I have the problem of weight loss&muscle atrophy for years,and I just found the info about Cachexia & AS which was rarely mentioned and discussed.

Living with a Chronic Disease – The Story of Ankylosing Spondylitis
https://www.physio-pedia.com/images/a/a4/Ankylosing_Spondylitis.pdf

5.CACHEXIA

This is defined as ‘an accelerated loss of skeletal muscle in the context of a chronic inflammatory response (Kotler 2000). This is a catabolic process which directly results in muscle atrophy, weakness, physical disability, increased infection rate and premature death (Kotler 2000). It is believed increased levels of tumour necrosis factor (TNF) and other pro-inflammatory cytokines in the body are attributed to cachexia in AS patients. A study by Marcora et al(2006) investigated muscle wasting in patients in AS. 19 patients with long standing AS (average duration 19 years) were compared to 19 age matched controls with similar levels of physical activity. Dual energy X-ray absorptiometry was used to assess body composition. Muscle strength was assessed by isokinetic knee extension, hand grip dynamometry and by 30s arm curl and chair sit-to-stand tests.Results demonstrated that patients with AS demonstrated a clinically and statistically significant 12% reduction in arms and legs and total body skeletal muscle mass in comparison to healthy controls. This muscle loss was significantly associated with reduced upper and lower body strength. Therefore it is evident that cachexia is a functionally relevant systemic complication of AS, especially those with severe disease.Nevertheless, body composition studiessuch as that above,have not consistently demonstrated a reduction in muscle mass in AS patients.

PHYSIOTHERAPY MONITORINGFOR CACHEXIA:
Physiotherapists should be aware of symptoms of cachexia such asreduced energy, weight loss, muscle wasting, and behaviours which additionally compromise energy intake, for example restlessness, malaise and anhedonia(the inability to experience pleasure from activities that are usually enjoyed. e.g. Hobbies, exercise, sexual activity or social interaction).
Resistance training and other interventions aimed at stimulating skeletal muscle growth might be of benefit for this population.