Background/Purpose:

Fatigue is one of the cardinal features of ankylosing spondylitis (AS). The clinical and laboratory correlates of fatigue in AS however are not well defined. In the current study we undertake a systematic analysis of fatigue in a longitudinal observation cohort of AS patients.

Methods:

A systematic review of 950 AS patients (671 male and 279 female) followed in a longitudinal clinic which entails regular clinic visits using a standardized protocol. Fatigue was recorded using the Fatigue Severity Scales (FSS). T tests were used to compare continuous variables and Chi-Squared tests for categorical variables. Multivariate analysis was conducted using logistic regression to assess associations between FSS and various clinical features. P-value <0.05 was used to define statistical significance.

Results:

In the univariate analysis there were a number of clinical variables showing association with FSS. This was followed by logistic regression analysis. Figure 1 outlines selected covariates based on a p-value<0.05 in univariate models and included in the full model. Stepwise selection was used to determine the variables most associated with (FSS). In the reduced model the clinical domains with the strongest correlation with FSS were morning stiffness, Bath AS Functional Index (BASFI), and Short Form (36) Health Survey (SF36-MCS). For patients with morning stiffness, there is an expected 2.13 increase in the FSS. For every unit increase in the BASFI, there is a 0.37 increase expected in the FSS. For every unit increase in the SF36-MCS, there is a 0.11 decrease in FSS.

Conclusion:

Fatigue continues a frequent and sometimes disabling aspect of AS. The strong correlation with stiffness suggests that these two variables may reflect a common underlying process. Since morning stiffness is considered a surrogate indicator of inflammation, fatigue may fall into this conceptual framework as well. The level of disability as measured by BASFI shows stronger correlation with fatigue than with mSASSS suggesting this fatigue may impose greater functional restrictions on patients than structural progression of the disease does. The reversed SF36-MCS correlation with FSS indicates more vitality, bodily pain, emotional, social and mental health functionality impairment in fatigued AS patient.

TABLE: Linear Regression Models to Assess Variables associated with Fatigue (FSS)

	Full Model		Reduced Model
	Covariate Estimate (SE)	p-value	Estimate (SE)	p-value
Age	0.05 (0.05)	0.29	—	—
Age at diagnosis of AS	-0.04 (0.06)	0.53	—	—
Sex	1.28 (2.28)	0.58	—	—
ESR	0.005 (0.05)	0.92	—	—
CRP	0.02 (0.05)	0.71	—	—
Enthesitis	3.84 (2.32)	0.12	—	—
Morning stiffness	2.47 (1.61)	0.15	2.13 (0.79)	0.01
Peripheral arthritis	-0.89 (1.44)	0.55	—	—
Nocturnal back pain	0.49 (1.09)	0.66	—	—
MSASS	-0.02 (0.04)	0.64	—	—
BASDAI	0.03 (0.29)	0.92	—	—
BASFI	0.51 (0.30)	0.11	0.37 (0.13)	0.009
ASQoL	0.14 (0.20)	0.50	—	—
EQ5D	-1.58 (3.30)	0.64	—	—
SF36-PCS	0.05 (0.08)	0.57	—	—
SF36-MCS	-0.05 (0.05)	0.39	-0.11 (0.03)	0.0005
HAQ	-1.80 (1.38)	0.21	—	—
NSAIDs	1.79 (1.14)	0.14	—	—
Biologics	1.14 (0.87)	0.21	—	—

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/fatigue-in-ankylosing-spondylitis-a-multivariable-analysis-implicates-inflammation-as-the-key-determinant-of-disability/