We focus on low-power spike-based sensor node communication where runlength-limited (RLL) encoding is applied to map the information to the timing of the spikes. In this type of sensor communication, often rare events need to be communicated such that spikes are transmitted rarely, and the detection of those spikes has to be performed in an energyefficient way. Generally, standard analog-to-digital converter (ADC) based detectors are employed at the receiver, which are always active even if there is no spike being transmitted, resulting in unnecessary power consumption. To mitigate this issue, this paper studies an integrate-and-fire (IF) circuit followed by a counter and pre-processor as an energy-efficient spike detector. It counts the number of fires in each symbol interval, which is further used by the pre-processing unit to map the fire count within each symbol interval into transmitted RLL symbols’ loglikelihood ratios. We evaluate a lower bound on the mutual information (MI) rate and the bit error rate of the communication system using this spike detector. Numerical results indicate that the proposed spike detection enables to receive RLL encoded spike sequences at a significantly lower energy per communicated bit Eb than traditional ADC based spike detection and alternative low-power IF time encoding machine (IF-TEM) based detection methods. Moreover, the required Eb decreases when increasing the minimum runlength constraint jointly with the signaling rate. Furthermore, a comparative discussion on the power consumption between the IF circuit based analog-todigital conversions and standard ADCs is presented.