Generally speaking a normal platelet count ranges from 150,000 and 450,000 per mm³. These limits, however, are determined by the 2.5th lower and upper percentile, and a deviation does not necessary imply any form of disease.

Signs and symptoms

Often, low platelet levels do not lead to clinical problems; rather, they are picked up on a routine full blood count. Occasionally, there may be bruising, nosebleeds and/or bleeding gums.

It is vital that a full medical history is elicited, to ensure the low platelet count is not due to a secondary process. It is also important to ensure that the other blood cell types red blood cells, and white blood cells, are not also suppressed.

Diagnosis

Laboratory tests might include: full blood count, liver enzymes, renal function, vitamin B12 levels, folic acid levels, erythrocyte sedimentation rate.

If the cause for the low platelet count remains unclear, bone marrow biopsy is often undertaken, to differentiate whether the low platelet count is due to decreased production or peripheral destruction.

Causes

Decreased platelet counts can be due to a number of disease processes:

decreased production
- vitamin B12 or folic acid deficiency
- leukemia or myelodysplastic syndrome
peripheral destruction
- immune thrombocytopenic purpura (ITP)
- thrombotic thrombocytopenic purpura (TTP)
- hemolytic-uremic syndrome (HUS)
- disseminated intravascular coagulation (DIC)
- paroxysmal nocturnal hemoglobinuria
- antiphospholipid syndrome
- medication-induced:
  - Many of the commonly used drugs may cause thrombocytopenia or low platelet counts. Some drugs like anticancer drugs and valproic acid causes thrombocytopenia in a dose depended mechanism by causing myelosuppression. Some other groups of drugs cause thrombocytopenia by immunological mechanisms. Based up on the mechanism immunological drug induced can be caused by two types.
  - Example of the first mechanism is the quinidine group of drugs. This is caused by drug depended binding of Fab part of the pathological antibody with the platelets, causing the destruction of platelets.. Fc portion of the antibody molecule is not involved in the binding process.
  - Example of the second mechanism is heparin induced thrombocytopenia (HIT). In this type the Fab portion of the pathological antibody binds to platelet factor 4 (PF4).When complexed with heparin or other drugs, the Fc portion of the antibody molecule bind to platelet receptors causing platelet activation. Since Fc portion of the antibody is bound to the platelets, they are not available to the Fc receptors of the reticulo-endothelial cells. This may explain, why severe thrombocytopenia not commonly seen in patients with HIT.
  - A full list of known drugs causing thrombocytopenia is available at the linked website. Most of the elderly patients are on multiple medications and the intake of these drugs must always be considered in the differential diagnosis of thrombocytopenia.
  - heparin-induced thrombocytopenia (HIT or white clot syndrome): this is a rare but serious condition that may occur in a hospitalized population especially in the cardiac units where they are exposed to large quantities of heparin. HIT may occur with a delay of 4 to 14 days after exposure to heparin. As mentioned above the heparin-PF4 antibody complex will activate the platelets, and this will lead to clotting. A term known as paradoxical thrombosis (HITT, where the last T is for thrombosis) is often used to describe this condition.
  - abciximab-induced thrombocytopenia

In some tropical countries, dengue infection is a known rather common cause of thrombocytopenia associated with fever.

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Acute respiratory failure with thrombocytopenia in a 47-year-old woman after hiking in the Sierras - pulmonary and critical care pearls
From CHEST, 8/1/03 by Viji Sankaranarayanan

A 47-year-old previously healthy woman presented to the emergency department with a 1-week history of fatigue and abdominal discomfort. Four days earlier, right flank pain developed with radiation to the groin, for which she was evaluated with CT scan of the abdomen and pelvis and treated with acetaminophen/hydrocodone For renal colic, though no renal stone was seen. Two days prior, she continued to have abdominal and back pain with nausea, nonbloody emesis, and temperature of 38.5[degrees]C. On evaluation, she reported a dry cough of 1 day in duration, but denied dyspnea or chest pain.

She had no significant medical or surgical history. She had spent a week in the California Sierra Nevada range hiking and bird watching 4 to 5 weeks before presentation. She lived in a wooded neighborhood, and owned no pets. One week prior to this episode, during inspection of the plumbing in the crawl space of her home, she noted rodent droppings.

Physical Examination

Temperature was 38.2[degrees]C, pulse was 98 beats/min, BP was 100/60 mm Hg, respiratory rate was 18 to 22 breaths/min, and resting room air saturation was 94%. In general, she was a young woman in no acute distress. Lung examination revealed bibasilar inspiratory crackles. Cardiac examination was unremarkable except for tachycardia. No jugular distension was noted. Abdominal examination was entirely benign. The remainder of her physical examination was normal.

