prep4md

Mitosis vs. Meiosis

2012-04-22T18:27:00.001+02:00

Biology : Meiosis - cell division

2012-04-22T18:13:00.001+02:00

P-Prepare (by chromosomes condensing and copying)

M-Middle (line up in)

A-Apart (pulled)

T-Two Cells Created (haploid)

Secondhand Smoke and Lung Cancer

2012-04-07T02:16:00.001+02:00

huntingtons disease

2012-04-04T02:19:00.001+02:00

Upper Gastrointestinal Endoscopy Procedure

2012-03-30T00:51:00.001+02:00

Kidney Stones: Symptoms, Prevention, and Relief

2012-03-09T20:23:00.001+02:00

History presentation for medical students

2012-02-22T01:34:00.000+02:00

How to present a medical case?

2012-02-22T01:25:00.000+02:00

Understanding Citation Styles

2012-02-13T02:51:00.000+02:00

How to Cite Sources: Citing Without Quoting

2012-02-13T02:42:00.000+02:00

What Is Slippery Slope?

2012-02-13T02:00:00.000+02:00

What Is A Red Herring?

2012-02-13T01:49:00.000+02:00

What Is A Straw Man?

2012-02-13T01:39:00.000+02:00

What Syria Needs

2012-02-12T13:46:00.000+02:00

What Syria needs is an informed and mobilized public that is ready to

challenge authority on every stance. The days of obedience and

unquestionable loyalty are over. The walls of fear should be

demolished and the chains of submission must be dismantled. A

fundamental belief that all women and men are created equal needs to

be established. A celebration of diversity instead of suspicion in

others ought to be the attitude. The fact that Damascus and Aleppo

have not revolted should raise many questions. People of great

knowledge, wealth, and wisdom reside in these cities. Their views

should be studied and their concerns should be considered. Anti-regime

protesters have a grave task ahead of them. Not on the battlefield but

in the hearts and minds of every Damascene and Aleppine. Weapons of

pens and keyboards not swords and rifles should be in every hand. An

armed confrontation will only dramatically increase the bloodshed.

There is still a place for soft power, the word of mouth, debate, and

education. All that is worth it takes time. Because what Syria needs

is a countrywide reconstruction not an illusion of a revolution.

http://en.wikipedia.org/wiki/2011%E2%80%932012_Syrian_uprising

Whitney Houston and Syria

2012-02-12T13:13:00.001+02:00

Whitney Houston passes away and it triggers global grievances. A hundred people are killed in Syria on the same day and it doesn't receive a quarter of the media coverage that Whitney did. Is Whitney's life worth more than a hundred Syrians'? Probably not. I would argue the problem lies in the way the deaths have been reported. On the one hand we have pictures, videos, songs, friends' memories, families' speeches, and on the other hand we have mere statistics. Numbers are raw and cold. They tell us nothing about the individual. That's why one report of a boy with a name, a picture, and a story resonated more than a 100 or even a 1000 deaths reported as numbers. Gather names, collect pictures, and tell the story of the human behind every number. Perhaps soon Syrians will get the coverage they deserve. RIP Whitney Houston and Hamza Al-Khateeb.

What Is Evolution?

2012-02-03T14:18:00.000+02:00

What Does Open-mindedness Mean?

2012-02-03T13:46:00.000+02:00

What Is Critical Thinking?

2012-02-03T01:22:00.000+02:00

Women and Smoking: Seven Deadly Myths

2012-02-02T23:02:00.000+02:00

What Is Cryptorchidism? (undescended testis)

2012-01-29T18:27:00.001+02:00

Cryptorchidism is the absence of one or both testes from the scrotum. It is the most common birth defect regarding male genitalia. In unique cases, cryptorchidism can develop later in life, often as late as young adulthood. About 3% of full-term and 30% of premature infant boys are born with at least one undescended testis. However, about 80% of cryptorchid testes descend by the first year of life (the majority within three months), making the true incidence of cryptorchidism around 1% overall. Cryptorchidism is distinct from monorchism the condition of having only one testicle.

