Symptoms of PDD may include behavioral and communication problems such as:

- Difficulty using and understanding language.

- Difficulty relating to people, objects, and events; for example, lack of eye contact, pointing behavior, and lack of facial responses.

- Unusual play with toys and other objects.

- Difficulty with changes in routine or familiar surroundings.

- Repetitive body movements or behavior patterns, such as hand flapping, hair twirling, foot tapping, or more complex movements.

- Inability to cuddle or be comforted.

- Difficulty regulating behaviors and emotions, which may result in temper tantrums, anxiety, and aggression.

- Emotional breakdowns.

Children with PDD vary widely in abilities, intelligence, and behaviors. Some children do not speak at all, others speak in limited phrases or conversations, and some have relatively normal language development. Repetitive play skills and limited social skills are generally evident as well. Unusual responses to sensory information – loud noises, lights – they also are common.

CDD is a rare condition, with only 1.7 cases per 100,000.

A child affected with childhood disintegrative disorder shows normal development and he/she acquires "normal development of age-appropriate verbal and nonverbal communication, social relationships, motor, play and self-care skills" comparable to other children of the same age. However, between the ages of 2 and 10, skills acquired are lost almost completely in at least two of the following six functional areas:

- Expressive language skills (being able to produce speech and communicate a message)

- Receptive language skills (comprehension of language - listening and understanding what is communicated)

- Social skills and self care skills

- Control over bowel and bladder

- Play skills

- Motor skills

Lack of normal function or impairment also occurs in at least two of the following three areas:

- Social interaction

- Communication

- Repetitive behavior and interest patterns

In her book, "Thinking in Pictures", Temple Grandin argues that compared to "Kanner's classic autism" and to Asperger syndrome, CDD is characterized with more severe sensory processing disorder but less severe cognitive problems. She also argues that compared to most autistic people, persons with CDD have more severe speech pathology and they usually do not respond well to stimulants.

Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. People with autism may be severely impaired in some respects but normal, or even superior, in others. Overt symptoms gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. It is distinguished not by a single symptom, but by a characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis. Autism's individual symptoms occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.

Autistic individuals can display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R) categorizes as follows.

- Stereotyped behaviors: Repetitive movements, such as hand flapping, head rolling, or body rocking.

- Compulsive behaviors: Time-consuming behaviors intended to reduce anxiety that an individual feels compelled to perform repeatedly or according to rigid rules, such as placing objects in a specific order, checking things, or hand washing.

- Sameness: Resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.

- Ritualistic behavior: Unvarying pattern of daily activities, such as an unchanging menu or a dressing ritual. This is closely associated with sameness and an independent validation has suggested combining the two factors.

- Restricted interests: Interests or fixations that are abnormal in theme or intensity of focus, such as preoccupation with a single television program, toy, or game.

- Self-injury: Behaviors such as eye-poking, skin-picking, hand-biting and head-banging.

No single repetitive or self-injurious behavior seems to be specific to autism, but autism appears to have an elevated pattern of occurrence and severity of these behaviors.

The childhood disintegrative disorder (CDD), also known as Heller's syndrome and disintegrative psychosis, is a rare condition characterized by late onset of developmental delays—or stunning reversals—in language, social function, and motor skills. Researchers have not been successful in finding a cause for the disorder. CDD has some similarity to autism, and is sometimes considered a low-functioning form of it. In May 2013, the term CDD, along with other types of autism, was fused into a single diagnostic term called "autism spectrum disorder" under the new DSM-5 manual. Therefore, CDD is now also called "regressive autism", being that this term can now refer to any type of autism spectrum disorder that involves regression, including CDD.

CDD was originally described by Austrian educator Theodor Heller (1869–1938) in 1908, 35 years before Leo Kanner and Hans Asperger described autism. Heller had previously used the name "dementia infantilis" for the syndrome.

An apparent period of fairly normal development is often noted before a regression in skills or a series of regressions in skills. The age at which this regression can occur varies, but typically after 3 years of normal development. The regression can be so dramatic that the child may be aware of it, and may in its beginning even ask, vocally, what is happening to them. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost.

Many children are already somewhat delayed when the disorder becomes apparent, but these delays are not always obvious in young children. This has been described by many writers as a devastating condition, affecting both the family and the individual's future. As is the case with all pervasive developmental disorder categories, there is considerable controversy about the right treatment for CDD.

Additionally, requirements for a proposed diagnosis such as the number and duration of symptoms and the impact on functioning are continuing to be investigated. But there is no doubt that both ADHD and SCT are found in children and adults and are linked to significant impairment and a diminished quality of life (QoL). The research by Barkley suggests that this is especially true if ADHD and SCT occur together: In adults, those comorbid cases were more likely to be unmarried and to be out of work on disability compared to cases with ADHD alone. But SCT alone is also present in the population and can be quite impairing in educational and occupational settings, even if it is not as pervasively impairing as ADHD.

In many ways, those who have an SCT profile have some of the opposite symptoms of those with classic ADHD: instead of being hyperactive, extroverted, obtrusive, excessively energetic and risk takers, those with SCT are drifting, absent-minded, listless, introspective and daydreamy. They feel as if "in the fog" and seem "out of it".

The comorbid psychiatric problems often associated with SCT are more often of the internalizing types, such as anxiety, unhappiness or depression. Most consistent across studies was a pattern of reticence and social withdrawal in interactions with peers. Their typically shy nature and slow response time has often been misinterpreted as aloofness or disinterest by others. In social group interactions, those with SCT may be ignored. People with classic ADHD are more likely to be rejected in these situations, because of their social intrusiveness or aggressive behavior. Compared to children with SCT, they are also much more likely to show antisocial behaviours like substance abuse, oppositional-defiant disorder or conduct disorder (frequent lying, stealing, fighting etc.). Fittingly, in terms of personality, ADHD seems to be associated with sensitivity to reward and fun seeking while SCT may be associated with punishment sensitivity.

A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered "normal". Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.

An Autistic spectrum. In its simplest form this joins together Autism and Asperger syndrome, and can additionally include other pervasive developmental disorders (PDD). These include PDD 'not otherwise specified' (including 'atypical autism'), as well as Rett syndrome and Childhood disintegrative disorder (CDD). The first three of these disorders are commonly called the autism spectrum disorders; the last two disorders are much rarer, and are sometimes placed in the autism spectrum and sometimes not. The merging of these disorders is based on findings that the symptom profiles are similar, such that individuals are better differentiated by clinical specifiers (i.e. dimensions of severity, such as extent of social communication difficulties or how fixed or restricted behaviors or interests are) and associated features (e.g. known genetic disorders, epilepsy, intellectual disabilities). The term Specific developmental disorders is reserved for categorizing particular specific learning disabilities and developmental disorders affecting coordination.

Craniodiaphyseal dysplasia (also known as CDD or lionitis) is an extremely rare autosomal recessive bone disorder that causes calcium to build up in the skull, disfiguring the facial features and reducing life expectancy.

These calcium deposits decrease the size of cranial foramina, and can also decrease the hole in the cervical spinal canal. In the few cases recorded, most of the sufferers died in childhood.

The underlying genetics are uncertain.

Among the medical signs are dacryocystitis, seizures, intellectual disability, and paralysis, each of which is a complication resulting from the diminutive foramina. A common sign reported as a result of the disease has been a difference of the size of the eyes.