length-dependent PPR'' and ''quantity-of-light dependent PPR'' in IGE, localization-related epilepsy and symptomatic generalized epilepsy.  It is hypothesized that membrane depolarization of some of the neurons to light stimuli, especially over the posterior head region contribute to photosensitivity, but the exact molecular mechanism underlying PPR remains unknown. Antiepileptic drugs like valproate and levetiracetam are known to suppress PPR, but our patient was not on any of these drugs. We postulate that external ventricular drainage for the hydrocephalus reduced the intracranial pressure and presumably the aberrant neuronal depolarization to photic stimuli related to raised intracranial pressure also subsided.
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[Figure 1], [Figure 2], [Figure 3]