Obstructive Sleep Apnea (OSA), a common disorder resulting from repetitive pharyngeal collapse during sleep, is multifactorial. Usually, OSA is considered primarily a problem of upper airway anatomy, with the craniofacial structure or neck fat decreasing the size of the pharyngeal airway lumen. Obesity, male sex and genetics are well established pathogenic factors. In the last decade rostral fluid displacement (fluid shift) to explain the pathogenesis of upper airway collapsibility has been increasingly studied.

Individuals living with spinal cord injury are at increased risk for OSA, with a prevalence that is three- to fourfold higher than the general population. Individual with acute tetraplegia and undiagnosed or untreated OSA may participate less in rehabilitation due to sleepiness and fatigue and therefore be less engaged in activities that improve quality of life and maintain functioning over time. Intermittent hypoxia, sleep fragmentation and alterations of the autonomous nervous system induced by OSA are thought to delay or limit recovery and in the long term, increase cardio- and cerebrovascular morbi-mortality.

Redolfi et al have shown that overnight change in leg fluid volume correlated strongly with the Apnea Hypopnea index (AHI) and the time spent sitting. In SCI patients two mechanisms may underline fluid shift importance in the pathogenesis of OSA: first, time spent sitting is obviously increased in patients with no walking abilities (prolonged sitting position in wheelchair). Secondly, motor deficit lead to the loss of skeletal muscle pumping activity which could promote leg fluid accumulation during the day.

In our knowledge, no study has specifically assessed the impact of rostral fluid displacement on upper airway collapsibility among patients with spinal cord injury.

Better comprehension of upper airway collapsibility determinants in patients with spinal cord injury is mandatory to identify new therapeutic targets (diuretics, contention…) especially since CPAP, the first line treatment for severe OSA, continue to pose adherence issues in SCI patients. In the future, phenotyping OSA patients, especially those with SCI, will improve personalized management.

The main objective is to find if there is a correlation between the apnea-hypopnea index (AHI) and rostral fluid shift overnight, in non-obese spinal cord injured patients. The secondary aim is to find if there is a correlation between AHI and:

• Neck circumference
• Neck volume
• Time spent sitting down

All SCI patients meeting inclusion criteria with scheduled polysomnography within the framework of usual care and referred to the sleep laboratory will be proposed to participate. These patients are usually recorded in a dedicated room within PMR units during a 3-day to one-week hospital stay.

Information will be given to eligible patients to allow time for reflexion before written consent is collected during the inclusion visit (during the PMR hospitalization or at the sleep laboratory to carry out the night recording).

Collection of clinical and demographic data will be performed by a PMR practitioner or sleep physician according to a predefined and standardized evaluation grid.

After consent form signature patients willing to participate will undergo "before-sleep" measurement of:

• Leg fluid volume
• Neck circumference
• Neck volume
• Short questionnaire developed for the needs of the study. Then patients will undergo a full night polysomnography according to the American Academy of sleep Medicine (AASM) guidelines.

The sleep recording will be carried out in a dedicated room within the spinal cord injury unit with the immediate proximity of a care team with specific competence in the field of the management of the heaviest patients (e.g: urinary catheterization, stool evacuation, pressure sore prevention, safe transfers, etc.).

The reading of the polysomnography will be ensured by a doctor specialized in sleep pathologies without any knowledge of the different impedance measurements, circumference and cervical volume measurements.

"After-sleep" measurement will be performed immediately after patients awakening (before any wheelchair transfer):

• Leg fluid volume
• Neck circumference
• Neck volume
• Short questionnaire developed for the needs of the study.

In summary: Before going to bed and before getting up for the first time, each patient will benefit from a standardized evaluation by measuring bioelectrical impedance on the lower limbs, 3D cervical acquisition and measurement of the neck circumference.

All of these measurements are non-invasive.

Inclusion Criteria:

Specific criteria:

• Low cervical (< C6), thoracic or lumbar traumatic spinal cord injury (SCI);
• BMI < 30Kg/m² ;
• Clinically complete SCI (AIS A) or incomplete with no walking ability;
• Neurological level;
• Aged >18 years;
• Patients with a previous indication for polysomnography as part of routine care and referred to sleep laboratory.

No-specific criteria:

• Affiliated to the social security system;
• Absence of serious intercurrent event.

Exclusion Criteria:

Specific criteria:

• Lower limbs amputation ;
• Treated OSA;
• Ongoing diuretic treatment;
• Pregnant woman;
• Pacemaker or other (spinal) stimulation equipment.

No-specific criteria:

• Patient refusal;
• Patient in a period of exclusion from another protocol;
• Inability to sign informed consent;
• Medical or surgical emergency context;
• Vulnerable person or adult subject to legal protection: pregnant or lactating women, person deprived of their liberty by judicial or administrative decision, person hospitalized without consent, or admitted for purposes other than research, Articles L1121 -5 to L1121-8.