Real Test of OpenSeizureDetector

Summary

Benjamin had a seizure yesterday morning (03/05/2017).  It was not a ‘classic’ tonic clonic seizure (with rapid shaking of the arms).  Instead he was lying on his back with arms stiff in the air and ‘waving’.  This movement was on the borderline of detectability by OpenSeizureDetector.  It gave one WARNING and one ALARM.   On the positive side, it did detect the seizure.   On the negative, it only gave a single ALARM initiation, which might not be enough to wake you up if asleep.   Reducing the AlarmRatioThresh setting will increase sensitivity to improve detection reliability in the future.

Description

The seizure was not a ‘classic’ tonic clonic seizure (quite rapid shaking of the arms).  Instead Benjamin was lying on his back with arms stiff in the air and ‘waving’.

OpenSeizureDetector gave one WARNING notification at 06:24:24 on 03/05/2017 followed by a single ALARM initiation at 06:20:30, then it re-set.

The movement was right on the limit of detection using default settings which look at a frequency range of 3bHz to 8bHz as the Region of Interest.   The movement peak was at 4bHz, but there was quite a lot of power in the 1-3bHz range too and not much above.  This meant that the region of interest average power was quite low – see graph below showing the frequency spectrum of the movement during the alarm (the grey shaded area is the 3-8 Hz region of interest).

The calculated spectrum ratio (=10x[Region of Interest Power]/Spectrum Power) was 59, against a threshold of 56 (the threshold had been set higher than the default 50 to reduce false alarm rate).

We use a second android device running OpenSeizureDetector in ‘Network’ mode to relay the alarm notifications to other parts of the house over wifi.   The second device gave a WARNING but did not go into alarm.   I think this is because of a timing issue with the ALARM being raised for only a short period, so this is something to look at as we would have expected the alarm to sound on both devices.

Changes to Settings

Detection of this particular type of seizure could be improved by reducing the Region of Interest to be 3-6 Hz rather than 3-8 Hz, but this would reduce the sensitivity to seizures resulting in higher frequency movement.   Benjamin’s worst fits have been the higher frequency type, so I am minded to retain detection in that region so have retained 3-8 Hz region of interest for now.

I have reduce the AlarmRatioThreshold back to 50 rather than the 56 that we were using to increase sensitivity.

So the main settings we will be using for now are:

  • alarmFreqMin = 3 Hz
  • alarmFreqMax = 8 Hz
  • alarmRatioThresh = 50

Software Changes

I will update the phone app to give multiple alarm sounds rather than just 3 beeps in the near future to improve the chances of you noticing a single alarm annunciation.

In the meantime, users could consider switching on the Latch Alarms option in the Alarms settings – this will make the system alarm continuously until you press the ‘Accept Alarm’ button on the phone app.

Conclusion

OpenSeizureDetector did detect this slightly unusual seizure, but it was right on the limit of detection, so just gave a single WARNING and a single ALARM notification.

Increasing the sensitivity by reducing the AlarmRatioThresh setting should improve detection in the future.  Other changes would improve detection of this seizure type to the detriment of detection of higher frequency movement seizures, which is probably not a good idea.

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