Fragmentary Myoclonus on PSG in Ankylosing Spondylitis
There is no established association between fragmentary myoclonus on polysomnography and ankylosing spondylitis, and this finding should not be considered common or expected in AS patients.
Evidence Base
The available evidence does not support a relationship between excessive fragmentary myoclonus (EFM) and ankylosing spondylitis:
No documented association exists in the literature. The major rheumatology guidelines for AS management from the American College of Rheumatology, Spondylitis Association of America, and Spondyloarthritis Research and Treatment Network make no mention of sleep-related movement disorders, fragmentary myoclonus, or polysomnographic findings as features of ankylosing spondylitis 1.
AS clinical features are well-characterized and do not include EFM. The spondyloarthropathies, including AS, are characterized by sacroiliitis, inflammatory arthritis, enthesitis, associations with inflammatory bowel disease or psoriasis, aortitis, and heart block—but not sleep-related movement disorders 1.
What EFM Actually Represents
Understanding the true nature of EFM is critical to avoid misattribution:
EFM is predominantly associated with peripheral nerve pathology, not rheumatologic disease. In a study of 98 patients with EFM detected incidentally on PSG, 50% had electrophysiological abnormalities, most commonly polyneuropathy (65.3%), followed by L5 nerve root lesions (26.5%) 2.
EFM occurs with specific sleep and neurological disorders. The phenomenon has been documented in association with sleep-related respiratory problems, periodic limb movements in sleep, narcolepsy, Parkinson's disease (62.7% prevalence), and Machado-Joseph disease (50% prevalence)—but not inflammatory arthropathies 3, 4, 5.
EFM is more common in older patients and males. A large prospective study found that patients with EFM had polyneuropathy and nerve root lesions significantly more frequently than controls (31.5% vs. 21%), and the finding was associated with advanced age 6, 5.
Clinical Implications
If EFM is found on PSG in an AS patient:
Investigate for peripheral nerve pathology. Patients with incidental EFM should undergo electrophysiological workup including motor nerve conduction studies of the peroneal and tibial nerves, F-wave recordings, sensory nerve conduction studies of the sural nerve, and needle electromyography 2.
Consider alternative explanations. EFM in an AS patient likely represents a coincidental finding related to age, peripheral neuropathy, obstructive sleep apnea, or periodic limb movements—not the AS itself 2, 6, 5.
Do not attribute EFM to AS pathophysiology. EFM is not primarily a sleep-related phenomenon but rather persists during sleep and points to peripheral nerve pathology in at least a portion of cases 2.
Common Pitfall
The major pitfall is assuming that any finding on PSG in an AS patient must be related to their rheumatologic disease. EFM has no established connection to inflammatory arthropathies and should prompt evaluation for the conditions with which it is actually associated: peripheral neuropathy, sleep apnea, and age-related changes 2, 6.