Background/Purpose:

Clinicians utilize several different clinical measurements (metrology) in patients with Ankylosing Spondylitis (AS) to assess the spinal mobility.  We aimed to examine the pattern of change in metrology over time and to evaluate the factors associated with these changes.

Methods:

In a prospective study, we included participants from a multi-ethnic, multi-center cohort of AS patients meeting modified New York criteria.  The primary outcomes were eight individual clinical measurements, recorded at baseline and follow up visits every 6 months: cervical rotation and flexion, occiput to wall distance, chest expansion, modified Schober, lateral lumbar spinal flexion, and hip internal/external rotation.  The independent variables included demographic, health habits, medications, radiologic and the patient-perceived functional limitation index (Bath AS Functional and Disease Activity Indices [BASFI & BASDAI]).  Mixed models that accounted for correlation of repeated measures longitudinally as well as interaction with time were conducted to examine whether the spinal mobility pattern is changed by each of independent variables among AS patients.

Results:

Overall, 611 patients, including 177 female (29%) and 462 (75.7%) non-Hispanic white were followed up to 4 years.   The univariable analyses are shown in table 1.  Age, sex, disease duration, smoking habits, radiologic severity and patient perceived functional limitation and disease activity indices at baseline were associated with metrology over time.  However, ethnicity and treatment with NSAID and anti-tumor necrosis factor (TNF)-α medications at baseline did not correlate with metrology over time among AS patients. Multivariable analysis indicated that after adjusting for age, sex, employment status, BASFI and BASRI at baseline, age>40 at enrollment associated with worse outcome among all measurements, except hip rotation. Gender, race, and educational status were not associated with the longitudinal changes in metrology. Smoking at baseline visit was associated with greater worsening of lateral flexion and cervical flexion and rotation. Moreover, being employed as well as lower BASDAI, BASFI, and BASRI score at baseline were associated with slower progression over time, across all eight metrology indices.

Conclusion:

Demographics and disease severity indices at baseline were independent correlates of the worsening of clinical measurements among patients with AS. Further studies are encouraged to identify whether reversing the potentially modifiable factors such as smoking cessation can slow the rate of progression.

Table 1.  Univariable longitudinal analysis of demographic and clinical factors associated with deterioration of mobility over time among patients with ankylosing spondylitis

Predictor factor	Cervical flexion	Cervical rotation	Chest expansion	Hip external rotation	Hip internal rotation	Lateral lumbar spine flexion	Occiput to wall	Schober test
Age ≥40 years	-24.4 (<0.001) *	-29.6 (<0.001)	-1.2 (<0.001)	-1.6 (0.003)	-2.8 (<0.001)	-4.9 (<0.001)	-3.7 (<0.001)	-0.9 (<0.001)
Sex, male	-9.1 (0.001)	-7.3 (0.040)	-0.3 (0.065)	0.6 (0.557)	-2.8 (<0.001)	-1.5 (0.006)	3.2 (<0.001)	-0.3 (0.026)
Ethnicity, non-Hispanic white	1.8 (0.761)	-0.3 (0.908)	0.6 (0.011)	0.1 (0.319)	-0.3 (0.291)	0.6 (0.604)	-0.1 (0.662)	0.2 (0.393)
Educational level, college degree or above	-1.1 (0.814)	-0.5 (0.530)	0.3 (0.057)	1.3 (0.254)	1.8 (0.104)	0.9 (0.244)	-2.3 (0.027)	0.3 (0.029)
Employment status, disabled	-23.2 (<0.001)	-33.5 (<0.001)	-1.4 (<0.001)	-2.6 (0.001)	-3.8 (<0.001)	-4.9 (<0.001)	5.6 (<0.001)	-1.3 (<0.001)
Smoking , current smoker	-11.2 (<0.001)	-13.8 (<0.001)	-0.6 (<0.001)	-0.5 (0.126)	-1.4 (0.006)	-2.3 (<0.001)	3.0 (<0.001)	-0.4 (<0.001)
Disease duration, >17 years	-24.9 (<0.001)	-33.4 (<0.001)	-1.1 (<0.001)	-0.2 (0.797)	-2.2 (0.004)	-4.9 (<0.001)	4.2 (<0.001)	-1.1 (<0.001)
History of joint surgery	-20.6 (<0.001)	-34.4 (<0.001)	-0.8 (<0.001)	-5.0 (<0.001)	-4.9 (<0.001)	-3.8 (<0.001)	2.5 (<0.001)	-0.8 (<0.001)
BASFI** ≥26	-20.1 (<0.001)	-27.4 (<0.001)	-1.4 (<0.001)	-3.3 (<0.001)	-4.5 (<0.001)	-4.8 (<0.001)	3.9 (<0.001)	-1.2 (<0.001)
BASDAIβ ≥4	-4.9 (0.034)	-9.6 (0.001)	-0.5 (0.005)	-2.6 (<0.001)	-1.7 (0.011)	-1.2 (0.018)	1.4 (0.206)	-0.4 (0.002)
BASRIγ ≥6	-35.7 (<0.001)	-43.8 (<0.001)	-2.0 (<0.001)	-1.3 (0.022)	-4.4 (<0.001)	-7.4 (<0.001)	6.5 (<0.001)	-1.7 (<0.001)
mSASSSα≥5	-35.5 (<0.001)	-43.7 (<0.001)	-1.9 (<0.001)	-1.2 (0.078)	-3.9 (<0.001)	-7.4 (<0.001)	6.2 (<0.001)	-1.7 (<0.001)
NSAID¥	6.3 (0.786)	6.9 (0.241)	0.1 (0.121)	1.2 (0.344)	-0.2 (0.434)	1.2 (0.972)	-0.6 (0.665)	0.3 (0.343)
Anti TNF-α  agent use	-2.6 (0.838)	-4.1 (0.616)	-0.1 (0.012)	-1.4 (0.048)	-0.5 (0.364)	-0.2 (0.538)	0.2 (0.210)	0.1 (0.161)
* mean difference (p-value) ; **BASFI: Bath AS Functional Index ; βBASDAI: Bath AS Disease Activity Index γBASRI: Bath Ankylosing Spondylitis Radiology Index αmSASSS: modified Stoke Ankylosing Spondylitis Spine Score and ¥NSAID: Nonsteroidal anti-inflammatory drugs

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