Although not commonly used by clinical psychologists, criteria for five levels of hoarding have been set forth by the National Study Group on Chronic Disorganization (NSGCD) entitled the NGSCD Clutter Hoarding Scale. Using the perspective of a professional organizer, this scale distinguishes five levels of hoarding with Level I (Roman numeral one) being the least severe and Level V (Roman numeral 5) being the worst. Within each level there are four specific categories which define the severity of clutter and hoarding potential:
Household is considered standard. No special knowledge in working with the Chronically Disorganized is necessary.
Household requires professional organizers or related professionals to have additional knowledge and understanding of Chronic Disorganization.
Household may require services in addition to those a professional organizer and related professional can provide. Professional organizers and related professionals working with Level III households should have significant training in Chronic Disorganization and have developed a helpful community network of resources, especially mental health providers.
Household needs the help of a professional organizer and a coordinated team of service providers. Psychological, medical issues or financial hardships are generally involved. Resources will be necessary to bring a household to a functional level. These services may include pest control services, "crime scene cleaners," financial counseling and licensed contractors and handy persons.
Household will require intervention from a wide range of agencies. Professional organizers should not venture directly into working solo with this type of household. The Level V household may be under the care of a conservator or be an inherited estate of a mentally ill individual. Assistance is needed from many sources. A team needs to be assembled. Members of the team should be identified before beginning additional work. These members may include social services and psychological/mental health representative (not applicable if inherited estate), conservator/trustee, building and zoning, fire and safety, landlord, legal aid and/or legal representatives. A written strategy needs to be outlined and contractual agreements made before proceeding.
The following (edited) case study is taken from a published account of compulsive hoarding:
The client, #1 lived with her two children, aged 11 and 14 and described her current hoarding behaviour as a "small problem that mushroomed" many years ago, along with corresponding marital difficulties. D reported that her father was a hoarder and that she started saving when she was a child ... The volume of cluttered possessions took up approximately 70 percent of the living space in her house. With the exception of the bathroom, none of the rooms in the house could easily be used for their intended purpose. Both of the doors to the outside were blocked, so entry to the house was through the garage and the kitchen, where the table and chairs were covered with papers, newspapers, bills, books, half-consumed bags of chips and her children's school papers dating back ten years.
The following case study is taken from a published account of compulsive hoarding:
A 79-year-old woman recently died in a fire at her Washington, DC, row house when "pack rat conditions" held back firefighters from reaching her in time. A couple of days later, 47 firefighters from 4 cities spent 2 hours fighting a fire in a Southern California home before they were able to bring it under control. There was floor-to-ceiling clutter that had made it almost impossible for them to come in the house
It is not clear whether compulsive hoarding is a condition in itself, or rather a symptom of other related conditions. Several studies[specify] have reported a correlation between hoarding and the presence and / or severity of obsessive–compulsive disorder (OCD). Compulsive hoarding does not seem to involve the same neurological mechanisms as more familiar forms of obsessive–compulsive disorder and does not respond to the same drugs (which target serotonin). Hoarding behavior is also related to obsessive–compulsive personality disorder (OCPD). There may be an overlap with a condition known as impulse control disorder (ICD), particularly when compulsive hoarding is linked to compulsive buying or acquisition behavior. However, some people displaying compulsive hoarding behaviour show no other signs of what is usually considered to be OCD, OCPD or ICD. Those diagnosed with attention-deficit hyperactivity disorder (ADHD) often have hoarding tendencies.
Bibliomania is an obsessive–compulsive disorder involving the collecting or hoarding of books to the point where social relations or health are damaged. One of several psychological disorders associated with books, bibliomania is characterized by the collecting of books which have no use to the collector nor any great intrinsic value to a more conventional book collector. The purchase of multiple copies of the same book and edition and the accumulation of books beyond possible capacity of use or enjoyment are frequent symptoms of bibliomania.
Digital hoarding involves collecting files on one's computer beyond the point of usefulness. Often, files can be acquired through the Internet at no monetary cost, leading to extraordinarily large collections. Examples are music collections, often beyond what one enjoys or can listen to and television shows, movies and computer games. Hoarders, or "digital pack rats", often resort to buying optical media or new hard drives to store their collections, rather than deleting what they may never use.
Digital hoarders find it just as difficult to press delete as traditional hoarders find throwing items in the trash can. They have the same feeling of clutter and chaos, and feel that they might find the item useful "someday," and similarly spend large amounts of time acquiring and organizing their collections. However, unlike physical clutter, automated systems exist to organize digital clutter. Scientific American remarked that humanity's propensity for data collection is growing at a rate faster than their ability to store it.
Digital hoarding is not a currently recognized subtype of compulsive hoarding by the DSM. Animal hoarding
Animal hoarding involves keeping larger than usual numbers of animals as pets without having the ability to properly house or care for them, while at the same time denying this inability. Compulsive animal hoarding can be characterized as a symptom of obsessive–compulsive disorder rather than deliberate cruelty towards animals. Hoarders are deeply attached to their pets and find it extremely difficult to let the pets go. They typically cannot comprehend that they are harming their pets by failing to provide them with proper care. Hoarders tend to believe that they provide the right amount of care for their pets. The American Society for the Prevention of Cruelty to Animals provides a "Hoarding Prevention Team", which works with hoarders to help them attain a manageable and healthy number of pets. Along with other compulsive hoarding behaviours, it is linked in the DSM-IV to obsessive–compulsive disorder and obsessive–compulsive personality disorder. Alternatively, animal hoarding could be related to addiction, dementia, or even focal delusion.
Animal hoarders display symptoms of delusional disorder in that they have a "belief system out of touch with reality". Virtually all hoarders lack insight into the extent of deterioration in their habitations and the health of their animals, refusing to acknowledge that anything is wrong. Delusional disorder is an effective model in that it offers an explanation of hoarder’s apparent blindness to the realities of their situations. Another model that has been suggested to explain animal hoarding is attachment disorder, which is primarily caused by poor parent-child relationships during childhood. As a result, those suffering from attachment disorder may turn to possessions, such as animals, to fill their need for a loving relationship. Interviews with animal hoarders have revealed that often hoarders experienced domestic trauma in childhood, providing evidence for this model. Perhaps the strongest psychological model put forward to explain animal hoarding is obsessive–compulsive disorder (OCD).
Brain imaging studies (PET) have shown that the cerebral glucose metabolism patterns seen in OCD hoarders were distinct from the patterns in non-hoarding OCD. The most notable difference in these patterns was the decreased activity of the dorsal anterior cingulated gyrus, a part of the brain that is responsible for focus, attention and decision making. A 2004 University of Iowa study found that damage to the frontal lobes of the brain can lead to poor judgment and emotional disturbances, while damage to the right medial prefrontal cortex of the brain tends to cause compulsive hoarding. 
Obsessive compulsive disorders are treated with a various antidepressants: from the Tricyclic antidepressant family clomipramine (brand name Anafranil); and from the SSRI families paroxetine (Paxil), fluoxetine (Prozac), fluvoxamine (Luvox), sertraline (Zoloft) and citalopram (Celexa). With existing drug therapy OCD symptoms can be controlled, but not cured. Several of these compounds have been tested successfully in conjunction with OCD hoarding, but paroxetine in particular is indicated for treatment of compulsive hoarding. A 2006 study of this usage of the drug to do treat compulsive hoarding was conducted by the University of California, San Diego. OCD disorders are also treated with psychotherapy