A Summary of Chronic Fatigue Syndrome and Its Classification
in the International Classification of Diseases
Prepared by the Centers for Disease Control and Prevention,
National Center for Health Statistics, Office of the Center Director, Data Policy and Standards
March 2001
This document provides a summary of the classification of Chronic Fatigue Syndrome in the
International Classification of Diseases, ninth and tenth revisions, and their clinical
modifications.
ICD-9
The International Classification of Diseases, ninth revision (ICD-9), was published by the World
Health Organization (WHO) in 1975. WHO did not make revisions to the classification between
major updates, which usually occurred every ten years. The term “chronic fatigue syndrome” did
not have a specific code in ICD-9 nor did the term appear in the alphabetic index of ICD-9. The
only entry in the alphabetic index of the ICD-9 was “Syndrome, fatigue” and referenced code
300.5, Neurasthenia, a condition classified in Chapter V, Mental disorders.
The term “benign myalgic encephalomyelitis” appears in the alphabetic index and references
code 323.9, Encephalitis of unspecified cause. The code 323.9 did not include reference to
postviral syndrome. The term “postviral syndrome” was classified to code 780.7, Malaise and
fatigue, in Chapter 16, Symptoms, signs and ill-defined conditions.
It should be noted that while many terms are listed in the alphabetic index, all of the terms may
not appear in the tabular list of the classification. This is a standard convention of the ICD.
ICD-9-CM
For morbidity data the United States uses the International Classification of Diseases, ninth
revision, clinical modification (ICD-9-CM), a clinical modification of ICD-9. ICD-9-CM has
been used in the United States since 1979 and has an annual update process that has been in place
since 1985. The update process begins with the convening of the public forum, ICD-9-CM
Coordination and Maintenance Committee. Proposals to modify the classification are presented
and discussed during these public meetings. Information about future meetings of the ICD-9-CM
Coordination and Maintenance Committee may be found on the NCHS website at
http://www.cdc.gov/nchs/about/otheract/icd9/maint/maint.htm. In 1990, a recommendation to
create a specific code for chronic fatigue syndrome was presented. At that time, there was no
consensus about the etiology of the syndrome, which is needed to accurately classify a condition
in the ICD. A new code could not be created because of this problem; however, a modification
to the alphabetic index was made to direct users of the classification to code 780.7, Malaise and
fatigue. This is the same code used to identify cases of postviral syndrome. This change became
effective October 1, 1991.
In 1998, subcategory 780.7 was expanded to include new five-digit codes. The new codes created
included code 780.71, Chronic fatigue syndrome. The placement of this condition in this
category was consistent with the WHO version of ICD-9 and with its placement within ICD-9-CM.
ICD-10
WHO published ICD-10 in 1992 and included many modifications, among them relocation of
some diagnoses to different chapters within the classification. WHO created a new category
G93, Other disorders of brain, in Chapter VI, Diseases of the Nervous System, and created a new
code G93.3, Postviral fatigue syndrome, a condition which was previously in the symptom
chapter of ICD-9. WHO also moved benign myalgic encephalomyelitis to the new code G93.3.
The alphabetic index contains other terms, such as chronic fatigue syndrome, that WHO
considers synonymous or clinically similar.
Changes made in ICD-10 are unique to that version of the classification and not retrospectively
applied to previous revisions of the ICD. Therefore, any changes in ICD-10 such as the creation
of new categories or relocation of conditions from one chapter to another are not retroactively
added to ICD-9 or ICD-9-CM.
ICD-10-CM
In keeping with the placement in the ICD-10, chronic fatigue syndrome (and its synonymous
terms) will remain at G93.3 in ICD-10-CM.
While it appears most appropriate to classify chronic fatigue syndrome in ICD-10-CM in the
same way that it is classified in ICD-10, this placement is not without problems. The primary
concern with the current WHO placement in ICD-10 has been that the abnormalities of the brain
in chronic fatigue syndrome patients most often cited in the literature are not found in all chronic
fatigue syndrome patients. While chronic fatigue syndrome may be a heterogeneous group of
disorders, some but not all are neurological in nature. Likewise, not all patients have
experienced a viral infection prior to being diagnosed with chronic fatigue syndrome, nor are
immune system anomalies universally found. Also of potential concern is the similarity between
the type of neurological findings in chronic fatigue syndrome and in depression, which is a
psychiatric disorder. Involvement of multiple systems has complicated the classification of
chronic fatigue syndrome.
It should be noted that issues related to reimbursement have not been a factor in deliberations
regarding placement of chronic fatigue syndrome in ICD. Modifications to ICD-9-CM (the
classification currently in use) and in ICD-10-CM, its intended replacement, are based on
relevant clinical information and adherence to the structure and conventions of the ICD. relevant clinical information and adherence to the structure and conventions of the ICD. The decision of third party payers regarding their coverage and reimbursement policies are independent of the decisions regarding modification of the classification.