Section 6.2: Differentials in Dermatological and Psychiatric Conditions

Section 6.2: Differentials in Dermatological and Psychiatric Conditions

Atopic Dermatitis (Eczema) vs. Psoriasis

Differentiating inflammatory skin disorders is a common clinical task. Atopic dermatitis and psoriasis are two prevalent chronic inflammatory conditions with distinct etiologies, clinical features, and locations.

• Atopic Dermatitis (Eczema): Primarily a disease of barrier dysfunction (often related to filaggrin gene mutations) combined with immune dysregulation. Known as "the itch that rashes," pruritus is the hallmark symptom. Acute lesions present as erythematous, poorly demarcated, pruritic papules and vesicles that can weep and crust. Chronic scratching leads to lichenification (thickening of the skin with exaggerated markings). In children and adults, eczema characteristically involves the flexural surfaces (antecubital and popliteal fossae).
• Psoriasis: An immune-mediated disease characterized by T-cell driven epidermal hyperproliferation, leading to accelerated keratinocyte turnover (reduced from the normal 28 days to only 3–5 days). Classically presents as well-demarcated, erythematous plaques covered with adherent, silvery-white scales. It primarily affects the extensor surfaces (elbows, knees), scalp, and sacrum. Key diagnostic signs include the Auspitz sign (pinpoint bleeding when a scale is peeled off) and the Koebner phenomenon (development of psoriatic plaques at sites of physical trauma).

Herpes Zoster (Shingles)

Herpes zoster is caused by the reactivation of the latent varicella-zoster virus (VZV) within the sensory dorsal root ganglia.

• Clinical Presentation: Characterized by a unilateral, painful, vesicular eruption that follows a single dermatome and does not cross the midline. It is preceded by a prodrome of localized pain, burning, itching, or hyperesthesia in the dermatome for 2–3 days before the rash appears.
• Complications: The most common complication is postherpetic neuralgia (PHN), defined as pain persisting for more than 90 days after the onset of the rash. Involvement of the ophthalmic branch of the trigeminal nerve (CN V1) can lead to herpes zoster ophthalmicus, heralded by vesicles on the tip of the nose (Hutchinson's sign), which is a medical emergency requiring immediate ophthalmology referral to prevent blindness.
• Management: Oral antiviral therapy (e.g., valacyclovir 1000 mg three times daily for 7 days) should be initiated within 72 hours of rash onset. The recombinant zoster vaccine (Shingrix) is highly effective and recommended as a two-dose series for immunocompetent adults aged 50 years and older to prevent shingles and PHN.

Skin Cancers

Psychiatric Disorders: Depression, Anxiety, and ADHD

• Major Depressive Disorder (MDD): Diagnosed using the DSM-5 criteria, requiring 5 or more symptoms during the same 2-week period (including depressed mood or anhedonia). Symptoms are recalled using the SIGECAPS mnemonic (Sleep, Interest, Guilt, Energy, Concentration, Appetite, Psychomotor, Suicidal ideation). Screening starts with the Patient Health Questionnaire-2 (PHQ-2), escalating to the PHQ-9 if positive. First-line treatments are SSRIs. Practitioners must screen for history of mania/hypomania to rule out bipolar disorder before prescribing, as antidepressants can trigger manic episodes.
• Generalized Anxiety Disorder (GAD): Defined as excessive, uncontrollable worry about multiple events for at least 6 months, accompanied by at least 3 physical or cognitive symptoms (e.g., muscle tension, irritability, fatigue, sleep disturbance). First-line therapies include SSRIs/SNRIs and cognitive behavioral therapy (CBT). Benzodiazepines should be avoided for long-term use, especially in older adults due to falls and cognitive impairment risks (Beers Criteria).
• Attention-Deficit/Hyperactivity Disorder (ADHD): A neurodevelopmental disorder marked by persistent inattention and/or hyperactivity-impulsivity that interferes with functioning. Symptoms must be present before age 12 and occur in two or more settings. Diagnosis is supported by rating scales (e.g., Vanderbilt for children, ASRS for adults). Stimulants (e.g., methylphenidate, mixed amphetamine salts) are first-line. FNPs must monitor blood pressure, heart rate, and pediatric growth (weight/height) at every visit.

Clinical Traps & Exam Tips

• The Bipolar Trap: Always screen a depressed patient for a history of mania before starting an SSRI. Unopposed antidepressant therapy in patients with bipolar disorder can precipitate manic episodes or rapid cycling.
• Hutchinson's Sign: Vesicles on the tip of the nose in a shingles patient indicate involvement of the nasociliary nerve (CN V1) and predict ocular involvement. Refer immediately to ophthalmology.
• Actinic Keratosis Precursor: Actinic keratosis (rough, scaly, sandpaper-like lesions in sun-exposed areas) is a precursor to Squamous Cell Carcinoma, not Basal Cell Carcinoma.

Worked Clinical Scenario

A 34-year-old female presents with a 2-week history of worsening rash on her forearms and face. She describes it as intensely itchy, which keeps her awake at night. On examination, the FNP notes ill-defined, erythematous papules and plaques with overlying excoriations and dry, scaling skin in both antecubital fossae. The patient has a history of asthma and seasonal allergies.

• Clinical Reasoning: The presentation is consistent with atopic dermatitis (eczema) flare, supported by the patient's history of asthma and allergies (representing the atopic triad: eczema, asthma, allergic rhinitis). The distribution in the antecubital fossae (flexural surfaces) is classic for adult eczema. First-line management includes establishing a baseline barrier repair routine with thick, alcohol-free emollients applied within 3 minutes of bathing, and prescribing a medium-potency topical corticosteroid cream (e.g., triamcinolone acetonide 0.1%) for the body lesions. For the face, a low-potency corticosteroid (e.g., hydrocortisone 1% or 2.5%) or a topical calcineurin inhibitor (e.g., tacrolimus) should be used to avoid skin atrophy, telangiectasia, and striae.