Patients and healthy controls
The study group consisted in 67 male patients with AS who fulfilled the modified New York criteria for the classification of AS and who presented consecutively to our department between September 2014 and July 2015. A group of 67 age-matched (within 2 yrs) healthy subjects was recruited from the same local population (hospital staff members and their family members, visitors…etc.) to serve as controls according to exclusion criteria. A subjects’ written consent was obtained according to the Declaration of Helsinki and the study was approved by our local ethics committee (Military Hospital Mohammed V, Rabat). Exclusion criteria were the presence of a history of neuroendocrine disorders (thyroid, parathyroid disorders, anticonvulsant usage etc.), chronic renal or liver diseases, systemic high dose steroid use, and excessive alcohol intake. The following data were collected for all the subjects: age, height, weight, and body mass index (BMI). The time elapsed between the onset of first AS related symptoms and enrollment defined disease duration. Disease activity and the functional consequences of the disease were assessed by the Bath AS disease activity index (BASDAI) and the Bath AS functional index (BASFI), respectively. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were assessed using standard laboratory techniques. Sacroiliitis was assessed on anteroposterior pelvic radiographs and graded according to the New York Scale. Spine structural damage was assessed by the Stoke AS Spinal Score (SASSS) in which each corner is scored for the presence of squaring, sclerosis, erosions, syndesmophytes and bridging syndesmophytes with a maximal score of 72.
Bone mineral density measurements
The DXA scans were obtained by standard procedures using Lunar Prodigy Vision machine. All BMD measurements were carried out by 2 experienced technicians. Daily quality control was carried out by measurement of a Lunar phantom which showed at the time of the study stable results. The coefficient of variation of the phantom precision was 0.08 and the reproducibility assessment in clinical practice showed in a previous study a smallest detectable difference of 0.04 g/cm2 (spine) and 0.02 (hips) . Patient BMD was measured at the lumbar spine (anteroposterior projection at L1–L4) and at the femurs (i.e., femoral neck, trochanter, and total hips) and the classification system of the World Health Organization (WHO) was applied, defining osteoporosis as T-score ≤ −2.5 and osteopenia as −2.5 < T-score ≤ −1 according to the lowest T-score of the L1–4 lumbar spine, femur neck, or total femur.
Body composition parameters assessment
All anthropometric measures were taken following standard procedures by the same investigator (FBE). The subjects were weighed to the nearest 0.1 kg, and standing height was measured to the nearest 0.1 cm and BMI was then calculated from weight/height2 (kg/m2). In accordance with WHO standards, individuals with BMI values <18.5 kg/m2 were considered underweight, between 18.5 and 24.9 as normal, 25 and 29.9 as overweight and values greater than 30 indicated obesity . Mid upper arm circumference and waist circumference were measured using a plastic, inelastic, flexible belt-type measuring tape to the nearest 0.5 cm.
Body composition (total and regional fat mass and lean mass) was measured with total body DXA using the same machine. The whole body scan used the DXA system’s automated software, which provided compositional estimates of legs, arms, trunk, head, and whole body. Scans were performed with the subject wearing light indoor clothing. The precision of soft tissue analysis for a Lunar Prodigy is 1 % for fat-free mass (FFM) and 2 % for fat mass (FM) . FFM and FM were expressed in absolute kg, and FM also as percentage of total mass. The normal reference value for FM% is 20 % to 30 % for women and 12 % to 20 % for men . Fat free mass index (FFMI, kg/m2), fat mass index (FMI, kg/m2) and skeletal mass index (SMI, kg/m2, where appendicular skeletal muscle mass is standardized using the square of the individuals’ height) were also calculated.
Muscle strength and performance assessment
Maximal voluntary grip strength of the dominant hand was measured with a Grip-A dynamometer (Takey, Kiki Kogyo, Japan). This assessment could be easily done as only 4 patients among the study population had peripheral involvement and none of the patients had hand joints involvement.
Global muscle performance was assessed by the Timed Get-Up-And-Go test: the subject rises from a chair, walks 3 meters, turns around, returns to the chair, and sits down. The subject was instructed to: “Sit with your back against the chair and your arms on the arm rests. On the word ‘go’, stand upright, then walk at your normal pace to the line on the floor, turn around, return to the chair, and sit down.“ The stopwatch was started on the word ‘go’ and stopped when the subject returned to the starting position.
Mini Nutritional Assessment
The MNA (0–30 points) is a dietary questionnaire assessing the number of meals, food and fluid intake and autonomy of feeding. It is a subjective assessment of self-perception of health and nutrition which also includes questions related to lifestyle, medication and morbidity. MNA classifies individuals with adequate nutritional status (>23.5 points), with risk for malnutrition (17–23.5 points) and with malnutrition (<17 points) .
Definition of pre-sarcopenia, sarcopenia and cachexia
As a consequence of the lack of a simple and commonly accepted definition for these conditions, we choose arbitrarily one from the published definitions:
Pre-sarcopenia was defined according to Baumgartner definition  by SMI <7.25 kg/m2.
Sarcopenia was defined by the combined presence of the two following criteria according to the European Working Group on Sarcopenia in Older People (EWGSOP) : a low muscle mass (SMI <7.25 kg/m2) and a low muscle strength (assessed by a handgrip strength <30 kg) or a low muscle performance (assessed by a timed get-up-and-go test >10 s).
Cachexia was defined according to the International Working Group on Sarcopenia (IWGS) definition  by a BMI <20 kg/m  plus 3 from the 5 following parameters: anorexia, fatigue, handgrip strength <30 kg, CRP >5 mg/l, FFMI <7.25 kg/m2.
The study was conducted on several steps. Step one consisted on the description of the study population. We compared in step 2 anthropometric and densitometric (BMD and body composition) variables between patients and healthy controls. In step 3, 4 and 5, we compared patients with or without pre-sarcopenia, sarcopenia and cachexia. A regression binary analysis was conducted in step 6 where the dependant variable was the presence of pre-sarcopenia and the independent variables the potential risk factors. And finally, to study the potential impact of TNF inhibitors (TNFi), in step 7, we compared patients taking or not TNFi.
Statistics Package for Social Sciences (SPSS Inc., Chicago, IL) was used for statistical analyses. Results are expressed in mean ± SD for quantitative variables and n (%) for qualitative variables.