A testis absent from the normal scrotal position can be:

-found anywhere along the "path of descent" from high in the posterior (retroperitoneal) abdomen, just below the kidney, to the inguinal ring;
-found in the inguinal canal;
-ectopic, that is, found to have "wandered" from that path, usually outside the inguinal canal and sometimes even under the skin of the thigh, the perineum, the opposite scrotum, or the femoral canal;
-found to be undeveloped (hypoplastic) or severely abnormal (dysgenetic);
-found to have vanished (also see anorchia).

About two thirds of cases without other abnormalities are unilateral; one third involve both testes. In 90% of cases an undescended testis can be palpated in the inguinal canal; in a minority the testis or testes are in the abdomen or nonexistent (truly "hidden").

Undescended testes are associated with reduced fertility, increased risk of testicular germ cell tumors and psychological problems when the boy is grown. Undescended testes are also more susceptible to testicular torsion and infarction and inguinal hernias. To reduce these risks, undescended testes are usually brought into the scrotum in infancy by a surgical procedure called an orchiopexy.

Although cryptorchidism nearly always refers to congenital absence or maldescent, a testis observed in the scrotum in early infancy can occasionally "reascend" (move back up) into the inguinal canal. A testis which can readily move or be moved between the scrotum and canal is referred to as retractile.

Cryptorchidism is derived from the Greek words "crypto" (meaning "hidden") and "orchid" (meaning "testicle"). During the last century, cryptorchidism was sometimes restricted to the subset of undescended testes that were not palpable above the scrotum or in the inguinal canal – those that were truly hidden in the abdomen or completely absent. In recent decades the distinction is no longer made in most contexts, and the two terms are used interchangeably. Cryptorchism is an older variant of the same term.

Normal fetal testicular development and descent:

The testes begin as an immigration of primordial germ cells into testicular cords along the genital ridge in the abdomen of the early embryo. The interaction of several male genes organizes this developing gonad into a testis rather than an ovary by the second month of gestation. During the 3rd to 5th months, the cells in the testes differentiate into testosterone-producing Leydig cells, and anti-müllerian hormone-producing Sertoli cells. The germ cells in this environment become fetal spermatogonia. Male external genitalia develop during the 3rd and 4th months of gestation and the fetus continues to grow, develop, and differentiate.

The testes remain high in the abdomen until the 7th month of gestation, when they move from the abdomen through the inguinal canals into the two sides of the scrotum. It has been proposed that movement occurs in two phases, under control of somewhat different factors. The first phase, movement across the abdomen to the entrance of the inguinal canal appears controlled (or at least greatly influenced) by anti-müllerian hormone (AMH). The second phase, in which the testes move through the inguinal canal into the scrotum, is dependent on androgens (most importantly testosterone).Maldevelopment of the gubernaculum, or deficiency or insensitivity to either AMH or androgen therefore can prevent the testes from descending into the scrotum. Some evidence suggests there may even be an additional paracrine hormone, referred to as descendin, secreted by the testes.

In many infants with inguinal testes, further descent of the testes into the scrotum occurs in the first 6 months of life. This is attributed to the postnatal surge of gonadotropins and testosterone that normally occurs between the first and fourth months of life.

Spermatogenesis continues after birth. In the 3rd to 5th months of life, some of the fetal spermatogonia residing along the basement membrane become type A spermatogonia. More gradually, other fetal spermatogonia become type B spermatogonia and primary spermatocytes by the 5th year after birth. Spermatogenesis arrests at this stage until puberty.

Most normal-appearing undescended testis are also normal by microscopic examination, but reduced spermatogonia can be found. The tissue in undescended testes becomes more markedly abnormal ("degenerates") in microscopic appearance between 2 and 4 years after birth. There is some evidence that early orchiopexy reduces this degeneration.

Causes and risk factors

In most full-term infant boys with cryptorchidism but no other genital abnormalities, a cause cannot be found, making this a common, sporadic, unexplained (idiopathic) birth defect.

Severely premature infants can be born before descent of testes.

