The international classification of diseases ICD is a globally used tool by the medical fraternity for diagnosis in epidemiology, health management and clinical purposes. Originally designed as a healthcare classification system it is maintained by the World Health Organization (WHO) under the United Nations Systems, which encompasses a system of diagnostic codes for diseases, nuanced classification of a variety of clinical signs, symptoms, complaints, social circumstances, abnormalities and external causes of injury or disease .
A six character-long code may be generated for any such ailment comprising of its generic category, specific variations, thus mapping the health conditions of an individual or community. The system finds use globally by other health administration systems and healthcare services for morbidity and mortality statistics, reimbursement schemes, as well as automated decision systems employed in medical healthcare.
The ICD is published by the WHO with the latest being in 1992 titled ICD-10, with updated modifications introduced over certain periods of time, depending upon nuances in a particular field, simplification of pre-existing classification systems or treatment needs. However, as of January 2022, an updated ICD-11 has been put into effect, which had been adopted by the World Health Assembly (WHA) on May 25, 2019. However, the historical basis of this classification system dates back to the latter half of the 19th century with healthcare stalwarts in the field such as Florence Nightingale and Jacques Bertillon.
Nightingale in 1860 called for a methodical approach in recording healthcare data across hospitals, and the proposal led to the development of systematic data collection. Bertillon 1893 presented the Bertillon Classification of the Causes of Death that was presented and approved at the congress of the International Statistical Institute, held in Chicago that year.
The system was based on the principle of distinguishing the causes of death from general causes to diseases limited to affecting a single organ or anatomical site in the body. The system originally had 44 titles and was adopted wholly by the City of Paris in the year of its inception, however, as other countries began to adopt the system, significant modifications were done which led to the system comprising over 161 titles; a resultant from the revisions and incorporations by the English, German and Swiss systems.
On the advice of the American Public Health Association (APHA), the systems were adopted by Canada, Mexico and the United States in 1898. The APHA also recommended this system be updated every 10 years to keep in line with the advancements in medical technology. This practice was continued since 1900, as ICD-1.
No significant modifications were made to the publication every decade with a few minor changes recorded tabularly and alphabetically in a small book. This was maintained by a mixed committee of representatives from both the International Statistical Institute and Health Organization under the League of Nations. 
This all changed in 1949, with the advent of the 6th volume of the system, i.e., ICD-6, were in the addition of morbidity and mortality statistics, and the title was modified to reflect this change; International Classification of Diseases, Injury and Causes of Death. The WHO had in the previous year (1948) assumed responsibility for the publication of the ICD every decade, following through for the next two iterations ICD-7 and ICD-8 in 1958 and 1967 respectively, before concluding that a 10-year revision for such a classification system was too short for the rate of advancing medical technology at the time.
TABLE-1: A BROAD HISTORICAL TIMELINE FOR ALL VERSIONS OF THE ICD SINCE INCEPTION
Several modifications were made in and after the publications of ICD-9 and ICD-10 pertaining to the healthcare systems adopted by the US, Canadian, Australian and other governments suiting their needs over both healthcare-related statistics as well as the state reimbursement and claim systems introduced and in vogue during the time.
ADOPTION AND MODIFICATIONS
Although the WHO published the ICD nearly every decade, some countries require modifications before they may implement it. For instance, with the advent of ICD-9, a separate supplemental document was published in separate fascicles known as the International Classification of Procedures in Medicine, abbreviated as ICPM, which enlisted codes for surgical procedures.
This system however was introduced by the United States, whereas a separate procedure list had been included in the ICD in all its previous versions. Several countries across the globe now use a translated and/or modified version of this ICPM in some capacity or the other. 
The International Classification of Disease-Clinical Modifications was also a modified document, created by the National Center for Health Statistics in the United States, which adopts the ICD as well as enlists morbidity data into the system. 
Similarly, after the publication of the ICD-10, the clinical modifications counterpart ICD-10-CM was slow to be adopted by the National Centre for Health Statistics in the United States. This was due to governmental policies of Medicare and Medicaid following the ICM-9-CM for the healthcare industry across the country. This resulted in a mixed system comprising morbidity details being based on ICD-9-CM whereas the mortality statistics were based on the ICD-10 without clinical extensions.
The system was re-updated in October 2013, where the existing ICD-9-CM codes had to be interchanged with that of the ICD-10-CM, however, the delays in congressional debates resulted in the actual implementation of the system being delayed by a year to October 2014. 
