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Starch–iodine test: a diagnostic tool for Horner syndrome
  1. Luís Ribeiro1,
  2. Raquel Rocha1,2,
  3. João Martins1,3 and
  4. Ana Monteiro1,4
  1. 1Neurology Department, Hospital Pedro Hispano, Unidade Local de Saúde de Matosinhos, Matosinhos, Portugal
  2. 2Harvard Medical School, Boston, Massachusetts, USA
  3. 3MedicilLisboa, Lisboa, Portugal
  4. 4Faculty of Medicine of University of Porto, Porto, Portugal
  1. Correspondence to Dr Raquel Rocha; rmrocha84{at}

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We present the case of a 62-year-old woman that reported right hemibody excessive sweating, which slowly progressed over 15 years, and which caused great social embarrassment. On examination, right hemibody hyperhidrosis was observed. A very slight anisocoria was also present (ambient light: right 3 mm; left 2 mm; light stimulation: right 2 mm, left 1.5 mm; dark: right: 5 mm; left 2.5 mm). The patient previously underwent bilateral blepharoplasty for excess palpebral skin, complicating evaluation of ptosis; however, a slight left ptosis was apparent. A Harlequin sign was not reported by the patient, nor noticed after exercise.

A starch–iodine test was performed, revealing left hemibody anhidrosis. To perform the starch–iodine test, we first thoroughly dried the patient’s skin with a 70% alcohol solution, then applied an iodine solution and allowed the skin to dry completely. Next, we dusted the skin with a thin film of starch (corn flour) (figure 1). Finally, we encouraged sweating (hot room and exercise) and after 25 min we evaluated the results. The patient’s right hemibody readily turned a dark blue shade when in contact with sweat, while the left side remained unchanged (figure 2).

Figure 1

Thoroughly dry the skin with an alcohol solution. Then, apply an iodine solution and let it dry completely. Dust with a thin film of starch.

Figure 2

Encourage sweating (hot room, exercise): an unforgettable dark blue coloration develops on the normal hemibody, strictly respecting the midline, and thus demonstrating contralateral anhidrosis.

An apraclonidine test was performed, causing left pupil dilatation. Cervico–thoracoabdominal–pelvic CT scan, neuraxis MRI, carotid doppler ultrasound and extensive laboratorial study were unremarkable. Based on these results, the diagnosis of idiopathic Horner syndrome was made. The patient was then administered oxybutynin (an anticholinergic drug) to reduce the excessive sweating on the contralateral hemibody, resulting in a greatly improved quality of life.

Currently, the starch–iodine test is rarely used in clinical practice. However, it is simple, inexpensive and does not produce the common side effects associated with other tests used in this context (such as the cocaine or apraclonidine tests, quantitative sudomotor axon reflex and thermoregulatory sweat tests). Moreover, the starch–iodine test is readily available and easy to interpret, even for patients, provided they are told what to expect.1 2 In this case, the test demonstrated to the patient that the problem was in fact contralateral lack of sweating with compensatory excessive sweating on the right. Overall, the test aided immensely in clarifying the diagnosis.

For all of these reasons, we believe that the starch–iodine bedside test can be a very effective alternative for hospitals with few technical resources and/or where more specialised diagnostic testing isdifficult to obtain in a timely manner (an increasingly important consideration given the current pressures exerted on global health systems by the COVID-19 pandemic).

Learning points

  • Horner syndrome is clinically diagnosed when the patient presents decreased pupil size aggravated in scotopic conditions, ipsilateral drooping eyelid and decreased sweating on the affected side of the body.

  • The starch–iodine test is a simple but powerful tool for the diagnosis of Horner syndrome. It is readily available, inexpensive and devoid of side effects.

  • When positive, the normal hemibody acquires a dark blue or purple coloration, while the affected hemibody remains unchanged, thus confirming a lack of sweating on the affected side.


We thank Scott Lipkowitz and Christian Bright for proofreading.



  • Contributors LR and AM conceived the presented idea. LR and JM did the first draft. RR and AM supervised the findings of this work and help writing the final manuscript. All authors discussed and contributed to the final manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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