There is a strong association of cryptorchidism with low birthweight due to either prematurity or intrauterine growth retardation. In these infants there is usually no evidence of hormonal malfunction. Inguinal hernias associated with low birthweight are common.

A contributing role of environmental chemicals – endocrine disruptors – that interfere with normal fetal hormone balance has been proposed and is rarely possible to implicate a specific chemical exposure for an individual child.

Environmental risk factors may include exposure to regular alcohol consumption during pregnancy (5 or more drinks per week, associated with a 3x increase in cryptorchidism, when compared to non-drinking mothers).

A small percentage of cases of isolated cryptorchidism are familial.

Cryptorchidism occurs at a much higher rate in a large number of congenital malformation syndromes. Among the more common are Prader-Willi syndrome, Noonan syndrome, and cloacal exstrophy.

Diagnosis

The most common diagnostic dilemma in otherwise normal boys is distinguishing a retractile testis from a testis that will not/cannot descend spontaneously into the scrotum. Retractile testes are more common than truly undescended testes and do not need to be operated on. In normal males, as the cremaster muscle relaxes or contracts, the testis moves lower or higher ("retracts") in the scrotum. This cremasteric reflex is much more active in infant boys than older men. A retractile testis high in the scrotum can be difficult to distinguish from a position in the lower inguinal canal. Though there are various maneuvers used to do so, such as using a crosslegged position, soaping the examiner's fingers, or examining in a warm bath, the benefit of surgery in these cases can be a matter of clinical judgement.

In the minority of cases with bilaterally non-palpable testes, further testing to locate the testes, assess their function, and exclude additional problems is often useful. Pelvic ultrasound or magnetic resonance imaging performed and interpreted by a radiologist can often, but not invariably, locate the testes while confirming absence of a uterus. A karyotype can confirm or exclude forms of dysgenetic primary hypogonadism, such as Klinefelter syndrome or mixed gonadal dysgenesis. Hormone levels (especially gonadotropins and AMH) can help confirm that there are hormonally functional testes worth attempting to rescue, as can stimulation with a few injections of human chorionic gonadotropin to elicit a rise of the testosterone level. Occasionally these tests reveal an unsuspected and more complicated intersex condition.

In the even smaller minority of cryptorchid infants who have other obvious birth defects of the genitalia, further testing is crucial and has a high likelihood of detecting an intersex condition or other anatomic anomalies. Ambiguity can indicate either impaired androgen synthesis or reduced sensitivity. The presence of a uterus by pelvic ultrasound suggests either persistent müllerian duct syndrome (AMH deficiency or insensitivity) or a severely virilized genetic female with congenital adrenal hyperplasia. An unambiguous micropenis, especially accompanied by hypoglycemia or jaundice, suggests congenital hypopituitarism.

Treatment

The primary management of cryptorchidism is surgery, called orchiopexy. It is usually performed in infancy, if inguinal testes have not descended after 4–6 months, often by a pediatric urologist or pediatric surgeon, but in many communities still by a general urologist or surgeon.

When the undescended testis is in the inguinal canal, hormonal therapy is sometimes attempted and occasionally successful. The most commonly used hormone therapy is human chorionic gonadotropin. A series of hCG injections (10 injections over 5 weeks is common) is given and the status of the testis/testes is reassessed at the end. Although many trials have been published, the reported success rates range widely, from roughly 5 to 50%, probably reflecting the varying criteria for distinguishing retractile testes from low inguinal testes.

Hormone treatment does have the occasional incidental benefits of allowing confirmation of Leydig cell responsiveness (proven by a rise of the testosterone by the end of the injections) or inducing additional growth of a small penis (via the testosterone rise). Some surgeons have reported facilitation of surgery, perhaps by enhancing the size, vascularity, or healing of the tissue. A newer hormonal intervention used in Europe is use of GnRH analogs such as nafarelin or buserelin; the success rates and putative mechanism of action are similar to hCG, but some surgeons have combined the two treatments and reported higher descent rates. Limited evidence suggests that germ cell count is slightly better after hormone treatment; whether this translates into better sperm counts and fertility rates at maturity has not been established. The cost of either type of hormone treatment is less than that of surgery and the chance of complications at appropriate doses is minimal. Nevertheless, despite the potential advantages of a trial of hormonal therapy, many surgeons do not consider the success rates high enough to be worth the trouble since the surgery itself is usually simple and uncomplicated.

