What is the most common cause of acute red bumps or spots on the forearms in individuals of any age?

Most Common Cause of Acute Red Bumps/Spots on Forearms

Contact dermatitis—either irritant or allergic—is the most common cause of acute red bumps or spots on the forearms, with irritant contact dermatitis being more prevalent than allergic contact dermatitis and having a worse prognosis. 1

Primary Diagnostic Considerations

Contact Dermatitis (Most Common)

• Irritant contact dermatitis (ICD) results from direct inflammatory pathways without prior sensitization and is induced by cumulative effects of water, soaps, detergents, and other chemical or physical agents 1, 2
• Allergic contact dermatitis (ACD) is a type IV delayed hypersensitivity reaction requiring prior sensitization (5-16 days after first exposure), with subsequent exposures causing inflammation 1, 3
• The forearms are frequently exposed sites vulnerable to both airborne irritants and direct contact with environmental substances 3, 1
• Common allergens include nickel, fragrances, rubber accelerators, and biocides 1

Drug-Related Reactions (Important Differential)

• Drug hypersensitivity reactions can present as delayed cutaneous eruptions appearing days after medication exposure, with rashes varying from maculopapular to more severe patterns 4
• The latency period combined with rash characteristics helps identify the clinical phenotype 4
• Drug reactions should be considered if there is recent medication initiation or change 4

Infectious Causes (Critical to Exclude)

• Rocky Mountain Spotted Fever (RMSF) classically begins as small (1-5 mm) blanching pink macules on ankles, wrists, or forearms that spread centrally and become maculopapular with central petechiae 4
• RMSF appears 2-4 days after fever onset (incubation 3-12 days post-tick bite) and is accompanied by fever, headache, chills, and myalgia 4
• This diagnosis carries 5-10% mortality and requires immediate recognition and treatment 4

Clinical Approach Algorithm

Step 1: Assess for Systemic Symptoms

• If fever, headache, malaise present: Consider RMSF or other tickborne rickettsial diseases, especially with recent outdoor exposure in endemic areas 4
• If no systemic symptoms: Contact dermatitis is most likely 1

Step 2: Identify Exposure History

• Recent new exposures (soaps, lotions, jewelry, occupational chemicals, plants) suggest ICD or ACD 1, 3
• Recurrent/persistent pattern with no previous eczema history strongly suggests contact dermatitis 1
• Dorsal aspects of hands and forearms are classic sites for ICD from cumulative water and detergent exposure 1

Step 3: Examine Lesion Characteristics

• ICD: Erythema, scaling, possible fissuring in distribution of irritant contact; often involves finger webs if hands affected 1, 2
• ACD: Erythema, edema, vesicles possible; distribution matches allergen contact pattern 3, 1
• RMSF: Small blanching macules progressing to maculopapular with petechiae; involves palms/soles by day 5-6 4

Step 4: Consider Atopic/Seborrheic Dermatitis

• Atopic dermatitis typically affects flexural areas (antecubital fossae) rather than extensor forearms, with chronic pruritus, xerosis, and lichenification 4
• Seborrheic dermatitis presents with greasy yellowish scales, not typically on forearms 5, 6

Critical Pitfalls to Avoid

• Do not dismiss systemic symptoms: Fever with forearm rash requires immediate evaluation for RMSF, which has high mortality if untreated 4
• Do not overlook occupational exposures: ICD is the most common cause of occupational dermatitis 3, 1
• Do not assume all rashes are benign: Drug reactions can progress to severe phenotypes like DRESS if the culprit medication is continued 4
• Do not confuse acute contact dermatitis with chronic atopic dermatitis: Contact dermatitis occurs in individuals with no previous eczema history and has identifiable exposures 1

Initial Management Approach

For Suspected Contact Dermatitis (Most Common Scenario)

• Immediately identify and remove the suspected irritant or allergen 3, 1
• Apply emollients to restore skin barrier function 4, 3
• Use topical corticosteroids for inflammation reduction 3
• Refer for patch testing if ACD is suspected to identify specific allergens 1

For Suspected Infectious/Systemic Causes

• Urgent evaluation and empiric treatment if RMSF suspected based on fever, systemic symptoms, and appropriate epidemiology 4
• Do not wait for complete clinical triad (fever, rash, tick bite) as this is present in only a minority at initial presentation 4