Essential Tremor (ET), the most common movement disorder, causes shaking of the hands, voice or other parts of the body. Some people with ET also have mild cognitive dysfunction, but it’s unclear if it comes from the same origin in the brain as the uncontrollable movement.

Cognitive function is not the same as intelligence

When we talk about cognitive function, we don’t mean intelligence. A person with a normal – or even high – IQ who develops ET may develop cognitive dysfunction with no loss of intelligence. This distinction is evident from the way each ability is evaluated:

• IQ (intelligence quotient) tests evaluate mathematical ability, language ability, spatial ability, memory and recall. Intelligence level stays the same throughout life.
• Cognitive assessments measure perception, motor skills, attention, and executive (organization and planning) skills. Cognitive abilities change – they can be developed and improved, and they can also worsen due to disease, aging, accidents, etc.

Thus, ET does not affect intelligence but it can impair the brain’s information processing abilities.

ET complicates multitasking, including motor skills

In today’s world, we hear a lot about multitasking. The term comes from computer technology, since computers can run more than one application or program at a time. The operating system keeps track of multiple running threads, and the user can hop back and forth without loss of information or efficiency because the tasks run separately and don’t interfere with each other.

However, humans have success with multitasking. The human brain constantly processes input from the senses as well as the flow of ideas and emotions. It is also making decisions, retrieving memories organizing plans, carrying them out, etc. When faced with a situation or problem, the brain can selectively focus on specific details, big picture possibilities, or deep systematic analysis – but it has a hard time doing that all at once, since some can interfere with others as brain areas integrate it all.

Now add in physical activity like walking, driving a car, mopping a floor, playing table tennis, etc. How many people (even without ET) can simultaneously swim laps, rehearse lines from a Shakespeare play, and plan a dinner party? It would be a challenge do to two of those at the same time, let alone all three. But what will suffer more? The physical activity, or the mental task?

A study by Rao et al. (2013)ⁱ poses the question whether ET-related cognitive impairments affect large motor activities such as walking, which already requires some amount of cognitive attention. While walking, you have to stay on the route, keep proper speed to get where you’re going on time, avoid hazards, etc.

To test their theory that ET causes further cognitive deficits that compromise a normal motor task like walking, they recruited 151 ET patients and 62 controls. All ET participants were given a standardized assessment of cognitive function. Based on scores, they were divided into two groups: ET patients with high cognitive score (ETp-HCS) and those with low cognitive scores associated with mild functional deficits (ETp-LCS,). All participants, including controls, underwent gait measurement (speed, stride length, cadence) by walking on a computerized mat. The walking measurements were performed under two conditions:
    A. walking normally at their preferred speed, and
    B. walking at their preferred speed while performing a verbal cognitive task (e.g. name as many animals as possible starting with the letter “b”).

Each condition was repeated three times for every participant.

Results

All groups (controls, ETp-HCS and ETp-LCS) experienced a decrease in gait functionality during the verbal task walks. While the controls and ETp-HCS groups had roughly equal lower scores, there was a significant worsening for the ETp-LCS group. It appeared that the verbal cognitive task further complicated their gait, given that they already had mild functional (not intelligence) deficits.

While slowed gait has been previously demonstrated among elderly people performing a cognitive task while walking, the authors note that “… our results show that ET participants demonstrate gait impairments above and beyond those seen in the healthy elderly. These gait impairments worsen during the performance of a concurrent [simultaneous] cognitive task during gait, particularly in ET participants with lower scores on a cognitive test.”

Thus, ET patients who have diminished cognitive ability in addition to uncontrollable shaky movements may have less motor ability when walking. More research is needed to confirm the observations of this study.
NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you have health concerns or questions of a personal medical nature.

ⁱ Rao AK, Uddin J, Gillman A, Louis ED. Cognitive motor interference during dual-task gait in essential tremor. Gait Posture. 2013 Jul;38(3):403-9.