Canada also adopted a modified system for recording its health statistics titled ICA-10-CA that extends beyond hospital healthcare services into risk factors such as socio-economical factors, psychological factors, occupational factors, environmental factors and lifestyle. This aided the Canadian Institute for Health Information to record morbidity classification, extending beyond the scope of simple acute hospital healthcare. 
ICD-10: ADOPTION AND REPLACEMENT
The work on ICD-10 began in 1983 and was endorsed by the 43rd World Health Assembly in 1990. It is the 10th modification of the original ICD upheld by the League of Nations pre-World War II. The base document enlists about 14000 codes to encompass new diagnoses as compared to its predecessor the ICD-9 which had only about a few thousand base codes.
The clinical modification of the ICD-10 however enlists upwards of about 70000 codes including health statistics, morbidity, mortality and procedural codes. It allows for tracking the specificity of disease, through the use of optional sub-classification systems, the cause, manifestation, location, severity and type of disease. It has 22 chapters enlisting a wide array of diseases denoted in specific codes of alpha-numeric type. 
|I||A00-B99||Certain infectious and parasitic diseases|
|III||D50-D89||Diseases of blood & blood-forming organs and certain disorders involving the immune mechanism|
|IV||E00-E90||Endocrine, nutritional and metabolic diseases|
|V||F00-F99||Mental and behavioral disorders|
|VI||G00-G99||Diseases of the nervous system|
|VII||H00-H59||Diseases of the eye and adnexa|
|VIII||H60-H95||Diseases of the ear and mastoid process|
|IX||I00-I99||Diseases of the circulatory system|
|X||J00-J99||Diseases of the respiratory system|
|XI||K00-K93||Diseases of the digestive system|
|XII||L00-L99||Diseases of the skin and subcutaneous tissue|
|XIII||M00-M99||Diseases of the musculoskeletal system and connective tissue|
|XIV||N00-N99||Diseases of the genitourinary system|
|XV||O00-O99||Pregnancy, childbirth and the puerpurem|
|XVI||P00-P96||Certain conditions originating in the perinatal period|
|XVII||Q00-Q99||Congenital malformations, deformations and chromosomal abnormalities|
|XVIII||R00-R99||Symptoms, signs and abnormal laboratory and clinical findings, not elsewhere classified|
|XIX||S00-T98||Injury, poisoning and certain other consequences of external circumstances|
|XX||V01-Y98||External causes of morbidity and mortality|
|XXI||Z00-Z99||Factors influencing health status and contact with health services|
|XXII||U00-U99||Codes for special purposes|
TABLE-2: ICD-10 LIST OF CHAPTERS AND CODES
The common complaints in the United States about the ICD-10-CM are
1) The long list of potentially relevant codes for a given condition (such as rheumatoid arthritis) can be confusing and reduce efficiency
2) The assigned codes for seldom seen conditions 
Thus this system was replaced by the ICD-11 which has been in effect since January 2022.
It is the 11th revision of the International Classification of Diseases which has been in effect since January 2022. The development of this iteration was started in 2007 and spanned the expertise of 300 specialists from 55 countries divided into 30 workgroups. An alpha version was drafted in 2011 followed by a beta draft in 2012. The stable draft was released in June 2018 and endorsed by the WHO members in the 72nd World Health Assembly in May 2019.
It consists of about 85000 entities also termed as classes or nodes. An entity, relevant to healthcare may range from a disease or a pathogen to an isolated symptom or a developmental anomaly or disability. The ICD-11 has a foundation component that entails the basis of the WHO Family of International Classification (WHO-FIC) which assigns alphanumeric codes to a particular entity depending upon the parent and child nodes it has been taken from.
The ICD-11 is thus also termed as ICD-11 MMS, to prevent confusion from the foundation document, which also encompasses diseases, disorders, body parts, bodily functions, the reasons for a visit to a healthcare facility, medical procedures, and microbes, the causes of death and social circumstances of the patient.
The foundation document includes chapters from ICD-11 MMS, ICF and ICHI. It is thus a multidimensional collection of entities. An entity can have multiple parents and child nodes. For example, pneumonia can be categorized as a lung infection, but also as a bacterial or viral infection (i.e. by site or by etiology). Thus, the node Pneumonia (entity id: 142052508) has two parents:
Lung infections (entity id: 915779102) and certain infectious or parasitic diseases (entity id: 1435254666). The Pneumonia node in turn has various children, including bacterial pneumonia (entity id: 1323682030) and Viral pneumonia (entity id: 1024154490). The node classification should however not be confused with entity ID.
