Effect of Heat Exposure on Cognition in Persons With Tetraplegia

The ability to maintain normal body temperature (Tcore) is impaired in persons with tetraplegia: subnormal Tcore and vulnerability to hypothermia (\<95 F) have been documented in this population after exposure to even mild environmental temperatures. However, no work to date has addressed the effect of subnormal Tcore on cognitive performance in persons with tetraplegia despite studies with able-bodied (AB) individuals that have documented progressive decline in various aspects of cognitive performance associated with the magnitude of the depression in Tcore. The investigators' study will confirm and extend their initial observations in persons with higher cord lesions who have subnormal Tcore to show that cognitive performance will be improved by raising Tcore to euthermic levels. This improvement should be associated with greater function and independence, reintegration into society, and an improved quality of life.

Specific Aims: During exposure to 95 F for up to 120 minutes in the seated position, the investigators' aims are:

Primary Specific Aim: To determine if a modest rise in Tcore to euthermic levels has a positive effect on cognitive performance (attention, working memory, processing speed, and executive function) in persons with higher-level spinal cord injury (SCI).

Primary Hypothesis: Based on the investigators' pilot data: (1) 80% of persons with SCI will demonstrate an increase of 1 F in Tcore, while none of the AB controls will demonstrate such an increase; (2) 80% of persons with SCI will have an improvement of at least one T-score in Stroop Interference scores (a validated measure of executive function), while none of the AB controls will demonstrate a change in cognitive performance.

Secondary Specific Aim: To determine changes in: (1) The average of distal skin temperatures; (2) Sweat rate; and (3) Subjective rating of thermal sensitivity.

Secondary Hypothesis: Persons with SCI will have less of a percent change in average distal skin temperatures and sweat rate, and will report blunted ratings of thermal sensitivity compared to that of AB controls.

This study will investigate the physiological and cognitive responses to warm exposure in persons with SCI who have subnormal Tcore and in AB persons who are euthermic. The positive effect that an increase in Tcore will be expected to have on cognitive performance, specifically working memory and executive function, will be demonstrated. These two areas of cognitive performance are vital for the ability to optimally care for one's self, at which persons with higher cord lesions must excel in order to ensure health, and for the ability to attain the maximal degree of independence possible.

Preparation for Study Visits: Subjects will be instructed to avoid caffeine and alcohol, maintain normal salt and water intake, and avoid strenuous exercise for 24 hours prior to the study. Subjects will be asked to eat a light, standard meal (plain bagel or 2 pieces of toast) 2 hours prior to their visit. Subjects will be asked to empty their bladders prior to their visit and again upon arrival, if needed. Subjects will wear minimal clothing during the study to maximize skin exposure to the warm temperature.

Instrumentation: Subjects with SCI will be asked to remain in their wheelchairs and AB controls will be placed in a manual wheelchair where they will remain seated and relatively still for the duration of the study visit. A thin, flexible rectal sensor will be placed 4 inches beyond the anal sphincter for Tcore collection, and skin sensors will be secured at 15 sites above and below the level of lesion for collection of skin temperature (Tsk). Sweat collection capsules will be placed on the left bicep, the left forearm, the left thigh and the left calf for measurement of sweat rate by QSweat methodology (QS). Laser Doppler flowmetry will be used to measure changes in microvascular perfusion (MVP) by placing a Doppler probe on the back of both hands and both feet to confirm peripheral blood vessel dilation. An automated blood pressure cuff will be placed above the left elbow to measure brachial BP.

Baseline Collection (BL): At the end of a 30-minute acclimation period at 81 F, a baseline collection of the following parameters will be performed for 15 minutes: Tcore and Tsk will be continuously monitored; BP, heart rate (HR), and thermal sensitivity (TS) will be measured in 10-minute intervals; and MVP and QS will be measured in 15-minute intervals. After 15 minutes, a battery of cognitive tests will be administered to establish baseline cognitive performance.

Thermal Challenge (Heat): Following completion of the BL period, subjects will be wheeled into a pre-heated (95 F) thermal chamber for up to 120 minutes or until Tcore increases 1 F. Tcore and Tsk will be continuously monitored to ensure subjects' safety throughout the protocol, while HR, BP, and TS will be assessed at 10-minute intervals. MVP and QS will be measured every 30 minutes. The battery of cognitive tests utilized during BL collection will be administered when Tcore increases 1 F from BL values (for subjects with tetraplegia) or after 100 minutes (for controls) to determine any change in cognition after exposure to 95 F. An increase in Tcore 1 F, significant changes in BP, or subject discomfort will result in termination of the protocol.

Eligibility Criteria:
Inclusion Criteria:

* Duration of injury 1 year
* Level of SCI C3-T4, AIS A \& B
* Tcore at BL \<98.6 F (subnormal core body temperature)
* Euhydration
* Gender and age-matched ( 5 years) AB controls (between 18-68 years of age)

Exclusion Criteria:

* Known heart, kidney, peripheral vascular, or cerebral vascular disease
* High blood pressure
* History of traumatic brain injury or diagnosed cognitive impairment
* Untreated thyroid disease
* Diabetes mellitus
* Acute illness or infection
* Dehydration
* Smoking
* Pregnancy
* Broken, inflamed, or otherwise fragile skin