Laboratory Findings

Initial blood counts showed a leukocyte count of 3,400/[micro]L, with normal differential, hematocrit of 35%, and platelet count of 77,000/[micro]L. Findings of serum chemistries, liver function tests, and urine analysis were normal. Chest radiography revealed normal cardiac silhouette with bibasilar interstitial markings and prominent Kerley B lines (Fig 1). CT scan of the abdomen and pelvis was unremarkable, except for small bilateral pleural effusions and interstitial prominence of the lower lung fields that were new in comparison to an abdominal CT scan 4 days earlier.

[FIGURE 1 OMITTED]

Hospital Course

The patient was admitted to the hospital for further evaluation and was treated with IV fluids and moxifloxacin. Over 24 h, her clinical status declined with persistent fevers, increasing dyspnea, and worsening oxygenation requiring a 100% nonrebreather mask to maintain oxygen saturation of 92 to 94%.

Physical examination showed diminished air entry at both bases with dullness on percussion, scattered rales, and without jugular venous distension or peripheral edema. Repeat chest radiography revealed increased bibasilar interstitial edema and alveolar opacities, now with large pleural effusions (Fig 2). Transthoracic echocardiography findings were entirely normal. Resting room air arterial blood gas revealed pH 7.49, PaC[O.sub.2] of 25 mm Hg, and Pa[O.sub.2] of 55 mm Hg. Blood counts demonstrated an increased WBC count (9,100/[micro]L) with > 20% bands, a decreased platelet count (33,000/[micro]L), and increased hematocfit (40%) suggesting hemoconcentration. Blood smear showed > 10% reactive lymphocytes with immunoblastic features: metamyelocytes and bands with no toxic granulation in neutrophils and thrombocytopenia (Fig 3). Coagulation studies were normal except for a mildly prolonged activated partial thromboplastin time (41.9 s). Blood culture findings were negative. Bronchoscopy revealed normal airways and a predominantly monocytic BAL fluid with negative bacterial, fungal, and viral stains. A diagnostic right thoracentesis revealed a monocytic (cell count, 80/[micro]L) exudate by protein criteria: pleural fluid to serum ratios of 0.6 and 0.4 for protein and lactate dehydrogenase, respectively.

[FIGURE 2-3 OMITTED]

What is the diagnosis?

What further test would help in establishing the diagnosis?

Diagnosis: Hantavirus pulmonary syndrome

Diagnostic test: Hantavirus serology--enzyme-linked immunosorbent assay for IgM and paired IgG

DISCUSSION

Hantavirus pulmonary syndrome (HPS) is a rodent-borne viral infection presenting as acute febrile illness with progressive cardiorespiratory compromise and bilateral interstitial pulmonary infiltrates resembling ARDS. First described in 1993 after a cluster of deaths in Near Mexico, this disease is now recognized as a pan-American zoonosis; to date, 335 cases have been reported from 31 states, with a mortality rate of 38%.

HPS is caused by certain hantavirus species, a group of closely related but genetically distinct single-stranded RNA viruses that belong to the Bunyaviridae family, with distinct rodent species as primary reservoirs. In the United States, the most common causative virus is the Sin Nombre virus (SNV), which is transmitted to humans from the deer mouse (Peromyscus maniculatus), a rodent with widespread geographic distribution. Despite an immune response to eliminate viremia, infected rodents shed virus in urine, saliva, and feces for weeks to months. Infection is acquired by inhalation of aerosolized particles of virus-contaminated rodent excreta, though disease can also be transmitted through rodent bites. There is no report of person-to-person transmission of SNV. Risk of disease is related to the extent of exposure to rodents, with most documented cases occurring in rural areas with high rodent densities, usually during spring and summer months. Case-control study data suggest that most exposures occur during peridomestic activities such as cleaning rodent-infested houses or cabins. Symptoms usually begin 1 to 4 weeks after exposure, but incubation periods as long as 6 weeks have been reported.

Clinically, HPS is characterized by a short prodrome of 3 to 6 days with malaise, headaches, myalgia, abdominal symptoms, and sudden onset of fever. Typically, cough, rhinorrhea, sore throat, and nasal congestion are absent and the physical examination is normal. Most patients are treated symptomatically, and diagnosis is seldom established at this initial stage as illustrated in the present patient. Laboratory studies such as low platelet count may suggest HPS in a patient with a history of rodent exposure.

The second, or cardiopulmonary, phase of HPS begins about day 7 with onset of dyspnea, tachypnea, hypoxemia, and hypotension. Presentation during this phase typically leads to hospitalization. The tetrad of thrombocytopenia, elevated WBC count with left shift, circulating immunoblasts, and elevated hematocrit should suggest HPS. This stage is marked by increased vascular permeability and development within 24 to 48 h of noncardiogenic pulmonary edema. Most patients require mechanical ventilation for management of respiratory failure. Data from pulmonary arterial catheterization help to differentiate HPS from ABDS secondary to septic shock by finding low-to-normal pulmonary artery wedge pressures, low cardiac output, and high peripheral vascular resistance. Echocardiography may show depressed myocardial contractility. Pleural effusions are common in HPS, and thoracentesis reveals a sterile mononuclear transudate during the early cardiopulmonary phase and exudate during established capillary leak phase. In contrast to the more common causes of ARDS, HPS is marked by respiratory compromise with no evidence of multi-system failure.

