Generally while filing for US Citizenship one must demonstrate an understanding of English language, and also reading, writing and speaking ability. English and Civics requirements for naturalization include the understanding of the fundamental of American history and principles.

In certain cases applicant due to physical or development disability or mental impairment that is permanent, and last for 12 months are even expected to be permanent, they shall be exempted from knowing English and Civics requirement while filing for naturalization. But these people should submit Form N-648 along with N-400 for the exemption.

Important Note:  If failed to submit the Form N-468 with Form N-400, their application for the delayed for adjudication of Form N-400. Per Immigration and Nationality Act (INA) 312(b)(1), applicant with developmental disability or mental impairment are not required to fulfill English and/or Civics requirement.
Applicant with reasonable accommodations includes, sign languages interpreters, extending time for testing and off-site testing – may not be limited. Many people think English and Civic requirement is the main criteria to obtain Citizenship but some medical disability exception.

Only medical doctors, doctors of osteopathy or clinical psychologists licensed to practice in United States. Staff of the medical practice associated with medical professional may assist in the completion of the form and checking accuracy of the form's content.

Form N-648 must be certified by a licensed medical professional, except for the section "Applicant Attestation" and "Interpreter's certification".

Part III of Form N-648 - DSM-IV Codes

Diagnostic and Statistical Manual of Mental Disorder – Edition IV, currently recognized mental health disorders.

1. Mental Retardation (Mild, Moderate, Severe, Profound).
2. Learning Disorder (Reading, Mathematics, written expression).

3. Motor Skill Disorder (developmental co-ordination).
4. Communication Disorder (expressive languages, phonological, stuttering, mixed respective & expressive).
5. Pervasive Developmental Disorder (Autistic, Rett's, Childhood Disintegrative, Asperger's)
6. Attention-Deficit & Disruptive Disorder (Hyperactivity, conduct disorder)
7. Feeding & Eating Disorder (Pica, Rumination)
8. Tic Disorder (Tourette's, Chronic motor, Transient tic)
9. Elimination Disorder (Enuresis – not due to medical condition; Encopresis – with/without constipation and overflow incontinence)
10. Others Disorder (Separation anxiety, Selective mutism, Reactive attachment, Stereotypic attachment)
11. Delirium, Dementia, and Amnestic and other Cognitive Disorders.
12. Alcohol Related Disorder (Abuse, Dependence, Induced mood)
13. Amphetamine Disorder (Abuse, Dependence, Induced mood)
14. Caffeine-related Disorder (Induced anxiety & sleep, Intoxication)
15. Cannabis-related Disorder (Abuse, Dependence, Induced mood)
16. Cocaine-related Disorder (Abuse, Dependence, Induced mood, Intoxication)
17. Hallucinogen-related Disorder (Abuse, Dependence, Induced mood, Intoxication)
18. Inhalant-related Disorder (Abuse, Dependence, Induced mood, Intoxication)
19. Nicotine-related Disorder (Dependence)
20. Opioid –related Disorder (Abuse, Dependence, Induced mood, Intoxication)
21. Phencyclidine – related Disorder (Abuse, Dependence, Induced mood, Intoxication)
22. Schizophrenia & other psychotic Disorder
23. Mood Disorder (Depressive, Bi Polar, Anxiety, Somatoform, Factitious, Dissociative)     
24. Sleep Disorder (Primary sleep, Parasomnias)

Need to provide basic descriptive of the disability. For Example "Down syndrome is a genetic disorder that causes lifelong intellectual disability (mental retardation) & developmental delays".