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Adult ADHD looks very much like childhood ADHD – the symptoms include an inability to focus or concentrate, trouble completing assigned tasks, restlessness, impulsivity, mood swings, trouble with relationships, difficulty coping with stress or frustration, and a hot temper, according to the Mayo Clinic.
The Mayo Clinic points out that other mental disorders can exist (be comorbid with) adult ADHD. One of these disorders is Borderline Personality Disorder.
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Symptoms of Adult ADHD
Adults with ADHD contend with an inability to focus their attention for more than a short period of time – especially on rote, boring tasks. The adult with ADHD has a hard time prioritizing tasks, organizing time, and avoiding careless mistakes. While the hyperactivity of childhood tends to resolve itself in adulthood, other symptoms may persist, making normal adolescent and young adulthood milestones more difficult to achieve.
One childhood symptoms that persists into adulthood is impulsivity. As with young children the adult with ADHD struggles to contain impulses to act on a moment’s notice. These adults also find it difficult to delay gratifying an urge (much as in childhood, where children with ADHD find it difficult to wait their turn and avoid interrupting others, according to the Mental Help website. 
In addition, adult ADHD can be linked to alcohol/drug abuse, trouble with law enforcement, unstable relationships, frequent vehicle accidents (or other kinds of accidents), and poor performance in school and work, according to the Mayo Clinic.  The Mayo Clinic states that adult ADHD and personality disorders can be linked. These disorders include anxiety disorders, mood disorders and personality disorders.
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Borderline Personality Disorder
The Encyclopedia of Mental Disorders defines Borderline Personality Disorder as a mental disorder “characterized by disturbed and unstable interpersonal relationships and self image, along with impulsive, reckless, and often self-destructive behavior." 
Adults with Borderline Personality Disorder have a difficult time viewing people significant in their lives; those people seem either extremely unfair or uncaring or absolutely flawless. Individuals with BPD either devalue or idealize those important to them – this behavior is called “splitting," according to the Encyclopedia of Mental Disorders.
Some causes of BPD include childhood traumas such as physical or sexual abuse, loss of a parental figure, or parental neglect. The child begins to develop feelings of self-dislike and inadequacy as a result of his or her life situations; this may be one cause of the development of BPD. As the person with BPD grows into adulthood, he tries to compensate for the mistreatment or neglect by establishing highly idealized demands on himself and others.
The symptoms of BPD include efforts to avoid either real or perceived abandonment; a pattern of intense and unstable interpersonal relationships that are characterized by alternating between idealization and devaluation; impulsive behaviors in at least two areas (binge eating, sexual acting-out, substance abuse, or reckless driving); persistent and unstable self-image and sense of self; recurrent suicidal behaviors, including threats and gestures or acts of self-mutilation (cutting or burning); unstable moods – irritability, depression, or anxiety; always feeling “empty;" intense and inappropriate anger, difficulty controlling anger outbursts, which is characterized by sarcasm and/or physical fighting; paranoia caused by stress, which passes quickly; dissociative symptoms, or feeling disconnected from the physical self, as if observing actions from outside.
The Encyclopedia of Mental Disorders points out that, in diagnosis, BPD “commonly" occurs with mood disorders, PTSD, eating disorders, ADHD, and other personality disorders. 
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 Hoermann, Simone. Personality Disorders and Attention Deficit/Hyperactivity Disorder, in MentalHelp.net, retrieved at http://www.mentalhelp.net/poc/view_doc.php?type=doc&id=31347&w=11&cn=3 MentalHelp.net
 Mayo Clinic Staff at MayoClinic.com. Adult ADHD (Attention Deficit/Hyperactivity Disorder), retrieved at http://www.mayoclinic.com/health/adult-adhd/DS01161/DSECTION=complications
 Encyclopedia of Mental Disorders. Borderline Personality Disorder, retrieved at http://www.minddisorders.com/A-Br/Borderline-personality-disorder.html
Image Credit: sxc.hu/urbaneye