Oculomotor Apraxia

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Disease Entity

Oculomotor apraxia (OMA) was first described in 1953 by American Ophthalmologist David Glendenning Cogan as a defect in, or absence of, the ability to perform voluntary eye movements[1]. Affected patients are unable to willingly draw their eyes to an object that grabs their attention but can otherwise freely gaze left and right. When their attention is drawn, patients with OMA exhibit head thrusting or blinking in order to bring the object into their gaze[1]. Although Cogan’s initial case series describes OMA as specifically affecting horizontal gaze, OMA is not always confined to horizontal movement, and the various manifestations have been described as a part of many neurological disorders. There is debate as to whether OMA is truly an “apraxia”, as true apraxias are failures to voluntarily initiate learned movements. In contrast, movements of the eyes are not widely considered as learned.

Etiology

Congenital OMA

Idiopathic congenital OMA has been coined “Cogan-type” and presents with symptoms during the first year of life. It is thought to be a genetic condition; however, an inheritance pattern has not yet been identified. Possible risk factors include gestational and perinatal morbidity[2]. In addition to horizontal OMA, these children also often have developmental delay, hypotonia, and speech problems[1,2]. The pathophysiology of this disorder has yet to be elucidated. Problems in the development of the Frontal Eye Fields (FEF), Superior colliculus, Paramedian Pontine Reticular Formation (PPRF), and/or Medial longitudinal Fasciculus (MLF) have all been suggested as possible causes of OMA, as these are involved in volitional horizontal eye movements. This is supported by the fact that ocular symptoms in these children improve as they develop; however, developmental delay (if present) often persists[3].

Acquired OMA

Cases have been reported of patients developing OMA after bilateral lesions of the posterior cerebral hemisphere, frontal eye fields (FEF) and bilateral basal ganglia infarcts.[4]

Associated Conditions

Other disorders in which OMA has been described include: Ataxia with oculomotor apraxia type 1 (AOA1), Ataxia with oculomotor apraxia type 2 (AOA2), Ataxia-Telangiectasia, Abetalipoproteinemia (vitamin E deficiency), Alagille's syndrome, Cockayne syndrome, Gaucher disease, Joubert syndrome, Lowe's syndrome, Niemann-Pick type C, Neurofibromatosis type 1, Pelizaeus-Merzbacher disease, Tay-Sachs disease, and Wilson's disease. [5]

Diagnosis

OMA is diagnosed clinically.

Symptoms

Children with congenital OMA have difficulty fixating on targets and may initially be mistaken as blind. Once head control is established—usually by 6 months of age—an affected child will begin to exhibit head thrusting in order to fixate on an object that has their attention. Head thrusting turns the head well past the object of interest, activating the vestibulo-ocular reflex as fluid in the semicircular canals stimulates eye movement in the direction of the thrust. Once the object is in view, the head is then turned back to its normal position[6]. If the child has concomitant poor head control, head thrusts may be delayed or even absent.

OMA patients also exhibit failure in the quick phase of optokinetic and vestibular nystagmus, resulting in what Harris et al. describes as “locking up”. In this phenomenon, the failure of a quick phase to reset eye position leads to an unchecked slow phase, leading to a “locked up” extreme deviation of the eyes at their mechanical limit.[2].

Other findings that present concomitantly with Cogan-type OMA include saccadic hypometria, low gain smooth pursuit, and strabismus[6].

Imaging

Neuroradiological findings may be normal, however, abnormalities of the cerebellar vermis, the fourth ventricle, and less so the corpus callosum have been described in cases of OMA. CT, PET and MRI may be indicated to determine if any of these findings are present[7].

Genetic Associations

Ataxia-telangiectasia

Ataxia-telangiectasia (ATM) is an autosomal recessive disorder characterized by the development of ataxia, chorea, myoclonus and other neuropathies in childhood. ATM is caused by mutations in the ATM gene which regulates cell division and is needed for some forms of DNA double-strand break repair[8]. Ocular involvement includes telangiectasias that develop in the conjunctiva. OMA, both horizontal and vertical, is seen in about one third of patients[9]. Hypometric saccades, pursuit abnormalities, and nystagmus are also frequently seen[10]. As in other DNA repair deficiencies, neurodegeneration is a hallmark[11]. Other features of the disorder are chronic lung infections, leukemias and lymphomas due to problems with DNA repair. These patients will have high levels of AFP.

Ataxia with oculomotor apraxia type 1

Ataxia with oculomotor apraxia type 1 (AOA1) is also termed early-onset ataxia with ocular motor apraxia and hypoalbuminemia (EOAH)[12]. AOA1 is caused by mutations in the aprataxin gene, APTX, which is involved in nucleotide excision repair[13]. In contrast to Cogan-type OMA, OMA in AOA1 presents at a mean age of 6.8 years of age with limb ataxia, dysarthria, OMA, peripheral neuropathy, and progression of neurological deficits [14,15]. The disorder is characterized by early onset cerebellar ataxia and chorea due to atrophy of the cerebellar vermis, cognitive impairment, and sensorimotor neuropathy[16]. These patients do not seem to be at an increased risk of cancer, unlike those affected with Ataxia-telangiectasia. Another key differentiation between Cogan-type and AOA1 is the loss of vestibulo-ocular reflex (VOR) cancellation seen in AOA1. This presents as normal saccadic initiation but hypometric, successive saccades[14]. Key biomarkers for this disorder include hypoalbuminemia, hypercholesterolemia and elevated AFP[17]

