Syncope: Given the brief episode of loss of consciousness without post-ictal confusion, tongue biting, or incontinence, and the absence of epileptiform abnormalities on EEG, syncope is a strong consideration. The episode's description, including leaning back and stiffness, could be consistent with a syncopal episode, especially if it was precipitated by a sudden change in position or other factors that might lead to a transient decrease in cerebral blood flow.

Seizure (likely complex partial seizure): Despite the absence of epileptiform abnormalities on the EEG, which does not entirely rule out seizure activity, the description of the event and the EEG findings of intermittent focal slowing and frontal intermittent delta activity (FIRDA) could suggest an underlying seizure disorder, particularly if the episode was a complex partial seizure that did not generalize.
Transient Ischemic Attack (TIA): A TIA could present with a brief loss of consciousness, especially if it involved areas of the brain that regulate consciousness. The absence of lasting neurological deficits does not rule out a TIA, and the EEG findings could be consistent with ischemic changes.

Cardiac Arrhythmias (e.g., AFib with rapid ventricular response, VT): These conditions can lead to syncope due to inadequate cerebral perfusion. Given the potential for sudden death, identifying and treating any underlying cardiac arrhythmia is crucial.
Subarachnoid Hemorrhage or Other Intracranial Hemorrhage: Although the EEG does not show specific signs of hemorrhage, and the clinical presentation was not typical, any form of intracranial hemorrhage can present with loss of consciousness and must be ruled out due to its high morbidity and mortality.
Meningitis or Encephalitis: Infections of the CNS can present with altered mental status, including loss of consciousness, and the EEG findings of FIRDA could be seen in the context of an encephalopathy. These conditions require prompt diagnosis and treatment.

Creutzfeldt-Jakob Disease: This rare neurodegenerative disorder can present with myoclonus, seizures, and rapidly progressive dementia, but it could also have episodes that resemble syncope or seizures. The EEG might show periodic sharp wave complexes, but early in the disease, it might be non-specific.
Hashimoto's Encephalopathy: An autoimmune condition associated with Hashimoto's thyroiditis, which can cause seizures, stroke-like episodes, and altered mental status. The EEG findings can be non-specific, including slowing and FIRDA, similar to what is described in this patient.