The ICD-11 can thus be analyzed on the basis of the following headings
- Chapters included
- Notable changes from ICD-10
The WHO Family of International Classifications is a family of classification systems that include the International Classification of Disease, International Classification of Functioning, Disability and Health (ICF) and International Classification of Health Intervention (ICHI). This forms the foundation of the 11th ICD which encompasses a multi-dimensional approach to the classification of disease, disability, symptoms, socio-economic conditions, morbidities and so on which is the core of all other referenced and derived classification systems.
The derived classification systems are usually tailored pertaining to a particular specialty which guarantees consistency. Such classifications are listed as a tabular or hierarchical form having single parent nodes that entail every enlisted entity as mutually exclusive of each other. Such a form of classification is also referred to as linearization. 
The chapters included in ICD-11 MMS are the primary linearization of the foundation component in ICD 11. Unlike the ICD-10 codes, the ICD-11 MMS codes never contain the letters I or O, to prevent confusion with the numbers 1 and 0.The list of chapters is as follows;
|1||1A00-1H0Z||Certain infectious or parasitic diseases|
|3||3A00-3C0Z||Diseases of blood or blood forming organs|
|4||4A00-4B4Z||Diseases of the immune system|
|5||5A00-5D46||Endocrine, nutritional or metabolic diseases|
|6||6A00-6E8Z||Mental, behavioral or neurodevelopmental disorders|
|8||8A00-8E7Z||Diseases of the nervous system|
|9||9A00-9E1Z||Diseases of the visual system|
|10||AA00-AC0Z||Diseases of the ear or mastoid process|
|11||BA00-BE2Z||Diseases of the circulatory system|
|12||CA00-CB7Z||Diseases of the respiratory system|
|13||DA00-DE2Z||Diseases of the digestive system|
|14||EA00-EM0Z||Diseases of skin|
|15||FA00-FC0Z||Diseases of the musculo-skeletal system or connective tissue|
|16||GA00-GC8Z||Diseases of the genitourinary system|
|17||HA00-HA8Z||Conditions related to sexual health|
|18||JA00-JB6Z||Pregnancy, childbirth or the puerperium|
|19||KA00-KD5Z||Certain conditions originating in the perinatal period|
|21||MA00-MH2Y||Symptoms, signs or clinical findings, not elsewhere classified|
|22||NA00-NF2Z||Injury, poisoning or certain other consequences of external causes|
|23||PA00-PL2Z||External causes of morbidity or mortality|
|24||QA00-QF4Z||Factors affecting health status or contact with healthcare services|
|25||RA00-RA26||Codes for special purposes|
|26||SA00-SJ3Z||Supplementary Chapter Traditional Medicine Conditions Module-1|
|27||VA00-VC50||Supplementary section for functioning assessment (in line with WHODAS2)|
TABLE-3: LIST OF ICD-11 CHAPTERS
A host of changes are evident between the ICD-10 and ICD-11. These changes are effected under numerous categories with several implications. Broadly they may be studied under the following heads;
- Mental health
- Burn out
- Sexual health
- Traditional medicine
One of the primary and most notable changes seen in ICD-11 as compared to ICD-10 is a flexible coding system. The previous system comprised of an alpha-numeric code with a letter of the alphabet followed by two digits creating 99 blocks per chapter excluding blocks and sub-classifications. Although this was enough for most chapters, chapters I, II, XIX and XX (see table-2) are so voluminous that they expand beyond two letters.
This was eliminated in ICD-11 MMS by assigning a single first character for every chapter, with the first 9 chapters with codes 1 to 9 and the next 19 chapters from codes A to X. The letters I and O are excluded to avoid confusion with numbers 0 and 1 respectively. This is followed by a letter, a number and a fourth character starting with an alphabet but ending with either a number or an alphabet. This forced addition of a third number by the WHO ensures the elimination of undesired words. Unlike its predecessor, ICD-11 is a block that may or may not have a code, but every entity has been assigned a unique ID.
The next level is no longer denoted by a dot and a number, as the new coding system enables characters to continue from A to Z after the initial 0 to 9 has been exhausted. This also allows for later updates in the current codes assigned without having to significantly alter any code for the near foreseeable future. This allows for certain stability in codes assigned currently to the entities in question.
The ICD-11 also features 5 new chapters as compared to ICD-10. The splitting of blood and blood-forming organs into separate chapters 3 and 4 respectively, along with the inclusion of chapters pertaining to sleep-wake disorders, sexual health and traditional medicine have made categorization easier as compared to its predecessor.