Ataxia with oculomotor apraxia type 2

Ataxia with oculomotor apraxia type 2 (AOA2) is an autosomal recessive cerebellar ataxia and presents with ataxia and sensorimotor neuropathy. AOA2 is caused by a mutation in the SETX gene which codes for senataxin. These patients present with ataxia, sensory motor neuropathy, primary ovarian failure, chorea and OMA in half of patients affected. Mean age of presentation is 14.6[9]. Chorea is less common but tends to persist longer in AOA2 than in AOA1. OMA is present in approximately 51% of individuals with AOA2, and head thrusts are only seen in a subset of these patients[9]. Increased serum cholesterol is often seen, and elevated AFP is present in > 90% of cases.

Management

Supportive therapy is recommended for patients with all types of OMA. The compensatory head thrusting tends to improve over time, possibly due to either improved saccades or compensatory strategies. Treatment of OMA secondary to an underlying disorder should be focused on treatment of the disorder. When presentation and laboratory results suggest an underlying genetic source of OMA such as AOA1 or AOA2, gene testing is available. A team approach is suggested, involving the child’s family, nurses, pediatrician, neurologist, physical therapists, genetic counselors and educators.

Cogan-type OMA has been associated with juvenile nephronophthisis type 1. However, there are no current guidelines to suggest screening or genetic testing[18].

References

  1. Cogan, D.G., A Type of Congenital Ocular Motor Apraxia Presenting Jerky Head Movements: The Jackson Memorial Lecture. American Journal of Ophthalmology, 1953. 36(4): p. 433-441.
  2. Harris, C.M., et al., Intermittent horizontal saccade failure ('ocular motor apraxia') in children. The British journal of ophthalmology, 1996. 80(2): p. 151-158.
  3. Wente, S., et al., Nosological delineation of congenital ocular motor apraxia type Cogan: an observational study. Orphanet journal of rare diseases, 2016. 11(1): p. 104-104.
  4. Chung, P.-W., et al., Ocular motor apraxia after sequential bilateral striatal infarctions. Journal of clinical neurology (Seoul, Korea), 2006. 2(2): p. 134-136.
  5. American Academy of Ophthalmology, Ocular motor apraxia. 2019. Clinical education / basic and clinical science course excerpt. https://www.aao.org/bcscsnippetdetail.aspx?id=ec1f5667-04e0-42af-8c13-4d8776f9039a. Web. 2019
  6. Purves D, A.G., Fitzpatrick D, et al., editors., Types of Eye Movements and Their Functions. Neuroscience 2nd edition., 2001.
  7. Kondo, A., et al., Congenital ocular motor apraxia: Clinical and neuroradiological findings, and long-term intellectual prognosis. Brain and Development, 2007. 29(7): p. 431-438.
  8. National Institute of Health., Ataxia-telangiectasia. Genetics Home Reference. https://ghr.nlm.nih.gov/condition/ataxia-telangiectasia#genes. Web. 2019.
  9. Moreira MC, K.M., Ataxia with Oculomotor Apraxia Type 2. GeneReviews, 2004 Nov 15 [Updated 2018 Jul 12].
  10. Farr, A.K., et al., Ocular manifestations of ataxia-telangiectasia. American Journal of Ophthalmology, 2002. 134(6): p. 891-896.
  11. Le Ber I, Brice A Durr A. New autosomal recessive cerebellar ataxias with oculomotor apraxia. Curr Neurol Neurosci Rep. 2005;5:411-7.
  12. Inoue N, I.K., Mawatari S et al., ocular motor apraxia and cerebellar degeneration -report of two cases. Clinical Neurology, 1971. 11: p. 855-861.
  13. Date, H., et al., Early-onset ataxia with ocular motor apraxia and hypoalbuminemia is caused by mutations in a new HIT superfamily gene. Nature Genetics, 2001. 29: p. 184-188.
  14. Le Ber I, D.A., Brice A, Autosomal Recessive Cerebellar Ataxias with Oculomotor Apraxia. Handbook of Clinical Neurology, 2012. 103: p. 333-41.
  15. Ferrarini M, Squintani G, Cavallaro T, et al. A novel mutation of aprataxin associated with ataxia ocular apraxia type I: phenotypical and genotypical characterization. J Neurol Sci 2007;260:219-24.
  16. Strupp, M., et al., Central ocular motor disorders, including gaze palsy and nystagmus. Journal of neurology, 2014. 261 Suppl 2(Suppl 2): p. S542-S558.
  17. Renaud, M., et al., Clinical, Biomarker, and Molecular Delineations and Genotype-Phenotype Correlations of Ataxia With Oculomotor Apraxia Type 1. JAMA neurology, 2018. 75(4): p. 495-502.
  18. Betz, R., et al., Children with ocular motor apraxia type Cogan carry deletions in the gene (NPHP1) for juvenile nephronopthisis. The Journal of Pediatrics, 2000. 136(6): p. 828-831.