Seizure Disorder: Given the patient's history of a significant head injury, which can lead to the development of seizure disorders, and the presentation of an episode where the patient was found unable to speak and reportedly choking while sleeping, followed by an inability to walk and lower back tightness, a seizure is a plausible explanation. The episode's characteristics, such as loss of consciousness, possible seizure activity (indicated by choking and inability to speak), and post-ictal confusion or weakness (inability to walk), support this diagnosis.

Post-Traumatic Epilepsy: This condition is directly related to the patient's history of head injury. The symptoms described could be indicative of a seizure, which would align with post-traumatic epilepsy, especially given the patient's history of significant head trauma.
Sleep Apnea or Other Sleep Disorders: Although less directly related to the head injury, the fact that the episode occurred while sleeping and involved choking could suggest a sleep disorder. However, the inability to walk and lower back tightness afterward might not be directly explained by sleep apnea alone.
Transient Ischemic Attack (TIA) or Stroke: Although less likely given the patient's age and lack of other risk factors, a TIA or stroke could present with sudden neurological deficits. The history of head injury might increase the risk of vascular events due to possible vascular damage.

Meningitis or Encephalitis: Infections of the brain or meninges can present with sudden neurological decline, including seizures, and are critical to diagnose and treat promptly. Although the patient does not have a reported fever or recent illness, these conditions can sometimes present atypically.
Subarachnoid Hemorrhage: Given the history of head injury, there's a possibility, although small, of a hemorrhagic event, especially if the patient has developed an aneurysm or vascular malformation since the injury.
Spinal Cord Injury or Compression: The patient's report of lower back tightness and inability to walk could indicate a spinal issue, potentially related to the initial trauma or a new event.

Conversion Disorder (Functional Neurological Disorder): This condition involves neurological symptoms that cannot be explained by a medical condition. Given the patient's history of head injury and ongoing memory issues, psychological factors could contribute to the presentation.
Mitochondrial Myopathies: These are a group of disorders that affect the mitochondria, often presenting with neurological and muscular symptoms. While rare, they could potentially explain some of the patient's symptoms, especially if there's a familial component or other systemic symptoms.