Background Age-related diseases, particularly dementia, now known as major neurocognitive disorder (NCD), are a great health burden in Hong Kong and worldwide. Interventions that aim to ameliorate cognitive decline or prevent dementia offer a compelling alternative paradigm for reducing the impact of the disease, not only on individuals but also on their families and on society. In principle, to achieve its optimal benefits, intervention for dementia should begin at the earliest preclinical stage. However, no evidence has been found to support a pharmacological approach to the prevention, reduction, or postponement of cognitive decline during the stage of mild NCD. Besides pharmacological approaches, non-invasive brain stimulation (NIBS) is increasingly recognised as a potential alternative to tackle this problem. The typical examples of NIBS are transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS). Besides these, there is a new NIBS named transcranial pulse stimulation (TPS), also known as low-intensity extracorporeal shock wave therapy (Li-ESWT), which recently obtained CE marking in 2018 for the treatment of the central nervous system (CNS) in patients with mild to moderate Alzheimer's disease (AD).

The introduction of TPS TPS is using repetitive single ultrashort pulses in the ultrasound frequency range to stimulate the brain. With a neuro-navigation device, TPS can achieve this in a highly focal and precisely targeted manner. TPS differs from tDCS and TMS using direct or induced electric current. Using electric current to stimulate the brain may be limited by the problem of conductivity and failure to reach deep brain regions. Instead, low-intensity focused ultrasound provides good spatial precision and resolution to noninvasively modulate subcortical areas, despite the problem of skull attenuation. Using lower ultrasound frequencies TPS can successfully improve skull penetration in humans.

Biological mechanism of TPS The basic mechanism of TPS is mechanotransduction. It is a biological pathway through which the cells convert the mechanical TPS stimulus into biochemical responses, thus influencing some fundamental cell functions such as migration, proliferation, differentiation, and apoptosis. The ultrashort ultrasound pulse could enhance the cell proliferation and differentiation in cultured neural stem cell, which plays an important role in the repair of brain function in CNS diseases. The TPS may affect neurons and induce neuroplastic effects through several pathways including increasing cell permeability, stimulation of mechanosensitive ion channels, the release of nitric oxide resulting in vasodilation, increased metabolic activity and angiogenesis, stimulation of vascular growth factors (VEGF) and stimulation of brain derived neurotrophic factor (BDNF).

Clinical effects of TPS Focused ultrasound demonstrated the neuromodulation effect in the human brain. Focused ultrasound can modulate the amplitude of somatosensory evoked potentials (SEPs) when targeted at the cortical regions that generate these potentials and even the deep structure such as the thalamus. TPS, previously named as Li-ESWT was applied to five patients with unresponsive wakefulness syndrome. They received 4-week (3 times per week) treatment, 4000 pulses each, every 6 months for an average of two to four years. There was significant improvement in the vigilance and in three patients the percutaneous endoscopic gastrostomy (PEG) tube could be removed due to improved oropharyngeal motor function. In the most recent study, TPS was applied to 35 elderly with AD. They were treated in 3 TPS sessions (6000 pulses each) per week for 2-4 weeks, either over classical AD affected sites such as the dorsolateral prefrontal cortex, areas of the memory and language network, or over all accessible brain areas (global brain stimulation). Significant improvement in the CERAD (Consortium to Establish a Registry for Alzheimer's Disease) score was demonstrated (immediately as well as 1 and 3 months after stimulation. fMRI also showed significant increased connectivity within the memory network.

Safety issue of TPS TPS uses very low energy for the brain stimulation. In vivo animal TPS study did not cause any tissue damage despite using 6-7-fold higher energy levels compared with those in human studies. Furthermore, the intervention did not cause any serious adverse effects such as intracranial bleeding, oedema or other intracranial pathology, as confirmed with MRI in a previous AD study. Few subjects reported headache (4%), pain or pressure (1%) and mood deterioration (3%). The CE marked TPS system has proven to be safe in \>1500 treatments.