In cases where the testes are identified preoperatively in the inguinal canal, orchiopexy is often performed as an outpatient and has a very low complication rate. An incision is made over the inguinal canal. The testis with accompanying cord structure and blood supply is exposed, partially separated from the surrounding tissues ("mobilized"), and brought into the scrotum. It is sutured to the scrotal tissue or enclosed in a "subdartos pouch." The associated passage back into the inguinal canal, an inguinal hernia, is closed to prevent re-ascent.

When the testis is in the abdomen, the first stage of surgery is exploration to locate it, assess its viability, and determine the safest way to maintain or establish the blood supply. Multi-stage surgeries, or auto-transplantation and anastomosis, are more often necessary in these situations. Just as often, intra-abdominal exploration discovers that the testis is non-existent ("vanished"), or dysplastic and not salvageable.

The principal major complication of all types of orchiopexy is loss of the blood supply to the testis, resulting in loss of the testis due to ischemic atrophy or fibrosis.

Pathophysiology

At least one contributing mechanism for reduced spermatogenesis in cryptorchid testes is temperature. The temperature of testes in the scrotum is at least a couple of degrees cooler than in the abdomen. Animal experiments in the middle of the 20th century suggested that raising the temperature could damage fertility. Some circumstantial evidence suggests tight underwear and other practices that raise testicular temperature for prolonged periods can be associated with lower sperm counts. Nevertheless, research in recent decades suggests that the issue of fertility is more complex than a simple matter of temperature. It seems likely that subtle or transient hormone deficiencies or other factors that lead to lack of descent also impair the development of spermatogenic tissue.

The inhibition of spermatogenesis by ordinary intra-abdominal temperature is so potent that continual suspension of normal testes tightly against the inguinal ring at the top of the scrotum by means of special "suspensory briefs" has been researched as a method of male contraception, and was referred to as "artificial cryptorchidism" by one report.

An additional factor contributing to infertility is the high rate of anomalies of the epididymis in boys with cryptorchidism (over 90% in some studies). Even after orchiopexy, these may also affect sperm maturation and motility at an older age.

The most common type of testicular cancer occurring in undescended testes is seminoma. It is usually treatable if caught early, so urologists often recommend that boys who had orchiopexy as infants be taught testicular self-examination, to recognize testicular masses and seek early medical care for them. Cancer developing in an intra-abdominal testis would be unlikely to be recognized before considerable growth and spread, and one of the advantages of orchiopexy is that a mass developing in a scrotal testis is far easier to recognize than an intra-abdominal mass.

The risk of malignancy in the undescended testis is 4 to 10 times higher than that in the general population and is approximately 1 in 80 with a unilateral undescended testis and 1 in 40 to 1 in 50 for bilateral undescended testes. The peak age for this tumor is 15–45 yr. The most common tumor developing in an undescended testis is a seminoma (65%); in contrast, after orchiopexy, seminomas represent only 30% of testis tumors.

What Is Intussusception?

2012-01-29T00:46:00.001+02:00

An intussusception is a medical condition in which a part of the intestine has invaginated into another section of intestine, similar to the way in which the parts of a collapsible telescope slide into one another. This can often result in an obstruction. The part that prolapses into the other is called the intussusceptum, and the part that receives it is called the intussuscipiens.

Early symptoms can include nausea, vomiting (sometimes bile stained (green color), pulling legs to the chest area, and intermittent moderate to severe cramping abdominal pain. Pain is intermittent not because the intussusception temporarily resolves, but because the intussuscepted bowel segment transiently stops contracting. Later signs include rectal bleeding, often with "red currant jelly" stool (stool mixed with blood and mucus), and lethargy. Physical examination may reveal a "sausage-shaped" mass felt upon palpation of the abdomen.