The third stage, recovery, is marked by improvement in oxygenation and platelet count, with resolution of pulmonary edema. Radiographic progression of HPS is typical, with initial radiographs showing mild bibasilar interstitial infiltrates with normal cardiac silhouette, and subsequent studies demonstrating increased interstitial and alveolar opacities and pleural effusions.

Differential diagnosis includes atypical infections associated with a prodrome such as mycoplasma, chlamydia, leptospira, legionella, Q fever, tularemia, septicemic plague, and other noninfectious causes of ARDS. A history of rodent exposure, rapid progressive respiratory failure, radiographic evidence of pulmonary edema, and the tetrad of peripheral smear findings should raise clinical suspicion of HPS.

Diagnosis is established serologically by detecting hantavirus and SNV-specific IgM antibodies by enzyme-linked immunosorbent assay. Alternately, paired acute and convalescent serum IgG showing a fourfold elevation in titers is considered diagnostic. Cytomegalovirus, influenza virus, and mycoplasma infections may cause false-positive IgM results; serologic testing specific for these diseases should also be performed. Additional confirmatory testing can be done by Western blot or radioimmunoblot assay. Reverse transcriptase-polymerase chain reaction is available for detection of hantavirus RNA in tissue and blood, although it is not routinely performed. Viral isolation is difficult and is not essential for diagnosis.

Histopathologically, mild-to-moderate mononuclear interstitial pneumonitis is seen with minimal or no endothelial swelling or necrosis. The relatively preserved pulmonary parenchyma and the rapid resolution of infiltrate suggest an increased vascular permeability rather than direct cytopathic effect as the key event. Research implicates immune-mediated capillary leakage. Hantaviruses enter endothelial cells via [beta]-3 integlin and inhibit [beta]-3 integrinmediated endothelial cell migration that is crucial for capillary integrity. In addition, vascular permeability is increased by local production of tumor necrosis factor-[alpha] and interferon-[gamma] by infiltrating lymphocytes and mononuclear cells.

Treatment remains largely supportive with intensive monitoring, judicious use of fluids and vasopressors to maintain hemodynamic status, and management of hypoxemia with mechanical ventilation. Early trials with IV ribavirin have shown no benefit, although a prospective study is underway. A high index of suspicion is essential as early diagnosis and aggressive management may help reduce the high mortally. Disease prevention requires education of the community on limiting recreational, occupational, and domestic exposures to rodents, and following precautions while cleaning rodent droppings.

Clinical Course

With the development of respiratory distress, there was an improvement in the prodromal symptoms. With supplemental oxygen therapy, noninvasive positive pressure ventilation, and careful management of fluid and electrolytes, respiratory status improved over the next few days. Diagnosis of HPS was established serologically by detecting hantavirus-specific IgM and IgG antibodies using enzyme-linked immunosorbent assay (serology titers, IgM = 10.14, IgG = 12.62; positive test > 1.10). In addition, IgM and IgG antibodies to SNV were positive. Concurrent serologies for mycoplasma, cytomegalovirus, and influenza virus were negative. Moxifloxicin was discontinued on day 4 once HPS was confirmed. Hypoxemia improved, and the patient was discharged with normal oxygen saturation on day 8 of hospitalization. At a follow-up visit a week latter, she was asymptomatic and actively hiking. The platelet count was normal, and a chest radiograph showed complete resolution.

A subsequent interview of the patient revealed that dining her trip to the California Sierra Nevada range, she had opened up and stayed in a cabin that had been closed all winter. Following her case report, investigators from the state health department trapped deer mice in the area around the cabin. Twenty-three of the 27 mice (85%) tested positive for hantavirus. Deer mice in the Sierras have been known to carry hantavirus. The average prevalence across the state is 10 to 11%, with higher prevalence in the mountainous regions.

CLINICAL PEARLS

1. HPS is pan-American zoonosis that should be considered in the differential diagnosis of patients presenting with an acute febrile illness and rapidly progressive respiratory failure (ARDS) in the absence of clear precipitating cause.

2. A history of domestic, occupational, or recreational exposure to rodents should be sought.

3. Peripheral smear with thrombocytopenia, left-shifted clear myeloid series, circulating immunoblasts, and hemoconcentration is strongly suggestive of HPS.

4. HPS should be included in the radiographic differential diagnosis for acute onset bibasilar interstitial infiltrates with septal thickening/Kerley B lines.

5. Diagnosis is established by serology (delection of hantavirus-specific antibodies), demonstration of viral RNA by reverse transcriptase-polymerase chain reaction, or detection of viral antigens in tissue with monoclonal or polyclonal antibodies by radioimmunoblot assay or Western blot.