Several changes have been made in the mental health section of ICD-11. Apart from modifications of certain diseases, most notable are the revamping of the personality disorder section and the addition of the gaming disorder section in this iteration. 
The most noted changes are as follows
- The addition of disorders already existing in the American iteration of ICD-10-CM has been incorporated in ICD-11; binge eating disorder, bipolar type II disorder, body dysmorphic disorder, excoriation disorder frotteuristic disorder, hoarding disorder and intermittent explosive disorder
- Other than that, certain disorders have been newly added that were not a part of ICD-10-CM, namely avoidant/restrictive food intake disorder, body integrity dysphoria, catatonia, complex PTSD, olfactory disorder and prolonged grief disorder.
- Distinct personality disorders have been collapsed into a single entity with a dimensional model as opposed to the previous categorical model
- All types of schizophrenia have also been collapsed and the model has been changed from categorical to dimensional along with the symptoms category of psychiatric disorder, namely positive, negative, manic, depressive psychomotor and cognitive symptoms
- Deletion of permanent mood disorders that included cyclothymia and dysthymia
- Merging of separate phobic disorders into a group called Anxiety or fear-related disorders
- All pervasive disorders merged into one single category called Autism spectrum disorder
- Rett syndrome removed to developmental anomalies chapter
- Renaming of hyperkinetic disorders to attention deficit hyperactivity disorder, along with the elimination of hyperkinetic conduct disorder
- Acute stress is no longer considered a mental disorder and has been included in the factors influencing health status and contact with the healthcare services chapter
Apart from this, analogous to the Blue-book published under ICD 10 in 1992, a Clinical Descriptions and Diagnostic Guides subset, abbreviated as ICD-11 CDDG, is also in development. As of February 2022, this document is yet to be made public. 
Contrary to media reporting done in 2019, burn out is not a new section added in ICD11. ICD10 did have a burn out section with a single sentence definition, however, ICD11 makes a longer summary, specifically denoting that it be used only in occupational context and after ruling out disorders specifically related to stress and anxiety or fear-related disorders. As defined by WHO, burn out is not a mental disorder but an occupational phenomenon undermining the well-being of a person at the workplace.
The ICD 10 determined sexual health-related disorders in a Cartesian separation of organic or body related and non-organic or mental related. The organic causes were previously enlisted under genitourinary disorders and the non-organic causes under mental health. This outdated concept of mind-body split has been eliminated in ICD 11. Sexual disorders are now seen as an integrated brain and body disorders resulting from both physical and psychosexual factors. 
This eliminates the previous Cartesian separation of organic and non-organic sexual disorders. As compared to the previous three categories enlisted in ICD 10, namely, lack or loss of sexual desire, sexual aversion and lack of sexual enjoyment and failure of genital response, ICD 11 employs only two categories as hypoactive sexual desire dysfunction and sexual arousal dysfunction, further sub-classified for male and female. Five other etiological qualifiers have also been added to specify the diagnosis along with a temporal factor-like life-long, acquired or situational.
Excessive sexual desire in ICD 10 has been renamed to compulsive sexual behaviour disorder, and is listed under impulse control disorders and states that it is not a disorder unless and until it causes significant distress, impaired functioning or impaired behaviour control.
Disorders of sexual preference enlisted in ICD 10 have been renamed to paraphilic disorders in ICD 11. These disorders have now excluded fetishism and transvestitism which are no longer considered as sexual diseases. However, Frotteuristic disorders have been included in this category. 
Transgenderism and gender dysphoria have been now termed as gender incongruence. ICD 10 enlisted transsexualism, dual role transvestitism and gender identity disorder of childhood. Dual role transvestitism has been eliminated due to a lack of any public health or clinical concern. Transsexualism is now termed as gender incongruence of adults and gender identity disorder of childhood as sexual incongruence of childhood.
The reason behind this as stated by the WHO is both to provide better care for transsexuals who have reportedly suffered greater for both lack of distinct category as well as the obsolete psychopathological approach of medical science towards it. Thus eliminating gender-related disorders from mental health to the newly added sexual health chapter provides not only a unique coding system for such disorders for reimbursement but also prevents eliminating transgenderism from the ICD 11 altogether. 
The global antimicrobial resistance surveillance systems, devised by the WHO in 2015 have now been incorporated into the ICD 11. This aims to track the worldwide immunity of malicious microbes against medication. The coding block has also been significantly expanded to a 10 digit numerical key as compared to the U82 to U95 in the ICD 10 system. 