In children or those too young to communicate their symptoms verbally, they may cry, draw their knees up to their chest or experience dyspnea (difficult or painful breathing) with paroxysms of pain.

Fever is not a symptom of intussusception. However, intussusception can cause a loop of bowel to become necrotic, secondary to ischemia due to compression to arterial blood supply. This leads to perforation and sepsis, which causes fever.

Diagnosis

Intussusception is often suspected based on history and physical exam, including observation of Dance's sign. Per rectal examination is particularly helpful in children as part of the intussusceptum may be felt by the finger. A definite diagnosis often requires confirmation by diagnostic imaging modalities. Ultrasound is today considered the imaging modality of choice for diagnosis and exclusion of intussusception due to its high accuracy and lack of radiation. A target-like mass, usually around 3 cm in diameter, confirms the diagnosis. An x-ray of the abdomen may be indicated for evaluation of intestinal obstruction or the presence of free intraperitoneal gas; the latter finding would imply that bowel perforation has already occurred. In some institutions, air enema is used for diagnosis as the same procedure can be used for treatment.

Differential diagnosis

Intussusception has three main differential diagnoses. These are acute gastroenteritis, Henoch–Schönlein purpura, and rectal prolapse. Abdominal pain, vomiting, and stool with mucus and blood are present in acute gastroenteritis, but diarrhea is the leading symptom. Rectal prolapse can be differentiated by projecting mucosa that can be felt in continuity with the perianal skin, whereas in intussusception the finger may pass indefinitely into the depth of sulcus. Henoch–Schönlein purpura presents the characteristic rash.

Treatment

The condition is not usually immediately life-threatening. The intussusception can be treated with either a barium or water-soluble contrast enema or an air-contrast enema, which both confirms the diagnosis of intussusception, and in most cases successfully reduces it. The success rate is over 80%. However, approximately 5–10% of these recur within 24 hours.

If it cannot be reduced by an enema or if the intestine is damaged, then a surgical reduction is necessary. In a surgical reduction, the abdomen is opened and the part that has telescoped in is squeezed out (rather than pulled out) manually by the surgeon or if the surgeon is unable to successfully reduce it or the bowel is damaged, the affected section will be resected. More often, the intussusception can be reduced by laparoscopy, whereby the segments of intestine are pulled apart by forceps.

Prognosis

The outlook for intussusception is excellent when treated quickly, but when untreated it can lead to death within 2–5 days. Fast treatment is a necessity, because the longer the intestine segment is prolapsed the longer it goes without bloodflow, and the less effective a non-surgical reduction will be. Prolonged intussusception also increases the likelihood of bowel ischemia and necrosis, requiring surgical resection.

Kathryn Schulz: On being wrong

2012-01-26T01:05:00.000+02:00

Are Electronic Cigarettes Bad For You?

2012-01-20T13:01:00.000+02:00

What Is Burkitt's Lymphoma?

2011-12-28T22:34:00.001+02:00

Burkitt's lymphoma (or "Burkitt's tumor", Burkitt lymphoma or "malignant lymphoma, Burkitt's type") is a cancer of the lymphatic system (in particular, B lymphocytes).

Burkitt's lymphoma can be divided into three main clinical variants: the endemic, the sporadic and the immunodeficiency-associated variants. Burkitt's lymphoma is usually associated with over 90% of AIDS cases.

The endemic variant occurs in equatorial Africa. It is the most common malignancy of children in this area. Children affected with the disease often also had chronic malaria, which is believed to have reduced resistance to Epstein-Barr virus (EBV), allowing it to take hold. The disease characteristically involves the jaw or other facial bone, distal ileum, cecum, ovaries, kidney or the breast.

