## 2 Literature Review
### Monitoring Neurodegenerative Diseases  
Depending on the monitored condition, the following aspects have been found in the literature as relevant for monitoring neurodegenerative diseases:  
* **PD**: (1) _Motor symptoms_: PD can cause a variety of motor symptoms, with the most common ones including: akinesia (paucity of movements and delayed initiation of movement), bradykinesia (slowness of movement), hypokinesia (reduced amplitude of movement), postural instability (impaired ability to recover balance), rigidity (increased resistance to passive joint movement), stooped posture (a forward-hunched posture), tremor at rest (involuntary shaking, typically starting in the hands or fingers when the muscles are relaxed) [2; 5]. Postural instability, bradykinesia, stiffness, and tremors are the main symptoms. Wearables containing accelerometers are example technologies that can continuously track these motor signs [6]. Other movement disorders appear in patients with PD during the night [7], including nocturnal hypokinesia or akinesia, restless legs syndrome, periodic limb movement in sleep, among other abnormal movements during sleep; (2) _Gait analysis_: Patients with PD frequently have irregular gaits, which can be tracked by sensors worn on the body or placed in shoes [8]; (3) _Speech analysis_: Using microphones and voice analysis algorithms, voice alterations, such as reduced volume and pitch and rushed and slurred speech, can be observed [9]; (4) _Non-motor symptoms_: Sleep trackers, cognitive tests, and mood assessment apps can be used to monitor sleep disorders, cognitive deterioration, and mood swings [10]. _Sleep disorders_ (SD) are some of the most common non-motor symptoms of PD [7]. SDs may be caused or worsened by PD related motor issues that occur during sleep, as abnormal movement events related to PD, such as akinesia, rigidity and dystonia, occur during sleep as well. PD affects primarily motor functions with _mental decline_ generally appearing in later stages and the cognitive issues are more related to slowed thinking with memory loss problems appearing later in the progress of the disease [8]. * **AD:** (1) _Cognitive function_: Cognitive decline is usually the primary symptom of AD, with memory loss being one of the first noticeable symptoms [8]. Frequent cognitive assessments utilizing digital platforms can estimate cognitive impairments to gauge their degree and evolution over time, serving as screening assessments [11]; (2) _Daily activity monitoring_: Wearable technology and smart home devices can keep an eye on daily routines to spot any irregularities that can point to cognitive deterioration [12]; (3) _Sleep patterns_: Sleep-related health data (such as body movements, cardiac rhythm, respiration rate, snoring, discontinuances in the breathing episodes, etc.) is necessary in order to have an overview of the sleep quality, and patterns can be tracked using wearables to detect sleep disturbances common in AD [11]. Insufficient sleep is a risk factor for AD and AD leads to sleep deprivation and disruptions [13]; (4) _Movement disorders_: Although AD manifestations are typically related to cognitive functions, rigidity, slowness, and other movement disorders are common [14]. * **MS**: (1) _Neurological function_: Regular and comprehensive assessment of cognitive and motor functions utilizing digital platforms can help managing the symptoms [15]; (2) _Mobility and balance_: Mobility issues are also common in the majority of MS patients for whom decreased motor control, muscle weakness, balance problems, spasticity, fatigue and ataxia may lead to inability to walk long distances, impaired gait pattern, inability to support body weight [16; 17], problems which may lead to complete inability to walk, either permanent or during relapses.