Supplementary Chapter Traditional Medicines-Module 1 has been another new addition to the ICD 11. Although based on the Traditional Chinese Medicine, the term Traditional Medicine has been chosen by WHO as many such traditional medicine practices find their origin in Japan, Korea and Vietnam. A second and a third module named TM 2 and TM 3 are now in development which is to include Ayurveda and Homoeopathy practices. A fourth module called TM 4 is also in development as reported by several authors intending to incorporate other traditional medicine systems with independent diagnostic conditions in a similar fashion.
This decision however has been criticized by global journals such as Nature which allege these traditional practices to be a pseudoscience. Several authors also noted that this keeps speculations open for another classification system called International Classification of Traditional Medicine (ICTM), developed in 2008, to what actually encompasses its domain. The WHO argues that this is done only for statistical purposes and does not endorse such treatment. The intent of this TM1 and subsequent modules is done primarily due to 2 reasons 
- Several Asian healthcare practices involve traditional medicine in some form or the other in primary healthcare
- Nearly 70 to 80 per cent population worldwide use some form of traditional therapy for certain conditions such as acupuncture
However, American Scientific journal editorials blame the Chinese influence in this matter as alternative medical practices comprise a billion-dollar industry with a key share in China. However, these TM1 and subsequent module codes are to be used only in conjunction with the Western Medicine codes provided in chapters 1 to 25 of the ICD 11. 
Other notable changes seen in ICD 11 include
- Stroke is now considered a neurological disorder instead of a disease of the circulatory system
- Allergies are now coded under diseases of the immune system
- The obsolete concept of organic and non-organic sleep disorders enlisted in ICD 10 has been incorporated into a new chapter termed as Sleep-Wake disorders
- The supplementary section for functioning assessment is an additional chapter included that provides codes for the WHO Disability Assessment Schedule 2.0 (WHODAS2), the Model Disability Survey and the ICF.
Change pertaining to Dentistry and Dental Care
An assessment carried out by Sato et al (2019) compared the codes relating to dental care across ICD 10 and ICD 11. The authors have noted several significant distinctions which are enlisted as follows;
- ICD 11 has 24 categories of codes as compared to the 14 categories in ICD 10
- Gum and periodontal diseases have been subdivided into Gingival diseases and Periodontal diseases
- Dental caries and other hard tissue related diseases of teeth are now collapsed into one single category named diseases of hard tissues of teeth
- However, the stages of tooth decay are not categorized
- The extension codes system in ICD 11 has helped categorize cementum and enamel caries separately that may track the disease progression
- ICD 11 also makes note of secondary caries which were not present in its predecessor
Thus, in conclusion, ICD 11 enlists a host of changes as compared to ICD 10, with significant room for the addition of new codes, and extended codes for the tracking of disease progression. The revamped personality disorders and inclusion of gaming disorders have been a phenomenal change, along with the abolishment of several obsolete concepts of the mind-body split.
The aversion to sexual health-related issues from a psychopathological perspective to a more holistic mind-body duality concept has been significant and may provide for better healthcare as well as avoid a hassle in reimbursement and policy-related complicacies. This change may also be reflected in cases of sleep-wake disorders, anxiety-related problems and certain addictive and behavioural disorders in the near future.
The controversial addition of the supplementary traditional medicine modules and the subsequent modules in development may give rise to a pseudoscientific classification system in ICTM, however, if used wisely alongside the concepts of Western medicine, this concept might nuance both the statistical significance of classical healthcare procedures provided worldwide alongside holistic and alternative therapeutic practices.
The supplemental documents still in development in this regard are yet to unfold and a time-dependent consequence of this new classification system shall come to light in the days to come. The changes in government policies, the introduction of new treatment modalities and other real-world situations also have to be taken into account while assessing the merits and demerits of this system.
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- Reed GM, Drescher J, Krueger RB, Atalla E, Cochran SD, First MB, et al. (October 2016). “Disorders related to sexuality and gender identity in the ICD-11: revising the ICD-10 classification based on current scientific evidence, best clinical practices, and human rights considerations”. World Psychiatry. 15 (3): 205–221.
- The ICD‐10 classification of Sexual dysfunctions (F52) is based on a Cartesian separation of “organic” and “non‐organic” conditions.
- Reed et al. (2019): “The classification of sleep disorders in the ICD‐10 relied on the now obsolete separation between organic and non‐organic disorders (…) The ICD‐10 also embodied a dichotomy between organic and non‐organic in the realm of sexual dysfunctions”
- Sato, Yoko ET AL A Comparison of Representative Dental Disease Codes between ICD-10 and ICD-11; Diagnostic Sciences;2019
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