The sporadic type of Burkitt lymphoma (also known as "non-African") is another form of non-Hodgkin lymphoma found outside of Africa. The tumor cells have a similar appearance to the cancer cells of classical African or endemic Burkitt lymphoma. Again it is believed that impaired immunity provides an opening for development of the Epstein-Barr virus. Non-Hodgkin lymphoma, which includes Burkitt's, accounts for 30-50% of childhood lymphoma. The jaw is less commonly involved, compared to the endemic variant. The ileo-cecal region is the common site of involvement.

Immunodeficiency-associated Burkitt lymphoma is usually associated with HIV infection or occurs in the setting of post-transplant patients who are taking immunosuppressive drugs. Burkitt lymphoma can be one of the diseases associated with the initial manifestation of AIDS.

By morphology (i.e. microscopic appearance) or immunophenotype, it is almost impossible to differentiate these three clinical variants. Immunodeficiency-associated Burkitt lymphoma may demonstrate more plasmacytic appearance or more pleomorphism, but these features are not specific.

The tumor consists of sheets of a monotonous (i.e. similar in size and morphology) population of medium size lymphoid cells with high proliferative activity and apoptotic activity. The "starry sky" appearance seen under low power is due to scattered tingible body-laden macrophages (macrophages containing dead body of apoptotic tumor cells). The old descriptive term of "small non-cleaved cell" is misleading. The tumor cells are mostly medium in size (i.e. tumor nuclei size similar to that of histiocytes or endothelial cells). "Small non-cleaved cells" are compared to "large non-cleaved cells" of normal germinal center lymphocytes. Tumor cells possess small amount of basophilic cytoplasm. The cellular outline usually appears squared off.

Almost by definition, Burkitt's lymphoma is associated with a chromosomal translocation of the c-myc gene. This gene is found at 8q24.

Treatment includes dose-adjusted EPOCH with Rituxan (rituximab).

Effect of the chemotherapy, as with all cancers, depends on the time of diagnosis. With faster growing cancers, such as Burkitt's, the cancer actually responds faster than with slower growing cancers. This rapid response to chemotherapy can be hazardous to patient, as a phenomenon called "tumor lysis syndrome" could occur. Close monitoring of patient and adequate hydration is essential during the process.

Chemotherapy:
cyclophosphamide
doxorubicin
vincristine
methotrexate
cytarabine
ifosfamide
etoposide
rituximab

Other treatments are immunotherapy, bone marrow transplants, surgery to remove the tumor, and radiotherapy.

http://en.wikipedia.org/wiki/Burkitt's_lymphoma

AIDS Defining Clinical Condition

2011-12-26T23:40:00.005+02:00

AIDS Defining Clinical Condition

Candidiasis of bronchi, trachea, or lungs
Candidiasis esophageal
Coccidioidomycosis, disseminated or extrapulmonary
Cryptococcosis, extrapulmonary
Cryptosporidiosis, chronic intestinal for longer than 1 month
Cytomegalovirus disease (other than liver, spleen or lymph nodes)
Cytomegalovirus retinitis (with loss of vision)
Encephalopathy (HIV-related)
Herpes simplex: chronic ulcer(s) (for more than 1 month); or bronchitis, pneumonitis, or esophagitis
Histoplasmosis, disseminated or extrapulmonary
Isosporiasis, chronic intestinal (for more than 1 month)
Kaposi's sarcoma
Lymphoma, Burkitt's
Lymphoma, immunoblastic (or equivalent term)
Lymphoma, primary, of brain
Mycobacterium avium complex or Mycobacterium kansasii, disseminated or extrapulmonary
Mycobacterium, other species, disseminated or extrapulmonary
Mycobacterium tuberculosis, any site (extrapulmonary)
Pneumocystis jiroveci pneumonia (formerly Pneumocystis carinii)
Progressive multifocal leukoencephalopathy
Salmonella septicemia (recurrent)
Toxoplasmosis of the brain
Tuberculosis, disseminated
Wasting syndrome due to HIV
Cervical cancer (invasive)
Mycobacterium tuberculosis, any site (pulmonary)
Pneumonia (recurrent)

And in Children < 13 years:

Bacterial infections, multiple or recurrent
Lymphoid interstitial pneumonia or pulmonary lymphoid hyperplasia complex